Subspeciation in the Kangaroo Rat, Dipodomys ordii KU. Vol 1 No 23
Volume 1, No. 23, pp. 473-573, 27 figures in text, 7 tables
December 27, 1949
UNIVERSITY OF KANSAS Lawrence, Kansas
PRINTED BY FERD VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1949
22-6114
Subspeciation in the Kangaroo Rat, Dipodomys ordii
By
HENRY W. SETZER
CONTENTS
PAGE
Introduction 477
Methods and Acknowledgments 478
Paleontology 480
Phylogeny of the Species of the Genus 484
Dispersal of the Several Species 498
Subspeciation 499
Accounts of Subspecies 511 _Dipodomys ordii_ 511 _Dipodomys ordii richardsoni_ 511 _Dipodomys ordii oklahomae_ 514 _Dipodomys ordii compactus_ 515 _Dipodomys ordii sennetti_ 517 _Dipodomys ordii evexus_ 518 _Dipodomys ordii medius_ 519 _Dipodomys ordii obscurus_ 521 _Dipodomys ordii terrosus_ 523 _Dipodomys ordii fremonti_ 524 _Dipodomys ordii uintensis_ 525 _Dipodomys ordii sanrafaeli_ 526 _Dipodomys ordii panguitchensis_ 527 _Dipodomys ordii monoensis_ 528 _Dipodomys ordii ordii_ 530 _Dipodomys ordii luteolus_ 533 _Dipodomys ordii extractus_ 534 _Dipodomys ordii chapmani_ 536 _Dipodomys ordii montanus_ 538 _Dipodomys ordii cinderensis_ 540 _Dipodomys ordii fetosus_ 541 _Dipodomys ordii utahensis_ 543 _Dipodomys ordii columbianus_ 544 _Dipodomys ordii idoneus_ 546 _Dipodomys ordii priscus_ 547 _Dipodomys ordii celeripes_ 549 _Dipodomys ordii cineraceus_ 550 _Dipodomys ordii marshalli_ 551 _Dipodomys ordii inaquosus_ 552 _Dipodomys ordii attenuatus_ 553 _Dipodomys ordii fuscus_ 555 _Dipodomys ordii longipes_ 556 _Dipodomys ordii pallidus_ 558 _Dipodomys ordii nexilis_ 559 _Dipodomys ordii cupidineus_ 561 _Dipodomys ordii palmeri_ 562
Conclusions 563
Tables of Measurements 565
Literature Cited 571
INTRODUCTION
The geographic range of the kangaroo rats, genus _Dipodomys_, extends from southern Canada south to the southern limits of the Mexican Tableland and from the Pacific Coast east to the eastern limits of the Great Plains in Kansas, Oklahoma and Nebraska. These animals are usually restricted to sandy soils in semiarid regions. The species _Dipodomys ordii_, with which this account is primarily concerned, is, to the best of my knowledge, almost exclusively confined to sandy areas.
Since 1841, when Gray gave the generic name _Dipodomys_ to the kangaroo rats, basing the name on the four-toed species _Dipodomys phillipsi_, several other generic names have been applied. Fitzinger, in 1867, used the name _Perodipus_ for those animals with five toes on the hind foot, designating _Dipodomys agilis_ as the type of his genus. In 1890, Merriam proposed the generic name _Dipodops_ with _Dipodomys agilis_ as the type, apparently being unaware of Fitzinger's name, _Perodipus_. Trouessart, in 1897, through what was an apparent _lapsus calami_, applied the generic name _Cricetodipus_ Peale to all of the species of the then known genera _Perodipus_ and _Dipodomys_, but Trouessart later, 1904 or 1905, in his Supplementum, corrected this _lapsus_ and used the names _Dipodomys_ and _Perodipus_. Grinnell (1919:203) showed that some of the four-toed _Dipodomys_ had five toes on one hind foot and four on the other and that _Perodipus_ must fall as a synonym of the earlier generic name _Dipodomys_ which was to be applied to all of the kangaroo rats.
_Dipodomys ordii_ was named by Woodhouse in 1853, from specimens from El Paso, Texas, but between that time and 1919 the name _ordii_ was used in combination with all of the generic names mentioned above (see synonymies under the accounts of the subspecies).
The nearest approach to a revision of the genus was Grinnell's (1922) "A Geographical Study of the Kangaroo Rats of California." Since that time, Hall and Dale (1939) revised the _D. microps_ group and Durrant and Setzer (1945) reported upon the kangaroo rats of Utah. The present paper is a review of the species _Dipodomys ordii_. Some of the objectives in this review have been to learn: (1) What kinds of kangaroo rats are subspecies of the species _Dipodomys ordii_; (2) the limits of geographic range of this full species; (3) the extremes of color, and of size and shape of the skull in this one species; (4) the significance of different sizes, shapes and colors; (5) the reasons for the existence, or formation, of selected subspecies; and (6) the relationships of _Dipodomys ordii_ to other species in the genus.
METHODS AND ACKNOWLEDGMENTS
Available specimens were arranged according to geographic origin. These were segregated as to sex and then under each sex by age. Individual variation was next measured in each of several samples in which individuals were of like geographic origin, sex, age and season. Finally, comparable materials were arranged geographically for detection of variations of systematic worth. Following preliminary studies of material thus arranged, additional specimens were collected from critical areas.
When fully adult animals (see next paragraph) were segregated as to sex, and then measured, the degree of secondary sexual variation was found to be less than the degree of individual variation; therefore in the tables of indices, no distinction as to sex has been made.
The only external measurements of the animals used were those recorded by the collectors on the labels attached to the skins. These measurements were total length, length of tail and length of hind foot. Measurements of the ear have not been used since they were not in all instances recorded by collectors and since measurements of dry ears proved to be unsatisfactory. Only measurements of fully adult specimens have been used. The term fully adult is applied only to those specimens in which the auditory bulla is shiny and translucent, the permanent P4 is fully erupted and worn, and the tail is fully striped and penicillate. No one of these characters alone was accepted as proof of adulthood but only the three in combination.
The following measurements of the skull have been used in the tables:
_Greatest length._--From the most anterior tip of the nasals to the most posterior projection of the auditory bullae.
_Greatest breadth across bullae._--From the most lateral projection of the auditory bulla on one side to the corresponding position on the other bulla.
_Breadth across maxillary arches._--Greatest breadth across arches in a plane perpendicular to the long axis of the skull.
_Width of rostrum._--Width of the premaxillae and the nasals taken immediately anterior to the upper incisors (not greatest width of nasals which is attained farther anteriorly).
_Length of nasals._--Maximum length of a nasal bone.
_Least interorbital breadth._--Least width between the orbits immediately posterior to the lacrimal processes.
_Basilar length._--From the anterior margin of the foramen magnum to the posterior border of the alveolus of one of the upper incisors.
Capitalized color terms are from Ridgway, "Color Standards and Color Nomenclature," Washington, D. C., 1912. Color determinations were made by comparing a masked area of pure color on the side of the animal with a masked rectangle of named color on Ridgway's plates in natural light always from the same angle.
Abbreviations used for specimens examined from the various collections are as follows:
AMNH--American Museum of Natural History. BYU--Brigham Young University. CNHM--Chicago Natural History Museum. CM--Carnegie Museum. CMNH--Colorado Museum of Natural History. DJC--Dixie Junior College. DRD--Donald R. Dickey Collection. KU--Museum of Natural History, University of Kansas. LACM--Los Angeles County Museum. MHS--Collection of Myron H. Swenk. MVZ--Museum of Vertebrate Zoology, University of California. OU--Museum of Zoology, University of Oklahoma. RH--Collection of Ross Hardy. UM--Museum of Zoology, University of Michigan. UN--Museum of Natural History, University of Nebraska. USAC--Utah State Agricultural College. USBS--United States Biological Surveys Collection. USNM--United States National Museum. UU--Museum of Zoology, University of Utah. TCWC--Texas Coöperative Wildlife Collection.
This study is based on 3,732 specimens which were assembled at the Museum of Natural History, University of Kansas, or studied at other institutions. For the loan of this material and for the opportunity afforded for its study, I am extremely grateful to the authorities of each of these institutions and to the owners of the private collections.
Acknowledgement is made to the Office of Research and Inventions of the United States Navy for assistance with the field work which permitted the acquisition of essential specimens from several of the critical geographic areas while the author was research assistant on a larger over-all project (N6 ori-164-T02) of which the determination of the geographic range of this rodent species, a potential host of tularemia, was one facet. Tularemia was not detected in this genus.
I extend my thanks also to Professor Stephen D. Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh for the preparation of the drawings; to Professor E. Raymond Hall, of the University of Kansas, for guidance in the study and critical assistance with the manuscript; to Professors H. H. Lane and Worthie H. Horr for valued suggestions; to Mr. J. R. Alcorn for providing specimens for dissection when he was working under the University of Kansas endowment fund; and to the other friends and associates who have given of their time and criticism.
PALEONTOLOGY
The family Heteromyidae was defined by Wood (1935:81) essentially as follows: Cheek teeth brachydont to hypsodont and even rootless; usually six cusps per molar, three on each loph; enamel rarely divided into two plates, never reduced to one; skull light, thin and papery; mastoids inflated, mastoidal breadth often greatest, never appreciably less than zygomatic breadth; interorbital space wider than rostrum; palate nearly horizontal and little if any below level of zygomata; nasals extended beyond incisors; zygomata slender, with greatly reduced malar, almost, or quite, abutting against tympanic; frontals and parietals broad, with latter reaching, or nearly reaching, orbits; frontal trapezoidal; parietal quadrate, to pentagonal or triangular; interparietal primitively large, secondarily reduced; squamosal mostly or entirely confined to orbit; tympanic vesicular and inflated, in some forms highly inflated; mastoids inflated and bullous, reaching top of skull, and forming part of occipital surface; occipitals contracted and limited in area on occiput, but extending onto dorsum of skull; coronoid processes small, inclined caudad and lying below level of condyle; jaw small and weak with large, everted angle; tail as long as, or longer than, head and body; claws of manus elongate, fossorial, but forelimb slender; pelage usually coarse and frequently spinose; ears and eyes large; body murine in form; locomotion in many forms saltatorial.
This characterization of the family includes all of the members of the subfamilies Perognathinae, Heteromyinae and Dipodomyinae as well as the genus _Microdipodops_ which I am disinclined to place with any of the three subfamilies. Apparently it is more closely related to the subfamily Perognathinae.
The subfamily Dipodomyinae, which contains the genera _Dipodomys_, _Prodipodomys_ and _Cupidinimus_, might be characterized after Coues' (1875) original description of the subfamily as follows: Cheek teeth progressively hypsodont, in _Dipodomys_ becoming ever-growing; enamel progressively interrupted, eventually reduced to anterior and posterior plates; upper and lower third molars reduced in size; tooth pattern rapidly destroyed, leaving only an enamel oval; upper incisors smooth (some fossils) or grooved (living forms); progressive expansion of the auditory bullae and increase in saltatorial ability; pterygoid fossa double; calcaneal-navicular or even calcaneal-cuneiform articulation; tail tufted.
Owing to the fact that so little paleontological material is known and because even that is fragile and not easily accessible for study, knowledge of the fossil representatives has been drawn primarily from the literature, especially from Wood's (1935) account.
Heteromyids are known from the Chadron formation, of early Oligocene age, in which a single tooth was found. In the Orellan stage of the mid-Oligocene where the genus _Heliscomys_ occurs, it is notably generalized, in comparison with other members of the family, but it may not be ancestral at all. The lower premolar is tricuspidate and the first and second molars are quadritubercular with a broad cingulum. The teeth are bunodont and brachydont, with the cusps not uniting to form lophs. Wood (1935:78) shows _Mookomys formicorum_ (from the Arikeean) as the next heteromyid in the evolutionary sequence and postulates that this species arose from _Heliscomys gregoryi_. _Mookomys_ is judged by Wood to be the common ancestral form of the perognathines and the dipodomyines.
_Cupidinimus_, the genus next in line, is characterized by smooth upper incisors; lower molars with incipient H-pattern; cheek teeth progressively hypsodont and lophate (but always rooted); and calcaneal-navicular articulation.
The time range of this genus is from the late Miocene (Niobrara River, Local Fauna) of Nebraska to the medial Pliocene, Thousand Creek (Hemphillian) of Nevada.
Hibbard (1937:462) described _Dipodomys kansensis_ from the Ogallala formation (Hemphillian age) of Kansas. He redescribed his species, and made it the type of the new genus _Prodipodomys_ (Hibbard, 1939:458), differentiating it from _Dipodomys_ on the basis of the three-rooted p4, double-rooted m1 and m2 and the single rooted m3. It is shown to be closely allied to _Dipodomys_ by the form and position of a large foramen posterior and labial to m3, and by the development of the masseteric ridge.
The next youngest heteromyid fossils which have been described are of the genus _Prodipodomys_? from Arizona. Gidley (1922:123) described _Dipodomys minor_ from the Benson (Blancan) which Gazin (1942:486) refers to the genus _Prodipodomys_?. Wood (1935:156) described _Dipodomys gidleyi_ from the Curtis (Pleistocene). Both of these species are primitive as regards dentition; that is to say, the enamel ring of the tooth is complete and lacks any sign of a break. The limb bones of _D. gidleyi_ show lesser saltatorial ability, and therefore appear to be more primitive, than those of any living _Dipodomys_.
Several heteromyids which have not been assigned to any genus are known. Wilson (1939:36-37) recognized some from the Avawatz (Clarendonian) and the Ricardo (Clarendonian). Another, possibly of the genus _Diprionomys_?, from the Barstow (Barstovian) was described by Wood (1935:197) as follows: "The general shape of the tooth as figured strongly suggests either one of the most advanced species of _Dipodomys_ or else a Geomyid.... It is much more advanced than are any known contemporary heteromyids, and compares fairly well with such late Tertiary and Pleistocene geomyids as have been described. It certainly is not referable to any known heteromyid genus other than _Dipodomys_, and should probably be called a Geomyid." Wilson (_loc. cit._) refers to these specimens as Dipodomyine (?) n. gen. and sp. If these specimens referred to by Wood and Wilson are true heteromyids then a change in the phylogenetic scheme proposed by Wood (1935) would be necessary. Wilson (_loc. cit._) says, referring to the Avawatz specimen, "The cheek teeth are very hypsodont but are apparently not persistent in growth,... Wide enamel breaks are present in M/1 dividing the enamel into anterior and posterior bands. The enamel of P/4 is complete in the present stage of wear, but an examination of the tooth indicates that breaks would develop with additional attrition at the buccal and lingual margins of the metalophid, and at the buccal border of the protolophid. The incisor is of the slender heteromyid type."
Wood (1935:118) in referring to the ancestry of _Cupidinimus_ with regard to the grooving of the incisors says: "The philosophy of evolution which would prohibit its derivation from _Mookomys_, because of the grooved incisors in the latter genus, would require a separate line leading back at least to the Lower Miocene."
In view of the above statements, it is conceivable that additional material will be found carrying the dipodomyine line back into the early Miocene. Perhaps the line involving _Mookomys_ and _Cupidinimus_ which was regarded by Wood as the line of descent, is merely an aberrant side branch that parallels in its structures the main line of evolution of the dipodomyines (Figure 1).
As Wilson (1939:37) says: "Indeed it is hard to recognize such a form as _Cupidinimus nebraskensis_ as directly ancestral to _Dipodomys_ in view of the occurrence of the much more advanced Avawatz specimen in deposits that are at most only slightly later than those in which the former is found. The kangaroo rats were apparently much farther along in their development by lower Pliocene time than heretofore supposed."
