Part 3

For assistance with the illustrations I am indebted to the late Major Allan Brooks for Plate 1, to Mrs. Mary Blos for figures 25-31, to Miss Ann Murray for figures 11-13, to Mr. W. C. Matthews for all the photographs, to Mrs. Freda L. Abernathy for figures 2-9, 18-22, 24, and for retouching all the photographs except the following which were retouched by Mrs. Virginia Unruh: figs. _d_ of plates 2, 3, 4, 9, 10, 11, 16, 17; figs. _i_ of plates 5, 6, 7; figs, _h_, _j_, _k_ of plate 7; figs. _f_ and _g_ of plates 12 and 13; and figs. _c_ and _d_ of plate 14. To Mrs. Unruh I am further indebted for figures 1, 16, 17 and 23 and for much terminal assistance with preparing most of the illustrations for the engraver.

The methods of study, after specimens were assembled, included first comparisons of specimens of like age and sex from each of several localities to ascertain the constant features by which full species were distinguishable, one from the other. For example, it was found that in every individual from Trout Lake, Washington, of the species here designated _Mustela erminea_, the postglenoidal length of the skull amounted to more than 47 per cent of the condylobasal length whereas it was less than 47 per cent in all individuals here designated as _Mustela frenata_, from the same locality. Testing of specimens from other localities by means of this and other selected characters permitted the outlining of the geographic ranges of the full "species-groups." By comparing specimens of other nominal species and by examining specimens from localities geographically intermediate between the nominal species, I found intergradation and therefore arranged the nominal species as subspecies of a single species. Intergradation here is understood to be the result of crossbreeding in nature between two kinds of animals in the area where the geographic ranges of the two kinds meet. Presence of intergradation between two kinds of weasels was basis for according them subspecific rank. Absence of intergradation in nature at every place where the geographic ranges of two kinds met or overlapped, and absence of intergradation by way of some other kind, or chain of kinds, was basis for according each of the two kinds full specific rank. By thus applying the test of intergradation, or lack of it, I found that there were four full species of weasels, of the subgenus _Mustela_, in all of the Americas.

Next, the specimens of one species were arranged in trays in a geographic sequence. The specimens from any one locality were segregated by sex and under one sex from one place were arranged from oldest to youngest, that is to say by age. The four series with the largest numbers of individuals of a given age were selected. Seventeen cranial measurements and three external measurements were recorded for each individual of each of these four series. For each measurement, the coefficient of variation, standard deviation and probable error were computed. The four samples subjected to such analysis were a series of adult males, one of adult females, one of subadult males and one of subadult females. Also, studies of each sex were made to ascertain seasonal changes in pelage. After data were obtained on ontogenetic (age) variation, secondary sexual variation, seasonal variation, and degree of individual variation by studying specimens in the manner described above, tests were made for subspecific (geographic) variation by comparing series of specimens of like sex, age and season, from different localities. For each one of several geographically variable features noted, a map was prepared for animals of each sex. When all the data thus obtained were codified, subspecific ranges were, in a sense automatically, obtained. On the resulting map showing geographic ranges of subspecies for a species, a type locality was accurately plotted for each name that had been applied to the species, and names then were applied in accordance with the international rules of zoölogical nomenclature.

VARIATION

Variation with Age

The kind of variation which results from increasing age has been dealt with extensively for the skull (of the Old World _Mustela erminea_) by Hensel (1881) and for the external features and to some extent for the skull by Hamilton (1933) in the North American forms _M. erminea cicognanii_ and _M. frenata noveboracensis_.

The young of both _erminea_ and _frenata_ are hairless and blind at birth. In _M. frenata noveboracensis_, the eyes open on approximately the 37th day. When 2 to 4 months old, the tail is pointed at the tip. This is because the terminal hair of the tail, including the black tip, is short and lies flat on the tail. In subadults and adults the hair on the terminal part of the tail is as long as that on the basal part, and the tail appears to be of uniform diameter all the way out to the end.

In the western subspecies of _M. frenata_, and in its tropical subspecies, animals so young as to have pointed tails commonly have the underparts of the body more intensely colored than do adults. The young may have salmon-colored instead of yellowish fur on the underparts.

Otherwise, in animals that have attained approximately adult proportions--which appears to be at approximately 6 months of age in males--there are no variations which are ascribable to increasing age in the color-pattern or pelage that cause the systematist to confuse species or subspecies.

