Part 5

Many writers have commented on the yellowish color, sometimes with a greenish tinge, found on the fur of weasels in the white winter coat. The stain is more often found on the tail and hinder-parts of the body than elsewhere. Possibly, partly on this account, some have ascribed this color to the smearing of the fur with urine. Still others have thought it resulted from the smearing of the fur with secretions from the anal scent glands. Schumacher (1928) takes this point of view, and while it may be that he has not proved his point, still his conclusions fit the known facts and seem sound to me. Schumacher points out that the same soiling of the fur is present in summer as well as in winter, but that on the summer pelage the stain can be detected only on the light-colored underparts. It is from this point of view that he criticizes the systematic worth of white versus yellowish-white underparts in the summer pelage of geographic races of _Mustela erminea_ and _Mustela nivalis_. Although in the long-tailed weasels (_Mustela frenata_) the underparts of all the races are pigmented with some form of red, orange or yellow, it seems probable to me that the additional color resulting from the soiling effect of this glandular secretion explains the greater variation, found at a single locality, in the color of underparts than of upper parts in the summer pelage.

I have neither seen nor heard of a black weasel in any part of the New World or of the Old World. I have found only one albino among American specimens. It is an adult female, no. 121424, American Museum of Natural History, of _Mustela erminea richardsonii_, taken on August 30, 1935, at Hot Springs, Northwest Territory. This place, I am told by G. G. Goodwin who obtained the animal, is on the "Nahanni River where the rugged mountain ridges rise abruptly from the low mud flat lands, latitude 61, longitude 125." The shortness and coarseness of the hair corresponds to that of the summer pelage and not winter pelage. The pelage is everywhere white, even the tip of the tail. True, all except the nape and top and sides of the head has a faint yellowish-green tinge which has been supposed to result from staining by secretion of the anal scent glands but there is no pigment in the hair as in erythristic specimens. From the Old World, Farurick (1873:17) has recorded what he regards as an albino of _Mustela vulgaris_ since it had no black hairs on the tip of its tail. Flintoff (1935:228, 229) records what may have been an albino _Mustela vulgaris_ from Yorkshire and an albino _M. erminea_ from an unstated locality. Jäckel (1873:459) mentions specimens of _Mustela erminea_ and _Mustela vulgaris_, which were partly "albinistic" or "erythristic." Among the American specimens of _M. erminea_ I have not recorded any which appeared to be either partly or wholly erythristic or only partly albinistic. Among the 1550 skins of _M. frenata_ which were in summer pelage or brown winter pelage, five, described below, show marked abnormalities in color.

Two of these five are partly albinistic. One is an adult male, no. 223880, U. S. Nat. Mus., from Billy's Island, Okefinokee Swamp, Georgia, which has the nose as well as the area between the eyes white. Also there is a tuft of white hairs at the anterodorsal margin of each ear, scattering white hairs suggesting a postorbital bar on each side of the head, and a patch of white hairs on the mid-dorsal line behind the ears. Markings of this kind are not abnormal in _M. f. peninsulae_, the subspecies adjoining on the south, except for the white nose which clearly is an instance of partial albinism. The second specimen is a subadult male, of _M. f. noveboracensis_, no. 177679, U. S. Nat. Mus., in process of acquiring the brown winter coat, taken on November 27, 1911, at Gaylordsville, Connecticut. It has white markings on the nose, on the right side of the neck, on the right hind foot and right forefoot, and on the tip of the tail. The white area of the nose on the left side extends back to the eye, but on the right side barely encircles the nose-pad. On the right side of the neck, all that area between the foreleg and ear is white from the mid-dorsal line (including 7 or 8 millimeters to the left of the mid-dorsal line) down to the throat, which is white as it is also in normal individuals. The toes of the right hind foot are more extensively white than in normal specimens of _noveboracensis_, and all of the right forefoot as well as the wrist is white. The tail is of striking appearance because of its tricolor pattern. The proximal part is of the normal brown color. The black terminal part commences proximally at the usual place, but the distal 11 millimeters of the fleshy part of the tail bear only pure white hairs producing a terminal white pencil 35 millimeters long.

The three other specimens abnormally colored are erythristic individuals. An adult male of _M. f. latirostra_, no. 7574, coll. D. R. Dickey, taken on April 14, 1918, at Covina, Los Angeles County, California, has the color of the upper parts greatly restricted, and, in addition, has spots and blotches of the color of the underparts distributed over the back and rump. A spot of this same color occurs above each ear. Incidentally, this and other subspecies of _Mustela frenata_ from the Pacific Coast of North America obviously have the factor for erythrism operating over a larger part of the body than it does in _M. erminea_ or than in _M. f. noveboracensis_, where the underparts sometimes are white. In _M. f. latirostra_ and in other subspecies from the Pacific Coast the light color of the underparts always is tinged with this reddish color.

Another erythristic specimen is a young male of _M. f. nevadensis_, no. 23493, U. S. Nat. Mus., taken on August 6, 1890, at Birch Creek, Idaho. It has all of each foreleg, the axillary regions, and a saddle-shaped area over the shoulders of the same buff-yellow color as the underparts.