Wood (1935:78) suggested that _Dipodomys gidleyi_ gave rise to _Dipodomys spectabilis_ and _Dipodomys ordii_, and _Dipodomys minor_ gave rise to _Dipodomys compactus_. However, my own study indicates that _Dipodomys compactus_ is conspecific with _Dipodomys ordii_ and should stand as _Dipodomys ordii compactus_. Consequently a different phyletic arrangement than that proposed by Wood (_loc._ _cit._) is required. Since _D. compactus_ is more closely allied to _Prodipodomys? minor_ than _D. ordii_ is to _D. gidleyi_, it is possible that _P.? minor_ gave rise to _D. ordii_ and that _D. spectabilis_ is the end product of the phyletic trend of _D. gidleyi_ (Figure 1).
The trend of phyletic development in the dipodomyines has been toward the saltatorial habit. To acquire this habit from a scampering ancestor, certain morphological modifications were necessary. Among these modifications were a lengthening of the tail, a lengthening of the hind legs, the development of a calcaneal-navicular-ectocuneiform contact instead of a calcaneal-navicular contact for additional strength in leaping, a shortening of the forelimb, an increase in size and inflation of the mastoid and tympanic portions of the skull with a consequent reduction in size of the interparietal region and the fusion of certain of the cervical vertebrae. Late Miocene (_Cupidinimus_) and Pliocene (Avawatz specimen and _Prodipodomys_) forms had acquired certain of these morphological modifications that are present in the modern genus _Dipodomys_.
PHYLOGENY OF THE SPECIES OF THE GENUS
Representatives of nine species of _Dipodomys_ were dissected in an attempt to determine the degree of specialization and the relative systematic position of each species.
The myology was found to agree in detail as to origin, insertion and innervation with that of _Dipodomys spectabilis_ as reported by Howell (1932). The only variation noted in the muscular system was the size of the individual muscles in those animals of widely divergent body size.
_Dipodomys ordii_ is the most generalized and _Dipodomys deserti_ is the most specialized of the kangaroo rats (see Table 1), as judged by the osteology. Information gained by the study of the viscera of the various species supports this judgment. The visceral mass is relatively loose in _D. ordii_, but is markedly compact in _D. deserti_. This compactness appears to be brought about by the foreshortening of the mesenteries which support the entire gut and by the closer apposition of the large intestine to the caecum; both the intestine and caecum occupy a ventral position in the abdominal cavity. In _Dipodomys ordii_ the entire visceral mass is loosely interconnected and the caecum is relatively small as compared to the tightly compact viscera and the large caecum in _Dipodomys deserti_. Another striking feature is the size, proportion and position of the liver. In all animals dissected, even in _D. deserti_, the right lobe of the liver descends and forms a capsule around the anterior end of the right kidney. In the Ord kangaroo rat, the bulk of the liver lies on the right side of the body cavity. That is to say, there is a greater bulk of the liver on the right side and it is situated more dorsad than in any of the other species examined. In the most specialized condition, as in _Dipodomys deserti_, the bulk of the liver is almost equal on the right and left sides, and instead of having the greater bulk situated dorsally as in _D. ordii_ it is cup-shaped, with the dorsal and ventral parts of approximately equal size and situated on almost the same transverse plane. The entire mass of the liver is concave posteriorly.
TABLE 1
SKELETAL INDICES OF DIPODOMYS
====================================================== Humeroradial | Intermembral | | Crural | | | Tibioradial | | | | Femorotarsal- | | | | | metatarsal | | | | | Cranial | | | | | | ------------------------------------------------------ ordii 144.5 57.2 127.75 60.55 88.4 63.4
microps 138.5 56.17 132.3 57.27 90.95 60.8
panamintinus 146.1 55.3 132.0 57.5 90.5 60.8
agilis 147.0 55.05 133.65 57.25 94.55 62.65
heermanni 142.9 54.2 135.9 55.35 92.2 60.93
ingens 142.9 54.1 130.6 56.2 89.65 66.2
spectabilis 140.9 53.05 133.9 54.2 95.6 64.6
phillipsii 163.4 55.05 137.85 58.97 101.5 64.5
merriami 160.75 53.85 137.5 57.35 99.75 63.9
nitratoides 155.0 54.1 137.4 57.0 98.25 65.5
deserti 149.5 53.4 139.4 54.9 96.6 67.6 ------------------------------------------------------
The right kidney is variable in position in reference to the left. In all species the right kidney lies anterior to the left but in some, _D. deserti_ and _D. ingens_, it is markedly anterior.
In _Dipodomys agilis_, _D. merriami_ and _D. deserti_ there are small to large patches of lymphoid tissue on the caecum. These patches were not noted in any of the other species examined and I do not know their function. In the three above mentioned species, however, the large intestine is shorter in proportion to the small intestine than in any other species except _D. heermanni_ (see Table 2) and with the exception of _D. heermanni_, _D. venustus_ and _D. ordii_ the actual measurements are less.
Inasmuch as little is known of the food habits of the various species of kangaroo rats, any ascription of adaptive significance to the varying proportions of the digestive system would be only speculative.
Midgley (1938) describes the visceral anatomy of _D. ordii_ and _D. microps_. Except for the differences here noted the description of the viscera as given by Midgley (_loc. cit._) applies to the rest of the species studied.
TABLE 2
VISCERAL MEASUREMENTS (in Millimeters) of DIPODOMYS ================================================================= Column Headings:
H: heermanni M: merriami A: agilis D: deserti O: ordii S: spectabilis V: venustus P: panamintinus I: ingens
H M A D O S V P I ----------------------------------------------------------------- Large intestine 432 290 464 397 237 413 374 419 430
Small intestine 165 126 220 195 131 228 207 255 274
Percent of small to large intestine 38.2 43.4 47.5 49.2 55.2 55.3 55.4 60.9 63.7 -----------------------------------------------------------------
From the differences noted in the skeleton, in the entire visceral mass, and in the shape and position of the liver it appears that as a saltator becomes more specialized skeletally, there is a concurrent compacting and aligning of the viscera into a more or less bilaterally balanced mass. It seems that this alignment is for a stabilization in leaping. It seems reasonable that the individual that has a loose and unconsolidated visceral mass, or in which the viscera or at least the heaviest part of the viscera is relatively unilateral, would be thrown slightly off balance at the end of the jump. This would place the animal at a slight disadvantage before being able to make the next jump. Howell (1944:40) comments on the fact that kangaroo rats often land off balance, "owing apparently to clumsy use of the tail." Possibly the unilaterality of the visceral mass plus a shorter tail and a more clumsy use of that organ accounts for the off balance landings which Howell has observed.
The skeleton, particularly of the appendages, shows the most modification, ranging from a relatively generalized to a specialized condition. Skeletal indices, as established by Howell (1944:199) have been used in estimating the amount of such specialization.
These indices are obtained by dividing the length of one segment of a limb by the length of another segment and are expressed in percentages. The Femorotarsalmetatarsal and Cranial indices are not from Howell (_loc. cit._).
The Humeroradial index (radius/humerus ×100) in the generalized animal is theoretically 100 because the humerus and radius are of the same length. In kangaroo rats, which are saltators, the index rises to more than 100 owing to the lengthening of the radial component.
The Intermembral index (humerus and radius/femur and tibia ×100) in a generalized animal is theoretically 100, but, as Howell (1944:205) points out, the index in generalized mammals is probably nearer 75. If the hind leg elongates at the expense of the forelimb the animal will be a better saltator and the skeletal elements will yield a lower intermembral index.
The Femorotibial or Crural index (tibia/femur ×100) expresses the development of the tibia as an adaptation to the saltatorial habit and in generalized animals would be expected to be 100. As an adaptation to saltation the tibia would elongate at the expense of the femur and the index would be more than 100. The degree of divergence from 100 would be an expression of the degree of saltatorial ability.
The Tibioradial index (radius/tibia ×100) in the generalized animal also would be expected to approximate 100 but it is doubtful if any living mammals, except brachiating kinds, yield an index of more than 75. In saltators, the index is low because of the elongation of the hind appendages, whereas the forelimbs do not change their length or are shortened.
The Femorotarsalmetatarsal index (tarsometatarus/femur ×100) in the generalized condition would be less than 50 and an index approaching 100 would indicate a specialization for saltation owing to the elongation of the tarsometatarsal elements.
The Cranial index (breadth across bullae/length of skull ×100) reflects the development of the auditory or mastoid region of the skull as an adaptation for more acute hearing and possibly for more delicate balance. In heteromyids, the generalized condition would be represented by an index of 50 or less, and as the width across the bullae increases, the index rises toward 100.
TABLE 3
RELATIVE SPECIALIZATIONS OF THE SPECIES FOR EACH INDEX
Column headings:
A: Humeroradial B: Intermembral C: Crural D: Tibioradial E: Femorotarsalmetatarsal F: Cranial G: Average
============================================== A B C D E F G ---------------------------------------------- ordii 5 1 1 1 1 5 2.33 microps 1 2 4 5 4 2 3.0 panamintinus 6 3 3 3 3 1 3.1 agilis 7 4 5 6 6 4 5.3 heermanni 3 6 7 9 5 3 5.5 ingens 4 7 2 8 2 10 5.5 spectabilis 2 11 6 11 7 8 7.5 phillipsii 11 5 10 2 11 7 7.6 merriami 10 9 9 4 10 6 8.0 nitratoides 9 8 8 7 9 9 8.5 deserti 8 10 11 10 8 11 9.6 ----------------------------------------------
The figure 1 represents the least specialized condition for the index, while the figure 11 represents the most specialized condition. The remainder of the numbers indicate the relative degree of specialization of each species for each index.
The species that have been examined are listed in Table 3 in increasing order of specialization from top to bottom.
Usually animals of extreme morphological specialization are much restricted environmentally. Attempts to correlate the relative evolutionary position of the various species, as indicated by the degree of specialization interpreted from the indices with that of habitus has proven unsuccessful. For example, _Dipodomys merriami_, which is third from the top in the list as arranged above, is neither restricted to loose sandy soil as is _D. deserti_ nor to brush as are _D. agilis_ and _D. venustus_. _D. merriami_ does, nevertheless, inhabit a variety of habitats from loose sandy soils to rather hard rocky ground. Throughout the genus there is, however, a general trend toward increased specialization as the animals adopt the more open desert environment, as is indicated by the elongation of the tail and hind appendage and increase in size of the auditory region of the skull. A marked difference is noted in the size of the pinna of the ear in the various species. Generally, those species having small pinnae inhabit open desert country while those with large pinnae inhabit brushy country. This is in direct contradistinction to the hares and rabbits in which the small-eared kinds are brush dwellers whereas the large-eared kinds are inhabitants of open country. Three possible explanations for hares and rabbits having this specialization of the pinnae are: (1) To enable the open-dwelling animals with the larger pinnae to hear more readily the approach of an enemy when it is yet far away, while the brush-living forms, which rely for escape on a short dash into cover, do not need so large a "funnel"; (2) large pinnae have been developed by those animals which live in the open desert as an aid in dissipating the body heat; (3) large pinnae in brush-dwelling animals would be a decided disadvantage in rapid movement through the brush. Grinnell (1922:20) points out that animals with large pinnae usually have small auditory bullae and conversely, animals with small pinnae have large bullae. This compensatory factor, implying an auditory function, appears to be inoperative in _D. panamintinus mohavensis_ which has small ears and small bullae and in _D. elephantinus_ which has large ears and large auditory bullae. Grinnell (_loc. cit._) suggests that several additional factors enter into the problem, such as the amount of digging each species must do to gain safety, the texture of the soil for burrowing, the extent of forage area and the type of cover in connection with the mode of attack of predators. Of these factors, perhaps the most important are the two first mentioned.
Wood (1935:155), on the basis of structure of the teeth, listed the species which he examined in the following increasing order of specialization: _Dipodomys compactus_ (now _Dipodomys ordii compactus_), _D. nitratoides_, _D. merriami_, _D. ordii_, _D. agilis_, _D. herrmanni_, _D. spectabilis_, and _D. deserti_. This arrangement is at variance with that of Grinnell (1922) who listed the species in order of increasing specialization as: _Dipodomys herrmanni_, _D. panamintinus_, _D. ingens_, _D. spectabilis_, _D. merriami_, _D. nitratoides_, _D. ordii_, _D. agilis_, _D. venustus_, _D. microps_ and _D. deserti_. As noted, the only agreement between the two arrangements is the placing of _D. deserti_ as the most specialized. Relying on skeletal indices alone, I would accord the same position to _D. deserti_ but would not arrange the other species as have either Wood or Grinnell.
In this study, the amount of specialization of each species, as indicated by the skeleton, was determined by assigning consecutive numbers from 1 to 11 to each species in its place in each index, and then totaling and averaging these arbitrary numbers (Table 3). It will be noted that there is a tendency for each species to occupy the same relative position in each of the indices.
It is felt, however, that a more nearly correct arrangement, according to degree of specialization, is obtained by using the six skeletal indices plus the information obtained from the study of the viscera. On this basis the species may be arranged from least to most specialized as follows: _Dipodomys ordii_, _D. microps_, _D. panamintinus_, _D. agilis_, _D. herrmanni_, _D. ingens_, _D. spectabilis_, _D. phillipsii_, _D. merriami_, _D. nitratoides_ and _D. deserti_.
Grinnell (1922:95-96) arranged the Recent species of _Dipodomys_ in nine groups. Davis (1942:332) also proposed an arrangement of nine groups in which he combined the Compactus and Ordii groups of Grinnell, established a new Elator group by removing _Dipodomys elator_ from Grinnell's Phillipsii group, and in linear arrangement, Davis shifted the Spectabilis and what remained of the Phillipsii groups to new positions. Burt (1936:152) arranged Grinnell's groups into three (unnamed) groups solely on the basis of the structure of the baculum. In the arrangement proposed by Grinnell, two of his nine groups contained only one species each, one other, the Microps group, has since been shown to contain only one species and another, the Compactus group, contained only kinds which are, by me, regarded only as subspecies of _Dipodomys ordii_. To my mind neither Davis nor Burt added to or fundamentally changed the basic concepts as set forth in 1922 by Grinnell. Owing to the paucity of material at that time, especially from areas of intergradation, Grinnell's groupings and arrangement were as nearly natural as could be expected. With the accumulation of additional material and with the knowledge that certain kinds treated by Grinnell as full species are in actuality subspecies, it is felt that the several species of kangaroo rats can best be arranged in six groups which, from the least to the most specialized, are as follows:
ORDII GROUP.--Composed of the subspecies of _Dipodomys ordii_ and _Dipodomys microps_. Grinnell placed these two species in separate groups; Burt on characters of the baculum alone placed _D. microps_ with _Dipodomys deserti_ and _Dipodomys spectabilis_. Within the single species _D. ordii_, I find that the difference in shape and size of the baculum between the subspecies of _D. ordii_ is as great as the difference which Burt (1936:154-155) found between the full species _D. agilis_ and _D. microps_. The characters of the baculum are an aid, but not in and of themselves an adequate basis, for determining the natural relationships of the groups of species. Certainly the remainder of the morphological differences between _D. deserti_ and _D. microps_ are so great that I doubt that the similarity in the baculum is significant, at least in this one instance. The chisel-shaped lower incisors of _D. microps_ appear to be a specialization. They may enable _D. microps_ to utilize more woody types of vegetation than can _D. ordii_. Both species occupy the same territory over much of their geographic range, probably because they eat different kinds of food.