Of the several parts of the skull in juvenal animals, the braincase and width of the posterior part of the palate are most nearly of the size attained in the adult, the facial part of the skull at birth is the least developed, and the interorbital region is, in relation to its ultimate adult size, intermediate in stage of development. The permanent teeth are acquired when the animal is approximately eleven weeks old.

Four age groups, based on characters of the dentition and skull, have been recognized. They are:

Juvenile.--One or more deciduous (milk) teeth present. Birth to three months of age.

Young.--Sutures widely open between the maxillae and nasals and between the premaxillae and nasals. Three to seven and a half months of age.

Subadult.--Sutures between maxillae and nasals visible but indistinct. Seven and a half to ten months of age.

Adult.--Bones of rostrum coalesced with no traces of sutures visible to the naked eye. More than ten months old.

The skull as a whole increases in size until the animal is two-thirds of the way through the stage designated as young. After this time the width of the rostrum, as measured across the hamular processes of the lacrimals, increases until approximately a third of the way through adulthood. The interorbital breadth decreases from late subadulthood to adulthood and even in adults there appears to be a slight decrease in this part of the skull with increasing age.

The average zoölogist will readily distinguish skulls of juveniles and young from adults but usually fails to distinguish subadults from adults. Nevertheless, subadults must be distinguished from adults if geographic variation is to be measured accurately. The reason for this is that such differences in the form (not size) of the skull as result from increasing age equal and often exceed the differences of a geographic sort which serve for distinguishing subspecies that have adjoining geographic ranges. All sutures in the skull, except those between the tympanic bulla and the braincase, and those on the dorsal face of the rostrum, are obliterated while the animal is a subadult. Most kinds of mammals retain sutures throughout life or until the animals are well into adulthood. Therefore, skulls of weasels offer fewer features for estimating age than do those of most mammals and the skulls of weasels that are subadults or older are more difficult to classify accurately as to age than are the skulls of most other mammals. More reliance on shape of entire skull and less reliance on extent and shape of any individual bone is necessary in estimating the age of a weasel. Wright (1947:344) shows that the weight of the baculum (os penis) is a certain means of differentiating adults from males of lesser age. When approximately eleven months old, _Mustela frenata oribasus_ of western Montana molts from the white winter coat into the brown summer coat. At that time spermatogenesis starts for the first time and the weight of the baculum increases from less than 30 milligrams to more than 52 milligrams.

In the autumn and early winter, most of the specimens are subadults. Ordinarily the few adults obtained in these seasons can easily be segregated from the subadults because ontogenetic development in the twelve additional months of life of each of the older animals has obliterated the sutures on the rostrum, heightened (vertically) and lengthened (anteriorly) the sagittal crest, widened the rostrum, and produced still other changes in form that are revealed by direct comparison of specimens of the two ages.

Secondary Sexual Variation

The secondary sexual variation, which has been detected, is in size of the animal, relative length of the tail and shape of the skull. The female is the smaller. In the small _Mustela rixosa_ and apparently in _Mustela africana_ the secondary sexual difference in size is relatively slight. In _Mustela frenata_ and _Mustela erminea_, males are approximately twice as heavy as females, the degree of difference very definitely depending upon the subspecies. For example, in _M. e. richardsonii_ the recorded weights are 175 and 69 grams as opposed to 81 and 54 grams in _M. e. cicognanii_. In general, within one species the greatest difference in size of males and females is in those subspecies in which the animals are of large size. The secondary sexual variation in size is much more than the individual variation in either sex. The same is not true of secondary sexual difference in length of the tail (relative to the length of the head and body), which in eighteen subspecies of _M. erminea_ is from 1 to 7 per cent longer in males than in females. In two subspecies, _M. e. haidarum_ and _M. e. olympica_, the tail is a fraction of a per cent the longer in females if we may rely upon the few specimens for which collectors' measurements are available.

In both _M. erminea_ and _M. frenata_ the skull of the female is approximately 45 per cent lighter than that of the male, or put in the opposite way, the skull of the male is 83 per cent heavier than the skull of the female. The difference in this respect varies greatly depending on the subspecies. For example, the skull of the male is 127 per cent heavier than that of the female in _M. e. richardsonii_ but only 33 per cent heavier in _M. e. anguinae_. In _Mustela frenata_, the subspecies _noveboracensis_ shows most sexual dimorphism in weight of skull (3.6 and 1.7 grams) and _olivacea_ the least (5.3 and 3.8 grams). In general, the difference in this respect is less in subspecies the individuals of which are of small size.