PANAMINTINUS GROUP.--Composed of all the now known subspecies of _Dipodomys panamintinus_ and the species _Dipodomys stephensi_, if the latter is a full species. This group was included by Grinnell in the Heermanni group, with which it agrees in broadness of the maxillary arches and the configuration of the penis bone, but on the basis of the degree of specialization, as indicated by the indices (see Table 1), I feel that the Panamintinus group is more properly placed after the Ordii group and should be separated from the Heermanni group. Actually, animals in the Panamintinus group are intermediate between those of the Ordii and Heermanni groups.
HEERMANNI GROUP.--Composed of the subspecies of _Dipodomys heermanni_ and _Dipodomys agilis_, the species _Dipodomys ingens_, _Dipodomys venustus_ and _Dipodomys elephantinus_. _D._ _ingens_ even though larger in linear measurements than any of the other kinds included in this group, has almost the same degree of specialization as does _D. heermanni_. _D. agilis_, even though somewhat less specialized than the other kinds placed in this group, by the general nature of the indices, by the form of the visceral mass and to some degree by the shape of the baculum, shows itself properly to belong with this group. The species _D. venustus_, judged by characters of the visceral anatomy, also belongs here rather than with some other group or as a separate group. From the appearance of the visceral mass, _D. venustus_ is somewhat more specialized than either _D. heermanni_ or _D. agilis_, but _D. venustus_ does show its affinities with this group. The species _D. elephantinus_ has not been examined as thoroughly as have the other species but the external morphology and the configuration of the cranium place it with this group.
SPECTABILIS GROUP.--Composed of the subspecies of _Dipodomys spectabilis_. In two of the six indices, _D. spectabilis_ shows a high degree of specialization toward saltation, but in the other four indices it shows a relatively low degree of specialization or is average for the genus. Burt (1936:155) placed _D. spectabilis_ with _D. deserti_ on the basis of the baculum alone. I have not examined _D. nelsoni_ but place it with this group as did also Grinnell and Davis.
MERRIAMI GROUP.--Composed of the subspecies of _Dipodomys merriami_, _Dipodomys nitratoides_ and _Dipodomys phillipsii_, and the species _Dipodomys platycephalus_, _Dipodomys margaritae_, _Dipodomys insularis_, _Dipodomys mitchelli_, _Dipodomys ornatus_ and _Dipodomys elator_. I have not examined five of these species. However, the indices and characters of the viscera indicate that the three species first mentioned are closely allied. Owing to the lack of known intergradation between the three, I judge that they should be retained as full species, but the difference in degree of morphological specialization is no more than would be expected between subspecies. I have examined no specimens of _Dipodomys elator_ but from what I know of its morphology, I think that Grinnell better indicated its relations in allying it with _D. phillipsii_ than did Davis in erecting a new group for it on the basis of linear measurements.
DESERTI GROUP.--Composed of _Dipodomys deserti_ which has only two subspecies. In all morphological respects, _D. deserti_ is the most specialized species in the genus as shown by the reduced number (4) of toes on the hind foot, the bilateral arrangement of the viscera, the extreme development of the auditory region of the skull and by developing, early in life, the hiatus in the enamel wall of each molariform tooth.
The parallel arrangement below emphasizes the differences and similarities between Grinnell's (1922) arrangement and the one proposed in the present paper.
Grinnell's arrangement Present arrangement
HEERMANNI GROUP HEERMANNI GROUP _Dipodomys heermanni_ _Dipodomys heermanni_ _Dipodomys morroensis_ _Dipodomys agilis_ _Dipodomys mohavensis_ _Dipodomys ingens_ _Dipodomys leucogenys_ _Dipodomys venustus_ _Dipodomys panamintinus_ _Dipodomys elephantinus_ _Dipodomys stephensi_ _Dipodomys ingens_
SPECTABILIS GROUP SPECTABILIS GROUP _Dipodomys spectabilis_ _Dipodomys spectabilis_ _Dipodomys nelsoni_ _Dipodomys nelsoni_
PHILLIPSII GROUP NOW IN MERRIAMI GROUP BELOW _Dipodomys phillipsii_ _Dipodomys perotensis_ _Dipodomys ornatus_ _Dipodomys elator_
MERRIAMI GROUP MERRIAMI GROUP _Dipodomys merriami_ _Dipodomys merriami_ _Dipodomys nitratoides_ _Dipodomys nitratoides_ _Dipodomys platycephalus_ _Dipodomys platycephalus_ _Dipodomys margaritae_ _Dipodomys margaritae_ _Dipodomys insularis_ _Dipodomys insularis_ _Dipodomys mitchelli_ _Dipodomys mitchelli_ _Dipodomys phillipsii_ _Dipodomys ornatus_ _Dipodomys elator_
ORDII GROUP ORDII GROUP _Dipodomys ordii_ _Dipodomys ordii_ _Dipodomys microps_
COMPACTUS GROUP NOW IN ORDII GROUP ABOVE _Dipodomys compactus_ _Dipodomys sennetti_
AGILIS GROUP NOW IN HEERMANNI GROUP ABOVE _Dipodomys agilis_ _Dipodomys venustus_ _Dipodomys elephantinus_
MICROPS GROUP NOW IN ORDII GROUP ABOVE _Dipodomys microps_ _Dipodomys levipes_
DESERTI GROUP DESERTI GROUP _Dipodomys deserti_ _Dipodomys deserti_
WERE IN HEERMANI GROUP ABOVE PANAMINTINUS GROUP _Dipodomys panamintinus_ _Dipodomys stephensi_
Names of the subspecies are omitted from the groups named above and only the names of full species, as understood by Grinnell and as understood now, have been used. It will be noted that the phylogenetic order follows that of Grinnell rather than the one proposed herein.
The fossil record of the kangaroo rats is so scanty that one can but speculate on the evolutionary sequence. Wood (1935) presented a diagnosis of the early phyletic history up to and through _Cupidinimus_; this is probably as correct as can be made. I cannot, however, share his view that the recent species of _Dipodomys_ have originated from a descendant of _Cupidinimus nebraskensis_; instead, I think that the Recent species originated from some unknown ancestor in the southwest.
In view of the foregoing evidence it seems best to estimate the relationships and history of the various species and groups of species only as far back as the early Pleistocene (see Figure 21). Inasmuch as faunas of fossil mammals from the mid-Pleistocene contain few, if any, Recent species (see Hibbard, 1937:193), the living species of _Dipodomys_ have probably had a geologic history no longer than the period of time which has elapsed since the middle Pleistocene, or at the earliest the early Pleistocene. Of the Recent species, only _Dipodomys agilis_ is known as a fossil; it was found in the late Pleistocene tar pits of California. Under the heading "_Dipodomys near ingens_," however, Schultz (1938:206) recorded remains of kangaroo rats from the tar seeps of McKittrick in the San Joaquin Valley of California.
DISPERSAL OF THE SEVERAL SPECIES
If we assume the region of origin and center of dispersal of a group of animals to be the one in which the greatest numbers of the most specialized species of a given genus are found, then the northern Tableland of Mexico and the adjoining region of the United States in southeastern California and southwestern Nevada is the region of origin and the center of dispersal for the genus _Dipodomys_. _Dipodomys deserti_, _Dipodomys merriami_, _Dipodomys panamintinus_, _Dipodomys microps_, _Dipodomys phillipsii_ and _Dipodomys ordii_ are found in the region mentioned. That the aforementioned region may be the center of differentiation for this genus is further indicated by: First, the finding, in this region, of saline deposits of Cenozoic (Miocene) age, indicating aridity, which is thought to have been one of the essential stimuli for the development of the saltatorial habit in the genus _Dipodomys_; second, the recovery of the advanced heteromyids from the Avawatz and Ricardo of the Clarendonian (Pliocene) of this same region; and third, the present abundance and diversification of kangaroo rats in this same geographic region which has been more or less arid since Miocene time.
A secondary center of evolution has been the low, hot interior valleys and adjacent foothills of central California where _Dipodomys ingens_, _Dipodomys heermanni_, _Dipodomys venustus_, _Dipodomys agilis_, _Dipodomys elephantinus_ and _Dipodomys nitratoides_ are now found. Although there are as many species as in the principal center of origin, the amount of specialization and adaptive radiation in California is not so great. Probably during the Quaternary, when the process of mountain building was actively under way the animals that had reached central California from the parental center became isolated by the emergence of the Tehachapi Mountains. This mountain range separated the California animals from populations farther south and east. As a result, _D. nitratoides_ was differentiated from _D. merriami_, and _D. heermanni_ underwent an evolution of its own which resulted in animals having either four or five toes on the hind foot. At the same time _Dipodomys ingens_ developed there and has since been undergoing an evolution parallel to that of the large-sized species, _Dipodomys spectabilis_. The two species have paralleled each other not only in large size but to some extent in habits such as building large mounds that are kept free of vegetation and in occupying areas of rather hard clayey soil. Structurally, however, _D. ingens_ has not yet become quite so specialized as _D. spectabilis_, probably because _D. ingens_ has had less time in which to become so. A second species, if it is a full species, _Dipodomys elephantinus_, has also been isolated in central California but has not attained so high a degree of specialization as _D. ingens_. It is interesting to note that in each of the two stocks, two large-sized species have been evolved. In the parental stock the two species are _Dipodomys deserti_ and _Dipodomys spectabilis_; the former is the most specialized species in the genus. In the stock isolated in California, however, even though two large species have been formed they are still below the average in degree of specialization for the genus. As noted elsewhere in this paper, the species from these low, hot valleys, excepting _D. nitratoides_, are all closely related one to another. _Dipodomys venustus_ and _Dipodomys elephantinus_ are either closely related species or possibly only subspecies of one species, _Dipodomys agilis_.
It is worthy of note that as the distance away from the center of differentiation increases, the number of species decreases. For example, in the northern Great Basin there are only two species (_Dipodomys ordii_ and _Dipodomys microps_) and farther eastward, on the eastern side of the Rocky Mountains, there is only the one species, _Dipodomys ordii_. In north-central Texas, _Dipodomys elator_, perhaps a relict species, is found occupying an area farther east than that occupied by _Dipodomys ordii_ at that latitude.
_Dipodomys ordii_, _Dipodomys phillipsii_ and _Dipodomys merriami_ occupy the southern portion of the range of the genus. Instead of being generalized at this southern part of the periphery of the range as are the kinds found on the other parts of the periphery of the range of the genus, these three southern kinds are notably specialized there in the south. The subspecies _D. o. palmeri_ which occurs in the area, is the most specialized of the species _Dipodomys ordii_; and _Dipodomys phillipsii_ and _Dipodomys merriami_ stand high in the scale of specialization with respect to the other species of the genus. The reason for this is not clear.
SUBSPECIATION
_Dipodomys ordii_ is, without question, a valid species if one accepts Mayr's (1942:120) definition that "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups." _D. ordii_ is not known to hybridize with other species where their geographic ranges are adjacent or overlap. The first part of the definition "actually or potentially interbreeding populations" is substantiated by the 35 recognizable subspecies which can be defined as "a complex of interbreeding and completely fertile individuals which are morphologically identical or vary only within the limits of individual, ecological and seasonal variability. The typical characters of this group of individuals are genetically fixed and no other geographic race of the same species occurs within the same range" (after Rensch, 1934; from Mayr, 1942:106). Thus we find that certain populations of individuals differ from others and that in geographic areas between two of these populations, individuals (intergrades) are found which resemble those of both populations. In another instance, a population may be geographically isolated yet in its characters it may be recognizable as a subspecies without actual intergradation because of slight degrees of difference, or a group may be different from another without being geographically separated and may or may not show intergradation.
Subspeciation in _Dipodomys ordii_ almost certainly has been effected, by means of mutations, under the influence of natural selection. Natural selection enhanced by geographic and ~ecologic~ isolation, probably has retained mutations of evolutionary significance, thus permitting the development of the many recognizable subspecies.
In the subspecies of _Dipodomys ordii_ the color ranges from pale to dark. The difference in color is as pronounced as that between the full species _D. deserti_ and _D. heermanni_. The lightest-colored subspecies are _Dipodomys ordii celeripes_, _D. o. extractus_ and _D. o. compactus_; the darkest are _D. o. obscurus_ and _D. o. palmeri_.
There is a marked tendency for intergrades between a light-colored subspecies, such as _D. o. celeripes_, and a dark-colored kind, such as _D. o. utahensis_, to show varying degrees of blending in color. The insular population, _D. o. compactus_, has, however, two distinct color phases, a light phase and a slightly darker phase, and shows no tendency toward blending. In other kinds of mammals, blending of color is known to be the result of the action of multiple alleles, but in the insular kangaroo rat (_D. o. compactus_) the color appears to be the result of either a reduced multiple allelomorphic complex or even a unit factor. The two color phases of this insular subspecies, which might be an expression of a unit factor, more probably are specializations in which the multiple alleles for color have been reduced. The probability that there is either a unit factor or a reduced number of alleles at work is suggested by the taking of more dark-colored than light-colored animals and by the absence of blending of color. This insular population has undoubtedly been derived from the mainland kangaroo rat, _D. o. sennetti_, which has the usual range of variation but, to my knowledge, there are no individuals of _D. o. sennetti_ so light as the darkest animals of _D. o. compactus_ from the islands.
Populations from a given locality are remarkably stable in color except the animals from Samalayuca, Chihuahua, which vary in color from individuals almost as light as _D. o. compactus_ to animals that approach _D. o. ordii_ in darkness of pelage.
The subspecies of _D. ordii_ show no noticeable variation in the extent of the hip stripe, supraorbital and postauricular spots, basal white ring of the tail, lateral stripes of the tail or the extent of white on the venter and feet. There is, however, variation in the degree and extent of the arietiform facial markings. In _Dipodomys ordii utahensis_, _D. o. cupidineus_, _D. o. obscurus_ and _D. o. fuscus_ these markings are pronounced. In _D. o. celeripes_, _D. o. pallidus_, _D. o. compactus_ and _D. o. attenuatus_ these markings are either obliterated or nearly so.
In _Dipodomys ordii_, color does not seem to be correlated with amount of moisture or geography, but rather with color of soil. For example, all animals from the Bonneville Basin of western Utah, are light colored as are the soils; animals from the San Rafael Desert of eastern Utah are reddish, as is the soil. More striking extremes of this are shown by _D. o. compactus_ of Padre and Mustang islands, Texas, which is pale-colored as is the sand on which it lives, and _D. o. medius_ from east-central New Mexico and western Texas, which is reddish as is the soil there, which is derived from Permian rocks. In localities where alkaline soils are present, kangaroo rats may be found with a roseaceous cast to the pelage as a result of the action of the alkaline salts on the pigment of the hair. The roseaceous color is lost when the animal sheds the old pelage.
In the dorsal and ventral stripes of the tail, I find as much variation in the species _D. ordii_ as Grinnell (1922:Fig. E, p. 14) recorded in the whole genus. In _D. o. obscurus_, _D. o. fuscus_ and _D. o. utahensis_ the stripes are complete to the distal end of the tail and dark, whereas in _D. o. pallidus_ and _D. o. celeripes_ the ventral stripe is either absent or nearly so and the dorsal stripe is pale.
Color as a taxonomic character is valuable in a broad sense, and is useful in placing an individual or a group of individuals in the subspecies to which they pertain. In most subspecies studied, color was quite uniform throughout the range of the animals, but in _D. o. ordii_ and _D. o. columbianus_ color is so variable that cranial features were relied on almost exclusively for the final diagnosis.
Among the subspecies of _Dipodomys ordii_ there is relatively little variation in the length of the head and body. The smallest measurement is 95.5 mm. in _D. o. idoneus_ and the largest is 118.3 mm. in _D. o. richardsoni_. The shortest tail is found to be 112.0 mm. in _D. o. celeripes_ and the longest is 154.7 mm. in _D. o. terrosus_. The length of the hind foot varies from 35.0 mm. in _D. o. idoneus_ to 44.5 mm. in _D. o. nexilis_.
Allen's Rule is not operative in the species _D. ordii_. According to this rule, shorter tails and smaller feet in conjunction with a large body would be expected as the more northerly limits of the species are approached, and conversely, smaller body and larger appendages would be expected as the southerly limits of the species are approached. This is not the case, however, since the subspecies _D. o. terrosus_ ranges farthest north and has the longest tail, whereas _D. o. celeripes_, found in the central part of the range of the species, has the shortest tail. Again, in regard to the hind foot, the shortest is found in _D. o. idoneus_ which is at the extreme south of the range of the species, whereas the longest hind foot is found in _D. o. nexilis_ which occupies a nearly central position in the range.
Long tail and long hind foot would seem to be specializations for saltation and the two would be expected to be correlated. Actually there is no significant correlation in _D. ordii_. _D. o. celeripes_, in which the hind foot is near the mean for the species (39.8 as opposed to the mean of 40.7), has the shortest tail. _D. o. compactus_ has a short tail (117.0 mm.) but a medium-sized hind foot. _D. o. nexilis_ and _D. o. terrosus_ have both a long hind foot and long tail.
Cranial measurements vary less, probably because one person can measure a series with a uniformly subjective error. External measurements, however, are liable to a greater degree of subjective error. The total length of the skull varies from 35.4 mm. in _D. o. attenuatus_ to 41.3 mm. in _D. o. terrosus_. In no one series of adults from one locality, however, is the variation so marked as it is for the species as a whole. The usual range of variation in length of skull in any given series is not, as a rule, more than 2.5 mm.
Cranial indices (breadth across bullae/length of skull × 100) as established for random samples of the different species of the genus (exclusive of _D. ordii_) ranged from 60.8 to 67.6. In the subspecies of _D. ordii_ the same index varies from 59.7 to 65.2 with an average of 63.4. In other words, the degree of specialization indicated by this one index, in a few subspecies of _D. ordii_, is almost as great as that in _Dipodomys deserti_, which on the basis of total morphology appears to be the most specialized species in the genus. Also, on the basis of this same index, some subspecies of _D. ordii_ are more generalized than is any other species in the genus.
There is a general tendency for the nasals to decrease in length and the rostrum to decrease in width as the southern limits of the range of _D. ordii_ are approached. In ascertaining the decrease in length of the nasals an index was obtained as follows: nasals/interorbital width × 100 (see Table 4). The width of the rostrum, however, does not decrease in the same degree, nor at the same rate, as does the length of the nasals. This decrease in length of the nasals and in width of the rostrum may be correlated with the mean annual relative humidity of the environment. It is known (Howell and Gersh, 1936:8) that desert rodents, more exactly kangaroo rats, have a water retention mechanism in the kidneys and walls of the urinary bladder which enables them more efficiently to conserve metabolic water. The significance of the decrease of the area of the nasal mucosa, which seems to be related to relative aridity, is not yet properly understood.
In no cranial feature other than shortened nasals and narrowed rostrum, does _Dipodomys ordii_ show a gradation such that it might be termed a cline. Other parts of the skull that were measured do not vary greatly.
Perhaps the greatest amount of variation in the skull is in features which are not readily measurable by the usual physical means. The shape and size of the pterygoid fossae vary from almost round to rather ovoid in a given series of animals from one locality; the size and configuration of the zygomatic arches vary from slender to robust and from straight to curved laterally; the size of the lacrimal processes varies much in any given series, as do also the degree of expansion of the distal end of the nasals, the convexity of the braincase and the curvature of the upper incisors. In all instances where these features varied much, one size or shape was more pronounced in the series than any other size or shape. Thus, when comparisons were made, the size and certain shapes were the criteria used in assigning the animals under consideration to a given subspecies.
Subspeciation in _Dipodomys ordii_ seems to have been influenced by water barriers. It is known (Grinnell, 1922:28) that kangaroo rats lack the ability to swim. Large stable rivers such as the Colorado, Snake and Columbia serve as effective barriers to further dispersal of kangaroo rats. Streams that freeze over in the winter months, however, are not efficient barriers. This is indicated by the "blending" of morphological characters of _D. o. nexilis_ and _D. o. sanrafaeli_ along the Green River which freezes over.
Any mountain which has vegetational belts above the Transition Life-zone would serve as a barrier to the dispersal of these animals. The Uinta Mountains, lying in an east-west direction, are interposed between the ranges of _D. o. priscus_ and _D. o. uintensis_. The high Wasatch Mountains and their associated outliers, lying in a north-south direction in Utah, serve as an efficient barrier to the east-west movement of kangaroo rats and as a result, the subspecies east of the mountain mass are remarkably different from those to the west.
TABLE 4
PROPORTIONATE DECREASE OF NASALS
=========================================================== Width Length Least Nasals of of interorbital ------------ rostrum nasals orbital Interorbital width × 100 ----------------------------------------------------------- terrosus 4.1 14.75 13.5 91.6 luteolus 4.35 13.9 12.95 93.1 evexus 4.3 14.35 13.75 94.8 montanus 4.1 13.5 12.65 93.8 ordii 3.5 13.3 13.0 97.7 idoneus 3.7 13.2 13.75 103.5 palmeri 3.3 12.8 13.0 101.1 -----------------------------------------------------------
The first three columns represent the actual measurements of the various elements; the fourth column is the index established.
Six different complexes (groups) of subspecies of _D. ordii_ have probably arisen as a result of geographical separation.
The Great Plains complex consisting of _D. o. richardsoni_, _D. o. oklahomae_, _D. o. evexus_, _D. o. terrosus_, _D. o. luteolus_, _D. o. priscus_ and _D. o. medius_ are, with the exception of _D. o. priscus_, inhabitants of the high plains grassland habitat. _D. o. priscus_ inhabits the red Desert of Wyoming.
The Gulf Coast complex, comprising _D. o. sennetti_ and _D. o. compactus_ are separable from all others by small auditory bullae and short tail. _D. o. compactus_ probably has differentiated from _D. o. sennetti_ since the cutting off, by wave action, from the mainland, of the islands on which _D. o. compactus_ lives.
The Mexican complex consisting of _D. o. obscurus_, _D. o. fuscus_, _D. o. idoneus_ and _D. o. palmeri_ have probably differentiated by natural selection acting on fortuitous variations, but I lack first hand knowledge of the region concerned.
The southwestern complex consists of _D. o. chapmani_, _D. o. extractus_, _D. o. attenuatus_ and _D. o. ordii_. _D. o. attenuatus_ and _D. o. chapmani_ are subspecifically distinct owing to geographic isolation, although both kinds show intergradation where their ranges approach that of _D. o. ordii_.
The western desert complex, composed of _D. o. monoensis_, _utahensis_, _cineraceus_, _columbianus_, _cinderensis_, _fetosus_, _celeripes_, _marshalli_, _inaquosus_, _pallidus_, _panguitchensis_ and _fremonti_ was isolated from the other complexes of _D. ordii_ by the Quaternary uplift of the Wasatch Mountain mass, consisting of the Wasatch, Fish Lake and San Pitch mountains and the Wasatch, Aquarius, Paunsaugunt and Kaiparowits plateaus, and the concurrent reëstablishment of drainage systems. The drainages are those of the Colorado and Columbia rivers and that of the Snake River from Blackfoot, Idaho, to the junction with the Columbia. _D. o. fremonti_ has been isolated on the upper reaches of the Fremont River which arises from the eastern side of the Wasatch Divide. _D. o. panguitchensis_ has been isolated in Panguitch Valley as a result of the canyons formed by the Sevier River in Utah. _D. o. cineraceus_, although its subspecific and insular status are in doubt, appears to have been isolated on Dolphin Island, Great Salt Lake, Utah.
The intermontane complex consisting of _D. o. montanus_, _longipes_, _cupidineus_, _nexilis_, _sanrafaeli_ and _uintensis_, like the western desert complex, has become separated from the remainder of the subspecies of the species _D. ordii_ by Quaternary geologic events. _D. o. cupidineus_ has been cut off by the gorges of the Colorado River to the south and the Virgin River to the north. _D. o. sanrafaeli_ is separated from _D. o. uintensis_ by the Tavaputs Plateau and by the Roan and Book cliffs, and is separated from the range of _D. o. nexilis_ by the Colorado River although there is intergradation between _D. o. nexilis_ and _D. o. sanrafaeli_. _D. o. longipes_ has been separated from the rest of this intermontane complex by the San Juan and Colorado rivers, but to the east it intergrades freely with adjacent subspecies. _D. o. montanus_ has been relatively isolated in the San Luis Valley of Colorado and New Mexico, but in the southern part of its range it does show intergradation with other subspecies.
The complexes mentioned above are represented graphically in Figure 22, in a way that expresses some of my ideas as to their genetic relationships.
The indices used to determine the amount of specialization that each complex of subspecies has undergone are as follows:
The Body index (head and body/length of tail × 100) is the expression of the elongation of the tail as an organ of balance while the length of the head and body remain relatively constant. As the tail elongates the index decreases and as the tail becomes shorter the index increases.
The Pedal index (hind foot/head and body × 100) is the expression of the development of the hind foot as an element essential for the saltatorial habit. As the hind foot elongates the index will increase; elongation of the hind foot is interpreted as a specialization.
The Cranial index (breadth across bullae/length of the skull × 100) reflects the degree of development of the tympanic or mastoid region, or both, and is thought to be an adaptation for more acute hearing and possibly for more delicate balance. Inflation of the tympanic bullae is thought to be a specialization. As the auditory bullae become more inflated, the index increases toward 100.
The Bullar index (width of maxillary arches/breadth across bullae × 100) also expresses the degree of inflation of the auditory bullae. In a generalized mammal, at least in the heteromyids, the index would be 100, but as the auditory bullae become larger the index will decrease from 100.
In attempting to arrange the subspecies of _D. ordii_ according to degree of specialization, the geographic positions of the subspecies have been considered along with the information provided by the above-mentioned indices. These indices were used in the same way as were the indices for the species of the genus. In Tables 5 and 6 and in the accounts and maps the subspecies are arranged from the least specialized to the most specialized.
TABLE 5
INDICES FOR THE SUBSPECIES OF _DIPODOMYS ORDII_
====================================================== Body Pedal Cranial Bullar ------------------------------------------------------ richardsoni 88.85 34.35 60.95 88.25 oklahomae 86.75 35.5 61.7 90.25 compactus 127.7 37.25 59.75 88.35 sennetti 94.25 34.0 62.85 85.95 evexus 80.1 35.7 60.5 92.9
medius 80.4 33.7 63.55 85.9 obscurus 62.95 86.4 terrosus 75.25 35.05 61.6 86.85 fremonti 80.55 34.7 62.9 85.5 uintensis 77.2 35.3 62.3 86.0
monoensis 85.4 36.4 63.4 85.6 ordii 79.05 37.6 62.75 86.9 luteolus 75.0 37.05 62.35 86.3 extractus 83.65 34.35 64.3 84.25 chapmani 75.05 36.35 62.9 85.65
montanus 80.4 36.15 64.25 82.5 cinderensis 85.1 37.2 65.15 84.75 fetosus 81.8 38.85 63.95 83.95 utahensis 80.2 36.95 64.45 84.35 columbianus 78.5 37.55 64.25 84.9
idoneus 72.3 36.6 64.2 85.0 priscus 74.9 39.45 62.3 84.95 celeripes 91.85 38.75 65.0 84.25 cineraceus 75.5 39.1 63.9 84.8 marshalli 81.5 37.3 65.2 83.0
inaquosus 78.05 37.9 64.25 83.05 attenuatus 73.5 37.35 64.0 83.4 fuscus 79.8 39.0 64.3 83.2 longipes 75.7 37.1 64.3 82.75 pallidus 76.9 40.75 64.35 84.65
nexilis 77.1 40.7 64.95 78.45 cupidineus 73.15 39.1 64.1 80.85 palmeri 72.25 37.15 65.1 80.45 ------------------------------------------------------
TABLE 6
NUMERALS (derived from Table 5) ARE INDICATIVE OF THE RELATIVE DEGREE OF SPECIALIZATION OF THE SUBSPECIES OF _DIPODOMYS ORDII_
========================================================= Body Pedal Cranial Bullar Average --------------------------------------------------------- richardsoni 4 3 3 4 3.5 oklahomae 5 8 5 2 5.0 compactus 1 19 1 3 6.0 sennetti 2 2 10 10 6.0 evexus 15 8 2 1 6.5
medius 12 1 15 11 9.75 obscurus 12 7 10.0 terrosus 25 6 4 6 10.25 fremonti 11 5 12 14 10.5 uintensis 20 7 7 9 10.75
monoensis 6 12 14 13 11.25 ordii 17 23 9 5 13.5 luteolus 27 15 8 8 14.5 extractus 8 4 25 22 14.75 chapmani 26 11 11 12 15.0
montanus 13 10 23 30 19.0 cinderensis 7 18 32 19 19.0 fetosus 9 26 17 24 19.0 utahensis 14 14 28 21 19.25 columbianus 18 22 21 17 19.5
idoneus 31 13 20 15 19.75 priscus 28 30 6 16 20.0 celeripes 3 25 30 23 20.25 cineraceus 24 28 16 18 21.5 marshalli 10 20 33 28 22.75
inaquosus 19 24 22 27 23.0 attenuatus 29 21 18 25 23.25 fuscus 16 27 24 26 23.25 longipes 23 16 26 29 23.5 pallidus 22 32 27 20 25.25
nexilis 21 31 29 33 26.0 cupidineus 30 29 19 31 27.25 palmeri 32 17 31 32 28.0 ---------------------------------------------------------
=Dipodomys ordii=
Ord Kangaroo Rat
_Dipodomys ordii_ is a medium sized, relatively short-tailed, five-toed species of a color about average for the genus. As in other members of the genus, the hind legs and feet are disproportionately long as an adaptation to the saltatorial mode of progression. The upperparts are buffy, reddish or blackish, depending on the subspecies, but the entire ventral surface, dorsal surfaces of the hind feet, supraorbital and postauricular spots, forelimbs, hip stripes, lateral stripes of the tail and the tail at the base are pure white. The skull has a relatively short rostrum, moderate to large auditory bullae, relatively wide interparietal, relatively wide maxillary arches and grooved upper incisors.
The only other five-toed kangaroo rats with which _Dipodomys ordii_, at places, shares its geographic range, are _Dipodomys panamintinus_ and _Dipodomys microps_. _Dipodomys ordii_ can be distinguished from _Dipodomys panamintinus_ by smaller size (for instance the hind foot is shorter instead of longer than, 44 mm.) and narrower expanse of maxillary arches in relation to breadth across the auditory bullae, and from _Dipodomys microps_ by the awl-shaped, instead of chisel-shaped, lower incisors.
The species _D. ordii_ is divisible into 35 subspecies, accounts of which follow:
=Dipodomys ordii richardsoni= (Allen)
_Dipodops richardsoni_ Allen, Bull. Amer. Mus. Nat. Hist., 3:277, June 30, 1891.