Therefore, as might be expected, the secondary sexual variation in weight of the skull is less in _M. rixosa_, individuals of which are of small size, than in _M. erminea_ or than in _M. frenata_, in general of larger size. Nevertheless, in _M. africana_, in which the individuals are of large size, there appears to be less sexual dimorphism in weight of the skull than in _M. frenata_ or than in _M. erminea_, although it should be remarked that there are too few data for _M. africana_ to allow of forming a trustworthy conclusion concerning the amount of secondary sexual variation in that species.

The secondary sexual variation in shape of the skull consists of a slenderness in the female. In relation to the basilar length the spread of the zygomatic arches is more in males and, except in the one subspecies _M. f. altifrontalis_, the rostrum is broader. Also the interorbital region is relatively broader in males of most subspecies. In most subspecies of both _M. frenata_ and _M. erminea_ the tympanic bullae are relatively (to the basilar length) longer in females. The maximum sexual dimorphism occurs in _M. erminea arctica_ and the minimum dimorphism in _M. e. haidarum_, _M. e. anguinae_ and _M. e. muricus_. Taking into account all of the subspecies of each of the North American species, the shape of the skull differs most in _M. erminea_ and least in _M. frenata_. In the latter species the greatest difference in shape of the skull, as was true also of its weight, is in the subspecies _M. f. noveboracensis_. In these two subspecies, _M. f. noveboracensis_ and _M. e. arctica_, in addition to the secondary sexual variation already mentioned in the skull, females have the braincase smoother and more rounded, the postorbital-, mastoid-, and lacrimal-processes relatively smaller, and the ventral face of the tympanic bulla at its anterior margin more nearly flush with the floor of the braincase.

In the weasels, subgenus _Mustela_, the disparity in size of the two sexes is almost or quite as much as in any other fissiped carnivore. It is because of this large degree of difference that the skulls of the two sexes are described separately in the following systematic accounts. The need for such treatment was recognized by Reinhold Hensel (1881:127) more than sixty years ago when he wrote in the introduction to his "Craniologische Studien," of _Mustela_, as follows: ". . . die Geschlechtsdifferenzen am Schädel vieler Säugethiere . . . so gross sind, dass man diese wie Schädel verschiedener species behandeln muss, während in anderen Ordnungen (Rosores, Edentaten) die Schädel solche Unterschiede nichtzeigen." In the past, failure to appreciate the large amount of secondary sexual variation has resulted in erroneous deductions as regards characters of certain geographic races and has been the cause of some nomenclatural confusion, as for example, in _Mustela frenata macrura_, where the female was named as a separate species (_Mustela jelskii_).

Individual Variation

Individual variation is here considered to be the variation in one species which can occur between offspring of a single pair of parents, after variation ascribable to differences in age, sex, and season is excluded. Individual variation, therefore, is a term here used in a composite sense; it includes variations which probably represent different genetic strains within certain populations and variations induced within one generation by environmental factors.

In skulls of weasels, the individual variation in size is more than it is in relative proportions. Hensel (_op. cit._) has stressed that weasels, like other carnivores, produced "dwarfed" individuals more than do herbivorous mammals. I cannot vouch for the accuracy of this view, but can say that individual variation is not greater than in some other fissiped carnivores. Impressions to the contrary probably result largely from failure to recognize age-variation. When skulls of a large series from any one locality are arranged first by sex, and under each sex according to probable age on the basis of extension anteriorly of the sagittal crest and of degree of postorbital constriction, individual variation is seen to be less than a cursory examination, even of only one sex, would suggest.

Study of a large series of one age of one sex of one species from one locality shows that some parts, of the skull for example, vary more than other parts. In illustration, among 22 male topotypes of _Mustela frenata washingtoni_ the least interorbital breadth varied 25 per cent (9.0 mm. to 12 mm.) whereas the length of the tooth-rows varied only 13.3 per cent (15.6 mm. to 18.0 mm.). In color the individual variation definitely is more in areas of intergradation between subspecies than in other areas. Details of one such instance of intergradation are given in the account of _Mustela frenata spadix_.