_Dipodomys phillippi_, Knox, Trans. Kansas Acad. Sci., 4:22, 1875, (part--the part from Osborne, Kansas).
_Dipodomys phillipsi ordi_, Coues and Allen, Monogr. North American Rodentia, p. 542, 1877 (part--the part from Ft. Cobb, Oklahoma).
_Perodipus richardsoni_, Allen, Bull. Amer. Mus. Nat. Hist., 7:260, August 21, 1895 (part--the part from Pendennis, Kansas).
_Cricetodipus richardsoni_, Trouessart, Catalogus Mammalium, 1:581, 1897.
_Perodipus montanus richardsoni_, Bailey, N. Amer. Fauna, 25:144, October 1905 (part--the part from Canadian, Texas).
_Perodipus ordii richardsoni_, Goldman, Proc. Biol. Soc. Washington, 30:113, May 23, 1917.
_Dipodomys ordii richardsoni_, Grinnell, Journ. Mamm., 2:96, May 2, 1921.
_Type._--Male, no. 3025/2345, Amer. Mus. Nat. Hist.; on one of the sources of the Beaver River, Beaver County, Oklahoma; obtained on October 26, 1887, by Jenness Richardson and John Rowley, Jr. (After Allen, original description, type not seen.)
_Range._--Southwestern Nebraska, eastern Colorado, northeastern New Mexico, Panhandle of Texas, and western parts of Oklahoma and Kansas; marginal localities are: in Nebraska: Bladen, Haigler; in Colorado: Olney; in New Mexico: Clayton; in Texas: 6 mi. S and 1 mi. W Quitaque, Vernon; in Oklahoma: 3 mi. S Cleo Springs; in Kansas: Medora.
_Diagnosis._--Size large (see measurements). Color dark; entire dorsal surface Cinnamon-Buff, purest on sides and flanks, upper parts suffused with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, blackish; in some specimens the ventral stripe of tail does not extend to tip of pencil. Skull large; rostrum short and wide; nasals long; zygomata relatively heavy; auditory bullae well inflated and wide; thus with short rostrum giving appearance of nearly equilateral triangle; upper incisors long and robust.
_Comparisons._--From _Dipodomys ordii luteolus_, _D. o. richardsoni_ differs as follows: Size smaller in external measurements except length of body which is longer; color darker, except on plantar surfaces of hind feet and dorsal and ventral stripes of the tail which are lighter; ventral stripe of tail, in most specimens, continuous to end of pencil, whereas in _D. o. luteolus_ ventral stripe is present on only proximal two-thirds; skull larger in all measurements taken; zygomatic arch heavier; auditory bullae relatively as well as actually more inflated; external auditory meatus egg-shaped as contrasted to nearly round in _D. o. luteolus_; pterygoid fossae rounded as compared to ovate in _D. o. luteolus_.
From _Dipodomys ordii oklahomae_, _D. o. richardsoni_ differs as follows: Size larger in all measurements taken; color darker in all pigmented areas; skull larger in all respects; auditory bullae larger and more inflated ventrally; jugal straight or nearly so instead of bowed laterad; pterygoid fossae smaller; nasals straight instead of inflated as a "bulb" distally.
For comparisons with _D. o. montanus_ and _D. o. evexus_, see accounts of those subspecies.
Remarks.--This race of _Dipodomys ordii_ is readily distinguished from _Dipodomys ordii evexus_, from the valley of the upper Arkansas River, by larger size, larger skull and lighter color. Intergradation with _Dipodomys ordii luteolus_ occurs rather freely in northeastern Colorado, as indicated by specimens from 3 miles northeast of Fitzsimmons, 6 miles east and 1 mile north of Denver and Barr Lake. These specimens resemble _D. o. richardsoni_ in light color, greater inflation of the auditory bullae and the shape of the pterygoid fossae but resemble _D. o. luteolus_, to which they are here referred, in the length of the nasals, the least interorbital width and in the external measurements. In the southern part of the range of _D. o. richardsoni_ intergradation occurs with _Dipodomys ordii medius_, as at 6 miles southwest of Muleshoe, Texas. Specimens from there have the long, wide rostrum and narrow skull of _D. o. richardsoni_ but in the sum total of their characters more closely resemble _D. o. medius_. At Texline, Texas, the animals show intergradation in the length and shape of the nasals and degree of convexity of the cranium but are referable to _D. o. richardsoni_.
In fine, intergradation occurs at all points where the range of _D. o. richardsoni_ touches that of any other geographic race. No one series of it is as uniform as are most series of specimens of other known races. _Dipodomys ordii richardsoni_ shows a mixture of characters. Nevertheless, each of the populations studied has characters in most of the animals that make this form recognizable as a taxonomic unit--a unit that seems, as yet, not to have become stabilized even in the central parts of its range.
Coues and Allen (1877:542) list specimens from Fort Cobb, Arkansas. It is known that the Post Office Department, for administrative purposes, attached certain towns and military installations in Indian Territory (now Oklahoma) to the State of Arkansas. Thus it is apparent that Fort Cobb, Arkansas, as recorded by Coues and Allen (_loc. cit._) is Fort Cobb, Oklahoma. Specimens from Fort Cobb would be expected to be _D. o. richardsoni_.
_Specimens examined._--Total, 351, distributed as follows:
=Nebraska=: _Adams County_: Bladen, 10 (AMNH). _Dundy County_: Haigler, 1 (USBS).
=Colorado=: _Crowley County_: Olney, 1 (USBS). _Kiowa County_: Chivington, 3 (USBS). _Otero County_: 18 mi. S La Junta, 4 (AMNH); Higbee, 1 (USBS). _Bent County_: 4 mi. SE Las Animas, 4100 ft., 3 (MVZ). _Prowers County_: Lamar, 9 (LACM); 1 mi. S Lamar, 4000 ft., 11 (KU). _Baca County_: Gaumes Ranch, 4600 ft., NW Corner, 1 (USBS).
=Kansas=: _Cheyenne County_: 23 mi. NW St. Francis, 5 (KU). _Rawlins County_: 2 mi. NE Ludell, 2 (KU); 1-1/2 mi. W Ludell, 1 (KU). _Wallace County_: Lacey Ranch, 4-1/2 mi. E and 9 mi. S Wallace, 1 (KU); unspecified, 2 (KU). _Logan County_: 5 mi. W Elkader, 2 (KU); unspecified, 1 (UM). _Gove County_: unspecified, 1 (KU). _Trego County_: Banner, 8 (USNM); Parrington Ranch, 12 mi. S Collyer, 2 (KU); unspecified, 8 (USNM). _Ellis County_: Ellis, 1 (USBS). _Lane County_: Pendennis, 10 (USBS). _Hamilton County_: Coolidge, 2 (CNHM); 1 mi. E Coolidge, 5 (KU). _Pawnee County_: 1 mi. S Larned, 4 (KU); 2 mi. S and 1/4 mi. W Larned, 2 (KU); 3 mi. S and 1-1/2 mi. W Larned, 10 (KU). _Edwards County_: Kinsley, 3 (USBS); 3-1/2 mi. E Kinsley, 5 (KU); S side Arkansas River, 2 mi. S Kinsley, 1 (KU); 1 mi. W and 3-1/2 mi. S Kinsley, 9 (KU). _Stafford County_: Little Salt Marsh, 15 mi. N and 3 mi. E Stafford, 2 (KU). _Reno County_: Medora, 1 (UM); 2 mi. W and 1/2 mi. S Medora, 4 (KU). _Kiowa County_: 5 mi. N Belvidere, 1(KU). _Pratt County_: Cairo, 2 (USBS). _Sedgwick County_: Wichita, 6 (AMNH). _Morton County_: 10 mi. N and 3 mi. E Elkhart, 34-1/2 (KU). _Seward County_: 1 mi. E Arkalon, 7 (KU); Liberal, 1 (KU); unspecified, 1 (KU). _Meade County_: Meade, 1 (USNM); 13 mi. SW Meade, 13 (6 AMNH; 7 KU); 17 mi. SW Meade, 2 (KU). _Clark County_: 12 mi. S Ashland, 1 (UM); unspecified, 3 (KU). _Barber County_: Medicine Lodge, 4 (USBS); 1 mi. W Aetna, 3 (KU); 1/2 mi. W Aetna, 2 (KU); Aetna, 3 (KU); 1 mi. SW Aetna, 1 (KU); 1-1/2 mi. SW Aetna, 1 (KU); 1 mi. S Aetna, 5 (KU); unspecified, 2 (KU). _Harper County_: 4-1/2 mi. NE Danville, 12 (KU); 2 mi. NE Runnymede, 3 (KU).
=New Mexico=: _Union County_: Clayton, Apache Canyon, 1 (USBS). _Quay County_: Glenrio, 10 (LACM).
=Oklahoma=: _Cimmaron County_: Kenton, 1 (CM). _Beaver County_: 1-1/2 mi. N Beaver, 7 (KU); Beaver River, 8 (7 AMNH; 1 CNHM). _Harper County_: 3 mi. S of Englewood, Kansas, 2 (MVZ); 4-1/2 mi. N Laverne, 1 (UM). _Woods County_: 2 mi. W Edith, 1 (USBS); Alva, 12 (UM); Waynoka, 18 (UM); 3 mi. SW Waynoka, 1 (USBS). _Alfalfa County_: 4 mi. SE Cherokee, 1 (USBS). _Ellis County_: Shattuck, 1 (USBS). _Woodward County_: Woodward, 9 (USBS). _Major County_: 3 mi. S Cleo Springs, 1 (USBS).
=Texas=: _Dallam County_: Texline, 8 (USBS). _Lipscomb County_: Lipscomb, 8 (USBS). _Hemphill County_: 17 mi. NE Canadian, 1 (MVZ); 1 mi. W Canadian, along Red Deer River, 12 (MVZ); 1/2 mi. W Canadian, along Red Deer River, 7 (MVZ); Canadian, 5 (USBS). _Oldham County_: Tascosa, 6 (USBS). _Wheeler County_: 1 mi. W Mobeetie, 2 (MVZ); Mobeetie, 8 (USBS); Wallace Ranch, SW Wheeler County, 1 (TCWC). _Hall County_: Newlin, 1 (USBS). _Wilbarger County_: Vernon, 5 (USBS). _Floyd County_: 6 mi. S and 1 mi. W Quitaque, 1 (UM).
=Dipodomys ordii oklahomae= Trowbridge and Whitaker
_Dipodomys oklahomae_ Trowbridge and Whitaker, Journ. Mamm., 21:343, August 14, 1940.
_Dipodomys ordii oklahomae_, Davis, Journ. Mamm., 23:332, August 14, 1942.
_Type._--Female, young adult, no. 265454, U. S. Nat. Mus., Biol. Surv. Coll. (formerly Univ. of Oklahoma, Mus. Zool., no. 14517); north bank of South Canadian River, 2-1/4 mi. S Norman, Cleveland County, Oklahoma; obtained on March 16, 1934, by H. L. Whitaker, original no., X-catalog no. 29312 of U. S. Nat. Mus.
_Range._--Known only from the South Canadian River Valley west of Minco, Canadian County; and east to Lexington, Cleveland County, Oklahoma.
_Diagnosis._--Size medium (see measurements). Color light, entire dorsal surface near (_c_) Vinaceous-Buff, paler on sides with great suffusion of white; arietiform markings, pinnae of ears, plantar surfaces of hind feet, proximal ventral portion of tail and dorsal stripe on tail, brownish. Skull of medium size; rostrum wide; nasals short, projecting but slightly anteriorly to incisors; zygomatic processes of maxillae heavy; bullae not greatly inflated.
_Comparisons._--_Dipodomys ordii oklahomae_ differs from _D. o. richardsoni_ as follows: Size smaller; color lighter in all pigmented areas; ventral stripe of tail extending only one-fourth the length rather than three fourths or to end of tail; skull smaller in all measurements taken; rostrum heavier; auditory bullae less inflated; pterygoid fossae larger; braincase slightly more inflated; nasals more expanded distally; interparietal region wider.
From _Dipodomys ordii ordii_, _D. o. oklahomae differs_ in: Size larger in all measurements taken; color lighter in all pigmented areas; ventral stripe of tail extending one fourth length of tail rather than to end; skull larger in all respects; rostrum heavier; zygomatic arches heavier; bullae more inflated ventrally; cutting edge of upper incisors wider; pterygoid fossae larger; braincase more vaulted; nasals more expanded distally; orbital region larger; interparietal region wider.
_Remarks._--Trowbridge and Whitaker named this kangaroo rat as a full species. The diagnostic characters were the length and breadth of the rostrum and the relatively great inflation of the auditory bullae. Also, _Dipodomys oklahomae_ was not known to intergrade with any other named kinds. Davis (1942:332) treated _D. oklahomae_ as a subspecies of the earlier named species _Dipodomys ordii_. Certain characters in specimens from the type series of both _D. o. richardsoni_ and _D. oklahomae_, such as the shape and configuration of the nasals, the over-all proportion of the skull, tooth pattern and body proportions through individual variation overlap and indicate that these two groups of animals belong to the same species, even though animals from intermediate geographic areas are not available to show actual intergradation. My findings corroborate Davis' conclusion that _D. oklahomae_ should stand as _Dipodomys ordii oklahomae_. In spite of the great similarities shown by the two groups of animals there are still sufficient diagnostic characters between the two groups to enable them to be segregated easily as valid subspecies.
_Dipodomys ordii oklahomae_ is, for some unknown reason, restricted to a limited geographic range. Specimens examined from the upper reaches of the South Canadian River, farther westward, are all referable to _D. o. richardsoni_ rather than, as would be expected, to _D. o. oklahomae_ since the habitat for these animals is continuous from the type locality of _D. o. oklahomae_ to the upper reaches of the South Canadian River. In length and shape of the nasals, degree of inflation of the auditory bullae and width of the interorbital region these specimens from the upper reaches of the South Canadian River are intergrades between _D. o. richardsoni_ and _D. o. medius_. The range of _D. o. medius_ lies to the south of that of _D. o. richardsoni_ and to the southwest of that of _D. o. oklahomae_.
The present range of _D. o. oklahomae_, as now understood, is the most eastern part of the range of the species _Dipodomys ordii_ and of the genus _Dipodomys_. The existence of _D. o. oklahomae_ in this area is a precarious one since its habitat is limited in extent and is periodically flooded.
Although no specimens are known from the area where intergradation between _D. o. oklahomae_ and _D. o. richardsoni_ would be expected to occur, it would seem that when animals from this region become available, intergradation will be shown to occur.
_Specimens examined._--Total, 8, all from Oklahoma, distributed as follows: _Grady County_: 4 mi. N Minco, 1 (USBS). _Cleveland County_: 2-1/4 mi. S Norman, 7 (6 OU; 1 USBS).
=Dipodomys ordii compactus= True
_Dipodomys compactus_ True, Proc. U. S. Nat. Mus., 11:160, January 5, 1889.
_Cricetodipus compactus_, Trouessart, Catalogus Mammalium, 1:581, 1897.
_Perodipus compactus_, Elliot, Field Columbian Museum, Zool. Ser., 2:240, 1901.
_Dipodomys ordii compactus_, Davis, Journ. Mamm., 23:332, August 14, 1942.
_Type._--None designated but Poole and Schantz (1942:406) assumed it to be a female, no. 19665/35227, only the skin found, from Padre Island, Cameron County, Texas. April 3, 1888. Purchased from C. K. Worthen.