Statements to the effect that there is much individual variation in the color of weasels, were made mostly fifty years or so ago by writers who had but few specimens from widely separated localities. Where marked climatic differences exist between localities only a few miles apart, marked differences occur in coloration of the weasels from the different localities. Much of what formerly was mistaken for individual variation now proves to be geographic variation. Individual variation actually is of slight amount in comparison with that in mammals generally. Differences in size and relative proportions of parts usually are correlated with geographic differences in color. The color does fade slightly in the period between molts. Also as a result of the seasonal color change, in autumn along the upper margin of the Austral Life-zone, some individuals become white whereas others become white on only the underparts, the upper parts changing only to lighter brown. Probably it would be correct to say that this variation was a combination of seasonal and individual variation rather than either one alone.

As might be supposed, individual variation is not the same in all species or subspecies. For example, p2 is always absent in _Mustela africana_ and always present in certain subspecies of _M. frenata_. In some other subspecies of _M. frenata_, p2 is absent approximately as often as present. In the writer's experience, when only a few specimens are available for comparison, individual variation is more difficult to distinguish from specific and subspecific (geographic) variation than is age-variation or secondary sexual variation.

Among the larger series of specimens examined, only one instance of what might be called a mutation in the old sense of a large, sudden change, was detected. That was the loss of the second lower molar in many (less than a third) of the specimens from Newfoundland. The six instances of abnormal coloration described on pages 41 to 43, might be regarded as mutations of large magnitude but no evidence was found of repetition of an abnormality in any one population. Otherwise, in every instance where plotted, the manifestations of a variation arranged themselves about the mean in such a way as to form a smooth, unimodal curve.

Seasonal Variation

When subspecific and specific variations are the objectives of study, seasonal variation must be understood, in order to be excluded from consideration, in the same way that variations ascribable to age, sex and individualism must be understood in order to be excluded from consideration. In weasels, change in color of the pelage is the seasonal variation most important for the systematist to understand. Other seasonal variations in the pelage are hairiness versus nakedness of the pads of the feet, length of the pelage on the body, and possibly the density of the pelage on the body. In the northern half of North America, roughly speaking, seasonal change in color is so pronounced (white in winter and brown in summer) as to be easily recognized. South of this area, in the Austral and Sonoran life-zones, the color of the winter pelage differs only slightly from that of the summer pelage. In these more southern latitudes the winter pelage in almost all subspecies is of lighter color than the summer pelage and has a smoky suffusion. With material of the two seasons in hand for comparison, close attention to the variation will permit the systematist to recognize the difference in shade of brown as seasonal variation and not geographic or specific variation. Farther south still, in the Tropical Life-zone, seasonal difference in color was not detected in the material studied. Seasonal change in color is discussed in the section immediately following.

Variation in Coloration and Molt

In all American weasels (subgenus _Mustela_) the color, at least in summer, is brown with more or less white or whitish on the underparts. In one species, _Mustela africana_, there is a longitudinal stripe of brown on the middle of the light-colored underparts; this stripe is absent in each of the other three American species. Two species, _M. erminea_ and _M. frenata_, always have a black tip on the tail. Of the other two species, _M. africana_ lacks the black tip and _M. rixosa_ may or may not have a few black hairs in the tip of its tail. White or light yellowish facial markings occur in subspecies of _M. frenata_ from the southwestern United Stated to Central America. Subspecies having the most extensive light-colored facial markings have the remainder of the upper part of the head black. In weasels without light facial markings the upper parts of the head all are brown. In the two species, _M. erminea_ and _M. frenata_, the extent to which the light color of the underparts extends down the insides of the legs and out on the underside of the tail, or the absence of light color on these parts, is a matter of geographic variation. The same can be said for _M. rixosa_ except that first its tail is unicolored and second individual variation as well as geographic variation accounts for the color pattern on the underparts and legs in animals from the southeastern part of the range of the species.

The most remarkable feature of the coloration of weasels is the winter whitening. This occurs in the northern part of North America in each of the three species of weasels found on that continent. The black tip of the tail in _M. erminea_ and _M. frenata_ remains black in winter. If an individual of _M. rixosa_ has black hairs on the tip of its tail in summer, there are thought to be black hairs there also in winter. Otherwise the winter pelage is all white in northern areas in each of the three species. In this white winter coat the animal is known as ermine.