_Range._--Padre and Mustang islands, Cameron County, Texas.
_Diagnosis._--Size medium (see measurements); tail short. Color light; entire dorsal surface Light Ochraceous-Buff, purest on sides and flanks, upper parts but lightly suffused with black. A lighter color phase has entire dorsal surface Cartridge Buff, purest on sides and flanks, upper parts but lightly washed with black. In both phases, cheeks white; pinnae of ears, plantar surfaces of hind feet, dorsal stripe of tail, ventral stripe of tail (in most specimens) present on proximal third of tail only, brownish. Skull small; rostrum narrow and long; nasals long; auditory bullae inflated, but greatest breadth across bullae only slightly more than breadth across zygomatic processes of maxillae; interparietal region wide.
_Comparisons._--From _Dipodomys ordii sennetti_, _D. o. compactus differs_ in: Size slightly less; color lighter in all pigmented areas; skull smaller; auditory bullae slightly less inflated; interorbital width less; interparietal region wider; nasals longer.
From _Dipodomys ordii attenuatus_, _D. o. compactus_ differs in: Body larger; tail shorter; normal color phase darker, and lighter color phase lighter; skull larger; rostrum wider and longer; nasals longer; interorbital region wider; auditory bullae relatively as well as actually less inflated; interparietal region wider; pterygoid fossae large and round as opposed to small and ovoid.
Compared with _Dipodomys ordii medius_ and _Dipodomys ordii ordii_, _D. o. compactus_ is smaller, lighter in color, and has less inflated auditory bullae and a smaller skull.
_Remarks._--This subspecies of _Dipodomys ordii_ was originally described as _Dipodomys compactus_ by True in 1889 and stood as a full species until Davis (1942:332) relegated it to subspecific status under _Dipodomys ordii_. Davis (_op. cit._) observed close resemblances in external proportions, size of mastoid bullae, width of supraoccipital, and size and shape of the interparietal, between _Dipodomys ordii_ and _Dipodomys sennetti_ and therefore concluded that they were only subspecies of one species. He observed that the difference between _Dipodomys compactus_ and _Dipodomys sennetti_ was of approximately the same degree as that between _Dipodomys sennetti_ and _Dipodomys ordii_. From this he concluded that all three were subspecies of the one species _Dipodomys ordii_.
In any sizeable sample of _Dipodomys sennetti_ there are crania closely resembling those of _Dipodomys ordii ordii_ and others closely resembling those of _Dipodomys compactus_. The external proportions of both _D. sennetti_ and _D. compactus_ are duplicated in _D. ordii_ from El Paso and conversely, specimens with the proportions of typical _D. o. ordii_ occur in populations of _D. sennetti_ and _D. compactus_. Thus, it appears that Davis' usage of the name _Dipodomys ordii compactus_ should stand although there may be a hiatus in geographic occurrence between _D. ordii_ and _D. sennetti_, as of course there is between _D. sennetti_ and _D. compactus_.
In _D. o. compactus_ there is a complete enamel ring around the occlusal surface of each molariform tooth; in _D. o. ordii_ this ring is incomplete lingually on each of the molariform teeth and labially on the first three, and in some individuals of _D. o. sennetti_ the enamel ring is complete and in others it is incomplete labially and lingually as in _D. o. ordii_.
_Specimens examined._--Total, 44, all from Texas, distributed as follows: _Nueces County_: 19 mi. S Port Aransas, Mustang Island, 27 (17 TCWC; 10 MVZ); Mustang Island, 17 (LACM).
=Dipodomys ordii sennetti= (Allen)
_Dipodops sennetti_ Allen, Bull. Amer. Mus. Nat. Hist., 3:226, April 29, 1891.
_Cricetodipus sennetti_, Trouessart, Catalogus Mammalium, 1:581, 1897.
_Perodipus sennetti,_ Elliott, Field Columbian Museum, Zool. Ser., 2:239, 1901.
_Dipodomys ordii sennetti_, Davis, Journ. Mamm., 23:332, August 14, 1942.
_Type._--Male, no. 3478/2733. Amer. Mus. Nat. Hist.; near Brownsville, Cameron County, Texas; obtained on March 9, 1888, by J. M. Priour. (After Allen, original description, type not seen.) Type locality recorded by Bailey (1905:145) as "Santa Rosa, 85 mi. SW Corpus Christi."
_Range._--Southern Texas, south of Corpus Christi; marginal localities, all in Texas are: Somerset, 8 mi. NE Los Angeles, 8 mi. E Encinal, Santa Rosa, 28 mi. E Raymondville, 2 mi. S Riviera.
_Diagnosis._--Size small (see measurements). Color dark, entire dorsal surface (_c_) between Pinkish Buff and Cinnamon-Buff, purest on sides and flanks, upper parts mixed with black; arietiform markings, pinnae of ears, dorsal and ventral stripes of tail, plantar surfaces of hindfeet, brownish-black. Skull small; auditory bullae but slightly inflated in relation to size of skull; nasals slightly flaring distally; premaxillae extending but slightly posterior to nasals; interorbital width relatively great; external auditory meatus small; rostrum relatively long and wide; zygomatic arches relatively heavy.
_Comparisons._--From _Dipodomys ordii ordii_, _D. o. sennetti_ differs in: Size smaller, tail shorter; color darker; skull smaller; nasals longer; rostrum wider; auditory bullae less inflated; external auditory meatus smaller; pterygoid fossae more rounded; zygomatic arches heavier.
From _Dipodomys ordii medius_, _D. o. sennetti_ differs as follows: Size smaller; color darker, but with less red in pelage; skull markedly smaller in all respects.
From _Dipodomys ordii compactus_, _D. o. sennetti_ differs in: Size somewhat less; color darker; skull with total length greater; orbit smaller; least interorbital width greater; braincase more inflated; width across auditory bullae more; interparietal region wider; external auditory meatus larger; medial part of audital portion (see Howell, 1932) of auditory bullae larger.
_Remarks._--_Dipodomys sennetti_, along with _Dipodomys compactus_, was regarded by Davis as conspecific with _Dipodomys ordii_. Reasons for placing these two kinds of kangaroo rats as subspecies of _D. ordii_ are given in the account of _Dipodomys ordii compactus_.
This subspecies is known only from north of the Rio Grande which may serve as a barrier to the spread of the animal into northern Tamaulipas.
_Specimens examined._--Total, 20, all from Texas, distributed as follows: _Atascosa County_: Somerset, 2 (TCWC). _LaSalle County_: 8 mi. NE Los Angeles, 1 (TCWC); 8 mi. E Encinal, 1 (TCWC). _Kleberg County_: 2 mi. S Riviera, 9 (TCWC). _Jim Hogg County_: Hebronville, 8 (LACM). _Brooks County_: Falfurrias, 2 (LACM). _Willacy County_: 28 mi. E Raymondville, 2 (TCWC).
=Dipodomys ordii evexus= Goldman
_Dipodomys ordii evexus_ Goldman, Journ. Washington Acad. Sci., 23:468, October 15, 1933.
_Perodipus montanus richardsoni_ Warren, Mammals of Colorado, p. 76, 1910 (part--the part from Salida, Colorado).
_Type._--Male, adult, no. 150990, U. S. Nat. Mus. Biol. Surv. Coll.; Salida, Chaffee County, Colorado (altitude 7000 ft.); obtained on November 10, 1907, by Merritt Cary, original no. 1245.
_Range._--Upper Arkansas River Valley of south-central Colorado, from Salida to Pueblo.
_Diagnosis._--Size large (see measurements). Color dark, entire dorsal surface between (_16´´_) Pinkish Cinnamon and Cinnamon-Buff, purest on sides and flanks, upper parts strongly suffused with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet and dorsal and ventral stripes of tail, blackish. Skull of medium size; rostrum short and wide; nasals short; auditory bullae but slightly inflated; braincase but slightly vaulted.
_Comparisons._--From _Dipodomys ordii richardsoni_, _D. o. evexus_ differs as follows: Size smaller in all measurements taken; color darker; ears darker, dorsal and ventral stripes on tail darker, arietiform markings darker and more distinct, plantar surfaces of hind feet darker; skull smaller in all measurements; length, as expressed in percentage of width of skull, greater in _D. o. evexus_ (66 per cent in _D. o. evexus_, 62 per cent in _D. o. richardsoni_ which gives the appearance of a long, narrow skull as contrasted with a rather short, wide skull); auditory bullae less expanded laterally, posteriorly and ventrally; interparietal region relatively wider in proportion to greatest width across auditory bullae; cutting edge of upper incisors narrower; pterygoid fossae smaller and more nearly circular.
Compared with _Dipodomys ordii luteolus_, _D. o. evexus_ differs as follows: Size somewhat smaller in external measurements; color darker in all pigmented areas; skull smaller in two of the seven measurements taken but in the other five measurements somewhat larger; auditory bullae less inflated; cutting edge of upper incisors narrower; zygomatic arch heavier; pterygoid fossae smaller and more nearly circular; external auditory meatus ovoid as contrasted to nearly circular; paroccipital processes smaller.
From _Dipodomys ordii nexilis_, _D. o. evexus_ differs in: Color darker; rostrum wider and shorter; interorbital region wider; breadth across maxillary arches greater; auditory bullae less inflated; interparietal region larger; zygomatic arches heavy and bowed laterad; molariform teeth smaller; cutting edge of upper incisors narrower.
For comparison with _Dipodomys ordii montanus_ see account of that subspecies.
_Remarks._--This race of kangaroo rat, described from the upper Arkansas River Valley, closely resembles _Dipodomys ordii luteolus_ but differs in darker color, slightly smaller body and larger skull.
No evidence of intergradation with any other race was noted. To the south the range of _D. o. evexus_ is separated from that of _D. o. montanus_ by a high, transverse ridge of the Rocky Mountains which is inhospitable to these animals. Much territory inhospitable to _Dipodomys_ intervenes also between the ranges of _D. o. evexus_ and _D. o. luteolus_, but there are areas connecting the northern part of the range of _D. o. evexus_ and the southwestern part of the known range of _D. o. luteolus_, in which _Dipodomys_ may occur. If kangaroo rats occur in these areas it is to be expected that they will show intergradation between the two subspecies concerned.
_Specimens examined._--Total, 24, all from Colorado, distributed as follows: _Chaffee County_: Salida, 10 (3 USBS; 7 AMNH). _Fremont County_: Canyon City, 13 (USBS). _Pueblo County_: Pueblo, 1 (USBS).
=Dipodomys ordii medius= new subspecies
_Perodipus montanus richardsoni_, Bailey, N. Amer. Fauna, 25:144, October, 1905 (part--the part from Santa Rosa, New Mexico).
_Type._--Male, no. 118526, U. S. Nat. Mus. Biol. Surv. Coll.; Santa Rosa, Guadalupe County, New Mexico; obtained on October 5, 1902, by Jas. H. Grant, original no. 565.
_Range._--From north-central New Mexico, southeastward to west-central Texas; marginal localities are, in New Mexico: 15 mi. N Ojo Caliente, Gallina Mts., Deer Creek, San Pedro; in Texas: 20 mi. N Monahans, Colorado, 7 mi. E Post, 6 mi. SW Muleshoe.
_Diagnosis._--Size medium (see measurements). Color dark; entire dorsal surface (_14''_) between Orange-Cinnamon and Cinnamon, purest on sides and flanks, dorsal surface lightly washed with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish-black. Skull of medium size; nasals long; medial mastoid portion of auditory bullae well inflated caudad; braincase vaulted; external auditory meatus small; rostrum short and truncate; medial auditory portion of auditory bullae relatively little inflated; pterygoid fossae ovate; zygomatic arches slender and relatively straight; junction of jugal and zygomatic process of maxilla heavy.
_Comparisons._--From _Dipodomys ordii richardsoni_, _D. o. medius_ differs as follows: Tail longer; hind foot shorter; color darker; arietiform markings more distinct; white lateral stripes of tail narrower; ventral stripe of tail in most specimens complete to end of pencil; postauricular spots less pronounced; hip stripe narrower and in some specimens almost obliterated; skull smaller in all measurements taken; angle of dorsal extension of premaxilla with zygomatic process of maxilla more nearly 90°; braincase more vaulted; medial mastoid portion of auditory bullae more inflated, and coming to more of a point; medial auditory portion of auditory bullae more inflated ventrally; rostrum shorter and narrower; external auditory meatus smaller.
From _Dipodomys ordii montanus_, _D. o. medius_ differs in: Color lighter in all pigmented areas; skull larger in all respects; rostrum shorter and heavier; bullae more inflated; zygomata, while nearly straight, are bowed slightly laterally; pterygoid fossae more ovate; foramen magnum larger; pterygoid foramina smaller.
Compared with _Dipodomys ordii ordii_ and _Dipodomys ordii sennetti_, _D. o. medius_ is larger and darker. The skull is also larger in all measurements taken.
Compared with _Dipodomys ordii longipes_, _D. o. medius_ is darker and smaller.
_Remarks._--This hitherto undescribed race of _Dipodomys ordii_ can readily be distinguished from any of its near neighbors by the characters set forth under diagnosis and comparisons.
Intergradation is noted with _D. o. ordii_, _D. o. longipes_, _D. o. montanus_ and _D. o. richardsoni_. Among named races _D. o. medius_ shows closest affinities with _D. o. richardsoni_ but the two are easily separable. The northwestern extremity of the range of _D. o. medius_ is an area of intergradation in which no specimens are clearly of one subspecies or the other. In specimens from 5 miles east of Abiquiu, New Mexico, three-way intergradation occurs. These animals are like _D. o. medius_ in size, _D. o. longipes_ in color and their cranial proportions are as in _D. o. montanus_. At Deer Creek, New Mexico, and at Monahans, Texas, the animals show intergradation in size of body and agree with _D. o. ordii_ in cranial proportions. In specimens from 6 miles southwest of Muleshoe, Texas, intergradation with _D. o. richardsoni_ in the shape of the skull and width of the rostrum is noted. In the sum total of characters studied, however, these specimens are referable to _D. o. medius_.
_Specimens examined._--Total, 129, distributed as follows:
=New Mexico=: _Rio Arriba County_: 2 mi. SE El Rito, 2 (KU); Rio Alamosa, 15 mi. N Ojo Caliente, 1 (USBS); 6 mi. E Abiquiu, 4 (USBS); Rinconada, 5 (USBS); Espanola, 6 (USBS). _Sandoval County_: 12 mi. NW Alameda, 5500 ft., 3 (MVZ). _Santa Fe County_: Seton's Ranch, near Santa Fe, 1 (USBS); 8 mi. SW Santa Fe, 8 (KU); San Pedro, 3 (USBS). _San Miguel County_: Pecos, 2 (USBS); 3 mi. S Pecos, 2 (USBS); Rowe, 6 (LACM). _Bernallilo County_: Bear Canyon, Sandia Mountains, 7 (USBS); Pajarito, 3 (MVZ). _Guadalupe County_: Santa Rosa, 10 (USBS). _Lincoln County_: 44 mi. NW Roswell, 5 (MVZ). _De Baca County_: 8 mi. N Fort Sumner, 9 (USBS). _Roosevelt County_: Kenna, 4 (LACM). _Curry County_: 4 mi. W and 2-3/4 mi. N Clovis, 1 (MVZ). _Chaves County_: 40 mi. N Roswell, 2 (USBS); 35 mi. N Roswell, 2 (USBS); 15 mi. NE Roswell, 8 (LACM); Stinking Springs Lake, 3 (USBS).
=Texas=: _Bailey County_: 6 mi. SW Muleshoe, 5 (UM); 9 mi. SW Muleshoe, 2 (UM). _Garza County_: 7 mi. E Post, 5 (UM). _Martin County_: Stanton, 4 (USBS). _Howard County_: 6 mi. NE Coahoma, 7 (UM); 1 mi. S Coahoma, 1 (UM); 5 mi. W Big Springs, 2400 ft., 1 (MVZ). _Mitchell County_: Colorado, 5 (USBS). _Winkler County_: 20 mi. N Monahans, 1 (USBS). _Ward County_: Monahans, 1 (USBS).
=Dipodomys ordii obscurus= (Allen)
_Perodipus obscurus_ Allen, Bull. Amer. Mus. Nat. Hist., 19:603, November 12, 1903.
_Dipodomys ordii obscurus_, Grinnell, Journ. Mamm., 2:96, May 2, 1921.
_Type._--Male, adult, no. 20957, Amer. Mus. Nat. Hist.; Rio Sestin, northwestern Durango, Mexico; obtained on April 13, 1903, by J. H. Batty. (Type not seen.)
_Range._--Northwestern and northern Durango, Mexico; marginal localities are: Rosario, Rio Sestin, Mt. San Gabriel, Rio del Bocas, Villa Ocampo.
_Diagnosis._--Size small (see measurements). Color dark, entire dorsal surface (_16''_) between Pinkish Cinnamon and Cinnamon-Buff, purest on sides, flanks and cheeks, upper parts strongly suffused with black; arietiform markings, plantar surfaces of hind feet, pinnae of ears, dorsal and ventral stripes of tail, brownish. Skull of medium size, nasals long and flared distally; rostrum long and narrow; interorbital region relatively narrow; auditory bullae less inflated than in _Dipodomys ordii palmeri_; interparietal region narrow; zygomatic arches heavy and bowed laterad; pterygoid fossae ovoid; braincase but slightly vaulted.
_Comparisons._--From _Dipodomys ordii palmeri_, _D. o. obscurus_ differs in: Size larger; color lighter; nasals shorter and more flared distally; interorbital width less; lacrimal processes larger; auditory bullae less inflated; pterygoid fossae ovoid as opposed to subcircular; zygomatic arches heavier; rostrum shorter and wider.
From _Dipodomys ordii ordii_, _D. o. obscurus_ differs as follows: Size smaller; color darker; skull smaller; nasals longer; rostrum narrower and shorter; interorbital width greater; interparietal region narrower; narrower across auditory bullae; zygomatic arches heavier and more bowed laterally; pterygoid fossae more ovoid; breadth across maxillary arches greater; external auditory meatus smaller.
With _Dipodomys ordii attenuatus_ and _Dipodomys ordii sennetti_, _D. o. obscurus_ needs no comparison since it is larger and darker than either of those subspecies and can readily be told from the latter by the greater expansion of the auditory bullae.
For comparison with _Dipodomys ordii fuscus_ see account of that subspecies.
_Remarks._--_D. o. obscurus_ seemingly is not a far-ranging subspecies. The only examples referable to it come from a relatively restricted area of Durango, Mexico. One specimen from Rio del Bocas, Durango, is not typical and shows the characters described for the animals from Chihuahua City and from Casas Grandes. I have considered the possibility that this specimen is an intergrade between _D. o. obscurus_ and an unnamed subspecies ranging to the northeastward. The other specimens in the series from Rio del Bocas are typical of _D. o. obscurus_.
_Specimens examined._--Total, 69, all from Durango, distributed as follows: Rosario, 20 (AMNH); Villa Ocampo, 5 (AMNH); Rio Sestin, 30 (28 AMNH; 2 CNHM); Mt. San Gabriel, 2 (AMNH); Rio del Bocas, 11 (AMNH); Rancho Santuario, 1 (AMNH).
=Dipodomys ordii terrosus= Hoffmeister
_Dipodomys ordii terrosus_ Hoffmeister, Proc. Biol. Soc. Washington, 55:165, December 31, 1942.
_Dipodomys phillipsi ordi_, Coues and Allen, Monogr. North American Rodentia, p. 541, August, 1877 (part--the part from Yellowstone River, Montana).
_Perodipus montanus richardsoni_, Cary, N. Amer. Fauna, 49:124, December, 1926 (part--the part from Glendive, Montana).
_Type._--Male, no. 93477, Mus. Vert. Zool., Univ. California; Yellowstone River, 5 mi. W Forsyth, 2,750 ft., Rosebud County, Montana; obtained on June 2, 1940, by J. R. Alcorn, original no. 1528.
_Range._--Extreme southwestern Saskatchewan and southeastern Alberta, eastern half of Montana, northern Wyoming and probably extreme western North Dakota; marginal localities are: 50 mi. W Swift Current, Saskatchewan; "near Medicine Hat," Alberta; in Wyoming, Sheep Creek, and 23 mi. SW Newcastle; in Montana, Medicine Rocks (14 mi. NE Ekalaka), and Glendive.
_Diagnosis._--Size large (see measurements). Color dark, entire dorsal surface near (_c_) Ochraceous-Buff, purest on sides and flanks; upper parts mixed with black; arietiform markings, pinnae of ears, dorsal and ventral stripes of tail and plantar surfaces of hind feet brownish-black. Skull large; rostrum short, wide and deep; braincase slightly vaulted; auditory bullae markedly inflated ventrally; zygomatic arches heavy and bowed laterad; upper incisors long and robust.
_Comparisons._--From _Dipodomys ordii priscus_, _D. o. terrosus_ differs as follows: Size larger in all measurements taken, except for length of hind feet, which is less; color darker in all pigmented areas; skull larger in all parts measured except width of interparietal, which is less; auditory bullae more inflated ventrally; zygomatic processes of maxillae wider; rostrum deeper and shorter.
From _Dipodomys ordii richardsoni_, _D. o. terrosus_ differs as follows: Size larger; color darker in all pigmented areas; ventral stripe of tail extending farther distally; skull larger except in width across auditory bullae, which is the same.
For comparison with _Dipodomys ordii luteolus_ see account of that subspecies.
_Remarks._--As noted in the comparisons, _D. o. terrosus_ is larger and darker than _D. o. priscus_, _D. o. luteolus_ or _D. o. richardsoni_, its closest geographic neighbors, and does not resemble any of them, but rather resembles _D. o. longipes_ and _D. o. evexus_ in size and appearance, both of which are distantly removed geographically.
Like other subspecies of the species _D. ordii_, _D. o. terrosus_ prefers sandy soils to those of any other type. Two miles east and 1 mile south of Forsyth, Montana, animals were trapped on lenses of sandy soil. These lenses alternated with areas of black loam of similar size. It was noteworthy that burrows were found only in the areas of sandy soil, although paths used by the rats when foraging did extend onto and several crossed the lenses of black loam. We were not permitted to excavate any of these burrows, but conversation with farmers of the immediate vicinity indicated that the burrows were not deep. An eight-inch disc would frequently plow out nests and food caches. It was said that each of several caches contained as much as a peck of wheat.
Intergradation was noted in animals from 23 miles southwest of Newcastle and Arvada, Wyoming. In animals from both localities the pterygoid fossae are more as in _D. o. luteolus_ but referable to _D. o. terrosus_. The specimens from Arvada, although immature, possessed cranial characters which were intermediate between those of _D. o. terrosus_ and _D. o. luteolus_ but the specimens are referable to the former.
_Specimens examined._--Total, 74, distributed as follows:
=Montana=: _Petroleum County_: 24 mi. N Roundup, 8 mi. SW Flatwillow, 2 (UM). _Garfield County_: Jordan, 10 (1 UM; 2 MVZ; 7 AMNH). _Dawson County_: Glendive, 9 (USNM). _Musselshell County_: Harvey Ranch, Melstone, 3 (MVZ). _Rosebud County_: Yellowstone River, 5 mi. W Forsyth, 2750 ft., 7 (MVZ); 2 mi. E and 1 mi. S Forsyth, 2600 ft., 8 (KU). _Custer County_: Miles City, 1 (USBS). _Yellowstone County_: Billings, 2 (1 USBS; 1 MVZ). _Big Horn County_: Fort Custer, 1 (USBS); Crow Agency, 1 (USBS). _Powder River County_: Powderville, 4 (USBS). _Carter County_: Medicine Rocks, 15 mi. N Ekalaka, 2 (USBS); Medicine Rocks, 14 mi. N Ekalaka, 2 (USBS); Clark's Fork, 1 (USBS).
=Wyoming=: _Big Horn County_: Dry Creek, 10 mi. W Germania, 1 (USBS); 3 mi. E Germania, 1 (USBS); Greybull, 2 (USBS); Bighorn Basin, 1 (USBS). _Sheridan County_: Arvada, 8 (USBS). _Campbell County_: Little Powder River, 1 (USBS). _Weston County_: Newcastle, 2 (USBS); 23 mi. SW Newcastle, 4 (USBS). _Fremont County_: Wilson's Ranch, Sheep Creek, S base Owl Creek Mountains, 1 (USBS).
_Additional records._--=Canada= (Anderson, 1946:131): _Alberta_: near Medicine Hat, 1; _Saskatchewan_: near Shackleton, 45-50 mi. NW Swift Current, 1; near Tompkins, 50 mi. W Swift Current, 1.
=Dipodomys ordii fremonti= Durrant and Setzer
_Dipodomys ordii fremonti_ Durrant and Setzer, Bull. Univ. Utah, 35 (no. 26):21, June 30, 1945.
_Type._--Female, no. 15661, Carnegie Museum, Pittsburgh, Pennsylvania; Torrey, 7000 ft., Wayne County, Utah; obtained on July 19, 1938, by W. F. and F. H. Wood, original no. 1562.
_Range._--Known only from the type locality.
_Diagnosis._--Size small (see measurements). Color dark, entire dorsal surface Cinnamon-Buff, purest on sides, flanks and cheeks; upper parts strongly suffused with black; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish. Skull small; upper incisors long; rostrum deep; jugal bowed laterally; diastema long; upper molariform tooth-row long.
_Comparisons._--From _Dipodomys ordii panguitchensis_, _D. o. fremonti_ differs in: Color lighter in all pigmented areas, particularly ears which are light brown in _D. o. fremonti_ and black in _D. o. panguitchensis_; skull larger in all measurements taken; upper incisors longer; rostrum deeper; auditory bullae deeper; jugal bowed laterally rather than straight; diastema longer.
From _Dipodomys ordii cupidineus_, _longipes_, _nexilis_, _uintensis_ and _sanrafaeli_, _D. o. fremonti_ can readily be distinguished by its smaller size and generally darker color.
_Remarks._--This subspecies of _Dipodomys ordii_ inhabits the upper reaches of the Fremont River in west-central Wayne County, Utah. _D. o. fremonti_ appears to be isolated and is known only from the type locality. _D. o. fremonti_ is so remarkably different from any other subspecies of _Dipodomys ordii_ that a long period of isolation from the ancestral stock (which probably gave rise also to _Dipodomys ordii utahensis_ and _Dipodomys ordii panguitchensis_) is indicated. Although intergradation is not known to occur with other kinds, differentiation has not progressed far enough for these animals to be recognized as a distinct species.
The subspecies closest, geographically, to _D. o. fremonti_ is _D. o. cupidineus_ from which _D. o. fremonti_ differs more than from any of the other named forms.
_Specimens examined._--Total, 9, from Utah, as follows: _Wayne County_: Torrey, 7000 ft., 9 (CM).
=Dipodomys ordii uintensis= Durrant and Setzer
_Dipodomys ordii uintensis_ Durrant and Setzer, Bull. Univ. Utah, 35 (no. 26):27, June 30, 1945.
_Perodipus longipes_, Allen, Bull. Amer. Mus. Nat. Hist., 8:246, November 1896 (part--the part from Uncompahgre Indian Reservation, Utah).
_Dipodomys ordii luteolus_, Moore, Journ. Mamm., 11:88, February, 1930 (part--the part from Vernal, Utah).
_Type._--Male, adult, no. 11634, Carnegie Museum, Pittsburgh, Pennsylvania; Red Creek, 6,700 ft., 2 mi. N Fruitland, Duchesne County, Utah; obtained on August 15, 1936, by J. K. and M. T. Doutt, original no. 3433.
_Range._--Uinta basin of the White, Green and Duchesne river drainage in northeastern Utah; marginal occurrences are: 2 mi. N Fruitland, 10 mi. S Ouray, Vernal.
_Diagnosis._--Size large (see measurements); hind foot short. Color dark; entire dorsal surface, near (_c_) Cinnamon-Buff, purest on sides and flanks, with moderate suffusion of black on upper parts; cheeks white; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish. Skull large; frontomaxillary suture convex mediad; lacrimal process large; styloid process projects, on ventral surface of tympanic bulla, beyond middle of external auditory meatus; nasals flared distally.
_Comparisons._--From _Dipodomys ordii priscus_, _D. o. uintensis_ differs in: Hind foot shorter; color darker; styloid process projects on ventral part of tympanic bulla well anterior to middle of external auditory meatus as opposed to projecting to middle; depth of foramen magnum, expressed in percentage of width across posterior margin of occipital condyles, greater (86 per cent in _D. o. uintensis_ and 81 per cent in _D. o. priscus_); frontomaxillary suture convex mediad as opposed to nearly straight; lacrimal processes larger; nasals more flared distally.
From _Dipodomys ordii nexilis_, _D. o. uintensis_ differs as follows: Size smaller; color lighter; interorbital breadth greater; frontomaxillary suture convex mediad as opposed to concave; lacrimal processes larger; nasals more flared distally; narrower across auditory bullae; basal length greater; zygomatic arches bowed laterad as opposed to relatively straight.
From _Dipodomys ordii longipes_, _D. o. uintensis_ differs as follows: Size smaller; color darker; auditory bullae wider, longer and deeper; frontomaxillary suture convex mediad as opposed to nearly straight; greatest breadth across auditory bullae less.
For comparison with _Dipodomys ordii sanrafaeli_ see account of that subspecies.
_Remarks._--This large, rather dark race inhabits the desert valleys of the White, Green and Duchesne rivers in northeastern Utah. The race nearest geographically, as well as morphologically, is _Dipodomys ordii priscus_. Intergradation occurs with the latter subspecies at Vernal, Uintah County, in cranial measurements and in color. On the basis of color alone _D. o. uintensis_ can be distinguished from _D. o. sanrafaeli_, the geographic race to the south. Specimens from Jensen are intermediate in color and cranial measurements between _Dipodomys ordii nexilis_ and _D. o. uintensis_ but are referable to the latter.
_Specimens examined._--Total, 40, all from Utah, distributed as follows: _Duchesne County_: Red Creek, 6700 ft., 2 mi. N Fruitland, 4 (CM); 10 mi. S Myton, 1 (UU); 20 mi. S Myton, 1 (RH). _Uintah County_: Vernal, 1 (BYU); 20 mi. E Ouray, 5 (CM); Junction Green and White rivers, 4800 ft., 2 mi. S Ouray, 5 (CM); Pariette Bench, 5000 ft., 8 mi. S Ouray, 8 (CM); Desert Springs, 10 mi. S Ouray, 4 (CM); Pariette Bench, 12 mi. S Ouray, 2 (CM); Jensen, 5 (BYU); E side Green River, 3 mi. S Jensen, 4 (CM).
=Dipodomys ordii sanrafaeli= Durrant and Setzer
_Dipodomys ordii sanrafaeli_ Durrant and Setzer, Bull. Univ. Utah, 35 (no. 26):26, June 30, 1945.
_Dipodomys ordii longipes_, Stanford, Journ. Mamm., 12:360, November, 1931 (part--the part from King's Ranch, Utah).
_Type._--Female, adult, no. 4612, Museum of Zoology, University of Utah; 1-1/2 mi. N Price, 5567 ft., Carbon County, Utah; obtained on June 5, 1940, by Ross Hardy and H. Higgins, original no. 1901.
_Range._--East-central Utah, east into west-central Colorado. Marginal occurrences are: in Utah, 12 mi. E Price, 1-1/2 mi. N Price, Notom, King's Ranch, 12 mi. SW Green River, 16 mi. NW Moab; in Colorado, State Line and Grand Junction.
_Diagnosis._--Size large (see measurements). Color dark, entire dorsal surface Cinnamon-Buff, purest on sides and flanks with but slight suffusion of black on upper parts; cheeks white; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish-black. Skull large; pterygoid fossae ovoid; lacrimal processes small; width across maxillary arches relatively great; auditory bullae well inflated; diastema short.
_Comparisons._--From _Dipodomys ordii longipes_, _D. o. sanrafaeli_ differs as follows: Size smaller; color lighter, more cinnamon, pinnae of ears lighter; skull smaller; auditory bullae smaller; pterygoid fossae ovoid rather than round; wider across occipital condyles; narrower across zygomatic processes of maxillae.
From _Dipodomys ordii cupidineus_, _D. o. sanrafaeli_ can be recognized by its larger size, lighter color and larger skull.
For comparisons with _Dipodomys ordii nexilis_, _Dipodomys ordii priscus_ and _Dipodomys ordii uintensis_ see accounts of those subspecies.
_Remarks._--Intergradation between _Dipodomys ordii cupidineus_ and _D. o. sanrafaeli_ is noted in the intermediate size of body in a single specimen from Notom. Intergradation in color and cranial characters occurs between _Dipodomys ordii nexilis_ and _D. o. sanrafaeli_ in specimens from 16 miles northwest of Moab. All these specimens, however, are referable to _D. o. sanrafaeli_.
Animals from that part of the range of _D. o. sanrafaeli_ west of the Green River are typical while those to the east of the river are all intergrades. Animals from 16 miles northwest of Moab, Utah, and from three localities in Colorado, even though intergrades with _D. o. nexilis_, are all referable to _D. o. sanrafaeli_. It appears that the Green River does not act as a complete barrier in this area since in the winter it occasionally freezes over, thus allowing the animals to cross. It is thought that kangaroo rats do not hibernate but remain more or less active throughout the winter. Man-made conveniences, such as bridges, might also serve as means of dispersal, permitting these animals to cross otherwise prohibitive barriers. Where there are no bridges across the Green River, farther to the south, the rats apparently do not cross the river; steep, rocky canyon-walls and the lack of ice on the water in winter lessen the chances of small mammals crossing from one side to the other.
_Specimens examined._--Total, 30, distributed as follows:
=Utah=: _Carbon County_: 12 mi. NE Price, 2 (CM); 3 mi. NE Price, 1 (RH); 1-1/2 mi. N Price, 2 (1 RH; 1 UU); Wellington, 1 (RH). _Emery County_: "San Rafael, 21 mi. out," 1 (USAC); 12 mi. SW Green River, 2 (CM). _Grand County_: 1 mi. E Green River, 1 (MVZ); 16 mi. NW Moab, 2 (CM). _Wayne County_: Notom, 1 (BYU). _Garfield County_: King's Ranch, 4800 ft., 3 (2 UU; 1 USAC).
=Colorado=: _Mesa County_: State Line, 11 (MVZ); Fruita, 1 (USBS); Grand Junction, 2 (USBS).
=Dipodomys ordii panguitchensis= Hardy
_Dipodomys ordii panguitchensis_ Hardy, Proc. Biol. Soc. Washington, 55:90, June 25, 1942.
_Type._--Male, adult, no. 4375, Museum of Zoology, University of Utah; one mile south of Panguitch, 6666 ft., Garfield County, Utah; obtained on August 31, 1940, by Ross Hardy, original no. 2151.
_Range._--Known only from the type locality.
_Diagnosis._--Size small (see measurements). Color dark, entire dorsal surface near Olive-Brown, purest on sides and flanks, upper parts strongly suffused with black; cheeks white; arietiform markings, pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail which are wider than white lateral stripes, blackish. Skull small; rostrum relatively short and wide; interorbital region wide; interparietal region wide; foramen magnum elongate dorsoventrally; pterygoid fossae ovoid.
_Comparisons._--From _Dipodomys ordii utahensis_, which it closely resembles, _D. o. panguitchensis_ differs in: Size larger; color darker; interparietal region wider; foramen magnum elongate dorsoventrally as opposed to nearly round; pterygoid fossae ovoid as opposed to nearly round.
This subspecies can be distinguished from _Dipodomys ordii fetosus_, _Dipodomys ordii celeripes_ and _Dipodomys ordii cupidineus_ by its darker color and generally larger size.
For comparisons with _Dipodomys ordii cinderensis_ and _Dipodomys ordii fremonti_ see accounts of those subspecies.
_Remarks._--This geographic race inhabits the upper reaches of the Sevier River Valley in the vicinity of Panguitch, Utah. Natural barriers to kangaroo rats, such as the Cedar Mountains to the west, high plateau country to the south, the Paunsaugunt Plateau to the east and the narrow canyons of the Sevier River to the north prevent these animals from extending their range or from coming into physical contact with any adjacent geographic races. This isolation has resulted in a fairly stable population. Some animals, however, show characters, such as the width of the rostrum, and the shape and length of the nasals which are intermediate between those of topotypes of _D. o. utahensis_ and the type series of _D. o. panguitchensis_.
_Specimens examined._--Total, 3, all from Utah, distributed as follows: _Garfield County_: 1 mi. S Panguitch, 6666 ft., 3 (2 RH; 1 UU).
=Dipodomys ordii monoensis= (Grinnell)
_Perodipus monoensis_ Grinnell, Univ. California Publ. Zool., 21:46, March 29, 1919.
_Dipodomys ordii monoensis_, Grinnell, Journ. Mamm., 2:96, May 2, 1921.
_Type._--Female, adult, no. 27002, Museum of Vertebrate Zoology, University of California; Pellisier Ranch, 5 mi. N Benton Station, 5600 ft., Mono County, California; obtained on September 21, 1917, by J. Dixon, original no. 6384.
_Range._--Northeastern Inyo and Mono counties, California, north to southern Pershing County and east to eastern Nye County, Nevada; marginal occurrences are: in California, 5 mi. N Benton Station and Deep Spring Valley; in Nevada, Arlemont, 2 mi. NW Palmetto, 1 mi. N Beatty, 5 mi. W White Rock Spring, Big Creek at Quinn Canyon Mts., 2-1/2 mi. S Lock's Ranch, 2 mi. S Millett P. O., 13-1/2 mi. NW Goldfield, Fingerrock Wash, Eastgate, 1/2 mi. NE Toulon, 21 mi. W and 2 mi. N Lovelock, 1/2 mi. S Pyramid Lake, West Walker River in Smith's Valley, and 10 mi. S Yerington.
_Diagnosis._--Size medium (see measurements). Color pale, entire dorsal surface (_c_) between Pinkish Buff and Cinnamon-Buff, purest on sides, flanks and cheeks, with but slight suffusion of black in upper parts; pinnae of ears, plantar surfaces of hind feet, dorsal and ventral stripes of tail, brownish. Skull medium in size; rostrum relatively long and narrow; nasals relatively short; interorbital region narrow; interparietal region relatively wide; lacrimal processes small; auditory bullae relatively small; pterygoid fossae circular; zygomatic arches robust and relatively straight; foramen magnum nearly circular.
_Comparisons._--From _Dipodomys ordii columbianus_, _D. o. monoensis_ differs as follows: Size larger; color lighter; skull larger; rostrum longer and narrower; interorbital region narrower; breadth across auditory bullae less; lacrimal processes larger; braincase less vaulted; auditory bullae more inflated ventrally; pterygoid fossae smaller; zygomatic arches more robust; cutting edge of upper incisors wider.
From _Dipodomys ordii fetosus_, _D. o. monoensis_ differs in: Hind foot shorter; color lighter; skull smaller; rostrum shorter and narrower; interorbital width less; interparietal region larger; lacrimal processes smaller; auditory bullae less inflated.
For comparison with _Dipodomys ordii inaquosus_ see account of that subspecies.
_Remarks._--This subspecies retains all of its diagnostic characters throughout nearly all parts of its geographic range. Intergradation occurs in animals from the southern end of Pyramid Lake, Big Smoky Valley and near Toquima Peak, Nevada; these animals, although typical of _D. o. monoensis_ in coloration, resemble _D. o. columbianus_ cranially. Three-way intergradation between _D. o. columbianus_, _D. o. fetosus_ and _D. o. monoensis_ is noted in animals from east-central Nye County, Nevada. These animals resemble _D. o. monoensis_ in size, _D. o. fetosus_ in color and resemble _D. o. columbianus_ in certain cranial features. These animals are referred to _D. o. monoensis_. Animals from Toulon, Nevada, in the inflation of the auditory bullae, the vault of the braincase, the color and the total length show intergradation with _D. o. inaquosus_ but are referable to _D. o. monoensis_.
_Specimens examined._--Total, 264, distributed as follows:
=California=: _Mono County_: Pellisier Ranch, 5 mi. N Benton Station, 17 (7 DRD; 10 MVZ); Benton, 5639 ft., 2 (1 LACM; 1 MVZ); Taylor Ranch, 2 mi. S Benton Station, 5300 ft., 2 (MVZ). _Inyo County_: Deep Springs Valley, 1 (LACM).
=Nevada=: _Washoe County_: 1/2 mi. S Pyramid Lake, 3950 ft., 1 (MVZ); 1-1/2 mi. N Wadsworth, 4100 ft., 2 (MVZ). _Pershing County_: 21 mi. W and 2 mi. N Lovelock, 4000 ft., 2 (MVZ); 3-1/4 mi. NNE Toulon, 3900 ft., 1 (MVZ); 3 mi. NNE Toulon, 3900 ft., 6 (MVZ); 1/2 mi. NE Toulon, 1 (MVZ); Toulon, 3930 ft., 5 (MVZ). _Churchill County_: Truckee Canal, 2 mi. SW Hazen, 4000 ft., 1 (MVZ); 1 mi. NW Soda Lake, 4000 ft., 2 (MVZ); 1 mi. S Soda Lake, 4000 ft., 1 (MVZ); 5 mi. W Fallon, 1 (MVZ); 4 mi. W Fallon, 4000 ft., 3 (MVZ); 1 mi. W Mountain Well, 5350 ft., 3 (MVZ); Eastgate, 4400 ft., 13 (MVZ). _Lyon County_: 6 mi. N Fernley, 1 (MVZ); 1 mi. SE Wadsworth, 4200 ft., 7 (MVZ); 3/4 mi. N Fernley Underpass, Fernley, 1 (MVZ); 1/2 mi. N Fernley Underpass, Fernley, 1 (MVZ); Wilson Canyon, 8 mi. NE Wellington, 4700 ft., 1 (MVZ); West Walker River, Smith's Valley, 4700 ft., 4 (MVZ); 10 mi. S Yerington, Mason Valley, 4500 ft., 6 (MVZ). _Mineral County_: 8 mi. SE Schurz, 4100 ft., 18 (MVZ); Fingerrock Wash, Stewart Valley, 5400 ft., 4 (MVZ); Cat Creek, 4 mi. W Hawthorne, 4500 ft., 1 (MVZ); Huntoon Valley, 5700 ft., 1 (MVZ). _Nye County_: 2 mi. S Millett P. O., 5500 ft., 1 (MVZ); 4 mi. SE Millett P. O., 5500 ft., 11 (MVZ); 5 mi. SE Millett P. O., 5500 ft., 5 (MVZ); 4 mi. S Millett P. O., 5500 ft., 2 (MVZ); Millman Ranch, Moore Creek, 6400 ft., 19 mi. SE Millett P. O., 9 (MVZ); Meadow Creek Ranger Station, Toquima Mts., 2 (MVZ); Monitor Valley, 9 mi. E Toquima Mts., 7000 ft., 19 (MVZ); Fish Spring Valley, 1/2 mi. N Fish Lake, 6500 ft., 2 (MVZ); Railroad Valley, 2-1/2 mi. S Lock's Ranch, 5000 ft., 5 (MVZ); Hot Creek Valley 3-1/2 mi. E Hot Creek, 5650 ft., 1 (MVZ); Hot Creek Valley, 4/5 mi. S Hot Creek, 5900 ft., 1 (MVZ); 5-1/2 mi. NE San Antonio, 5700 ft., 1 (MVZ); San Antonio, 5400 ft., 2 (MVZ); 9 mi. W and 3 mi. S Tybo, 6200 ft., 2 (MVZ); Ralston Valley, 15-1/2 mi. NE Tonopah, 5800 ft., 2 (MVZ); Railroad Valley, 2-1/2 mi. S Lock's Ranch, 5000 ft., 5 (MVZ); Hot Creek Valley Creek, 5800 ft., 1 (MVZ); Ralston Valley, 34 mi. E and 1 mi. N Tonopah, 5650 ft., 2 (MVZ); Old Mill, N end Reveille Valley, 6200 ft., 6 (MVZ); 1-1/2 mi. S Silverbow, Kawich Mountains, 1 (MVZ); 5 7/10 mi. SE Kawich, 2 (MVZ); 5 mi. W White Rock Spring, 6950 ft., Belted Range, 2 (MVZ); 1 mi. N Beatty, 1 (DRD). _Esmeralda County_: 13-1/2 mi. NW Goldfield, 4850 ft., 3 (MVZ); 7 mi. N Arlemont, 5500 ft., 6 (MVZ); Arlemont, 11 (MVZ); Mouth Palmetto Wash, 7 (DRD); 2 mi. NW Palmetto, 7 (DRD); 1 mi. NW Palmetto, 1 (DRD); Palmetto, 7 (DRD); 1 mi. SE Palmetto, 7 (DRD); Pigeon Spring, 6400 ft., 1 (MVZ); Indian Spring, Mt. Magruder, 20 (DRD).
=Dipodomys ordii ordii= Woodhouse
_D(ipodomys) ordii_ Woodhouse, Proc. Acad. Nat. Sci. Philadelphia, 6:224, 1853.
_Dipodomys phillipsi ordi_, Coues and Allen, Monogr. North American Rodentia, p. 541, 1877 (part--the part from El Paso, Texas).
_Dipodops ordii_, Merriam, N. Amer. Fauna, 4:42, October, 1890 (part--the part from El Paso, Texas).
_Cricetodipus ordii_, Trouessart, Catalogus Mammalium, 1:581, 1897.
_Perodipus ordi_, Elliot, Field Columbian Museum, Zool. Ser., 2:238, 1901.
_Perodipus montanus richardsoni_, Bailey, N. Amer. Fauna, 25:144, October, 1905 (part--the part from Carlsbad, New Mexico).
_Perodipus ordii_, Goldman, Proc. Biol. Soc. Washington, 30:113, May 23, 1917.
_Type._--None designated. Species characterized from specimens obtained by Dr. Woodhouse at El Paso, Texas.
_Range._--Southeastern Arizona, southern New Mexico, western Texas and north-central Mexico; marginal occurrences are: in Arizona, 20