The third erythristic specimen is a subadult female, of _M. f. oregonensis_, no. 47149, Mus. Vert. Zoöl., taken on December 20, 1930, at Carlotta, Humboldt County, California. This specimen appears to be white and initially was thought to be merely an individual in the white winter coat. Closer examination, however, shows that it has a light wash of ochraceous or faint reddish color. Also, other specimens taken in winter at Carlotta show that weasels there do not acquire a white winter coat. The only normally brown area is approximately three millimeters in diameter at the anterodorsal margin of the pinna of the right ear. The tip of the tail is black as in a normal specimen. The specimen in question is actually pure white only on top of the head from a short distance behind the ears on over the forehead nearly to the eyes, and on the inside of the ears. In a normally colored animal this area is the dark area of the head. In this freak, the other parts of the head, which, in individuals of normal coloration are the white or light orange facial markings, have the reddish cast of the remainder of the body, although the color is less intense than on the back. The collector noted that the specimen had eyes of normal color. A possible explanation for the coloration of this specimen is that this species has three factors for color, one for the black tail tip, one for the reddish color, and a third, missing in the specimen in question, for the blackish brown.

For some more exact knowledge concerning this erythristic type of coloration, we are indebted to Pitt (1921:99), who describes a population of polecats, _Mustela putorius_, in Cardiganshire, England, in which this erythristic variation is maintained in a state of nature. In ferrets, _Mustela furo_, Pitt (_op. cit._:114) notes that ". . . erythrism is certainly dependent on a Mendelian factor, being dominant to albinism and recessive to the black-brown coloration. Both in the ferret and polecat, erythrism seems to be correlated with increased size, and certainly in the ferret is usually accompanied by a quick temper and general increase in vitality."

Variations of Taxonomic Worth

Variations of taxonomic worth usually are referred to as characters. For example, shortness of the tympanic bulla is a character, and the opposite condition, long tympanic bulla, is another character. Specific variations, that is to say specific characters, are provided by the color-pattern, length of tail, number of premolar teeth, shape of the tympanic bullae, and length of the braincase in relation to the length of the tooth-bearing parts of the skull. Subspecific characters are provided by color-pattern, color itself, size as measured by weight of the animal, and its linear measurements, size of the skull, and size and shape of parts of the skull. The characters distinguishing subspecies from one another are not of a different nature from those distinguishing species from one another.

Given any one of the above structural features, say, dorsal outline of the skull, several characters may be provided by it. For example, weasels of the species _Mustela frenata_ have the dorsal outline of the skull convex in southern Louisiana, straight in Missouri and concave in North Dakota, thus providing three characters. This is geographic variation. These variations, characters in zoölogical parlance, when plotted on maps, reveal the geographic occurrence of, say, the convex shape of the skull. In combination with other characters, for example, dark color and short tail, basis is provided for recognizing a subspecies, in this instance _Mustela frenata arthuri_ of Louisiana. Because the change from convex to flat skull takes place geographically at about the same place (in eastern Texas) as does the change from short tail to long tail, and the change from dark color to light color, it is easy to draw a line there marking the western geographic limit of occurrence of the _M. f. arthuri_. This same line marks also the eastern margin of the geographic range of the subspecies _Mustela frenata frenata_, the subspecies next adjacent to the westward. On this line and for several miles to either side of it weasels show varying combinations of these three characters or an intermediate condition as regards one or more of the characters, or both. For example, from a locality in eastern Texas a weasel may have (1) a facial pattern exactly intermediate between that of the unicolored face of _arthuri_ and that of the bicolored face of _frenata_, (2) the long tail of _frenata_ and (3) the convex skull of _arthuri_. In the sum of its characters this specimen is exactly intermediate between typical _arthuri_ and typical _frenata_. Another specimen from the same place may differ from the first specimen only in having the tail slightly shorter. The total "score" for the two specimens is, therefore, by a very slight margin in favor of _arthuri_. Let us suppose that we obtain a third specimen from the same place and that it has the face marked like that of _arthuri_ but the tail fully as long, and the skull as lacking in dorsal convexity, as in _frenata_. Now the score is definitely for _frenata_. For convenience of handling, the population is referred to _frenata_, providing that the average of specimens from a nearby locality to the westward is not in favor of _arthuri_. In event the average of specimens from a locality next adjacent to the westward is in favor of _M. f. arthuri_, the total evidence from the two localities may be weighed together and appropriate decision as to subspecific status of weasels from the area is made according to what the average is for the area as a whole.

The three individual animals of an intermediate sort are ordinarily termed _intergrades_. This implies that their characters are the result of mixed parentage--perhaps a female of _M. f. arthuri_ and a male of _M. f. frenata_ but probably each parent itself was an intergrade and the offspring, of which we examined three, owe their characters to reproductive processes operating in obedience to Mendelian laws of inheritance.

The two kinds of animals, _Mustela frenata arthuri_ and _Mustela frenata frenata_, are identified as subspecies because of the intergradation between them. If at this and all other places where the geographic ranges of _arthuri_ and _frenata_ met there was no crossbreeding (no intergrades), the two kinds would be treated as distinct species. Intergradation, and the lack of it, are accepted as the criteria of subspecies and species, respectively.

These criteria suffice for animals, in this instance weasels, which have a continuous geographic distribution. Some kinds of weasels are confined to islands, as for example the islands off the coast of Alaska and British Columbia. Because weasels are land animals, crossbreeding in nature between the weasels of two islands is, of course, impossible. A modified test (used in the study here reported upon) in deciding on specific versus subspecific status in these instances can be made as follows: On the adjacent mainland, ascertain the degree of difference between two subspecies whose geographic ranges meet (for example, _M. e. richardsonii_ and _M. e. alascensis_). Next ascertain the degree of difference between the insular kind of animal and the kind on the mainland. If the degree of difference is greater when the insular kind is compared than when only the kinds of the mainland are compared, the insular kind is to be regarded as a species. If the degree of difference is no greater between the insular kind and the mainland kind than it is between the two adjacent mainland kinds, the insular kind is to be regarded as a subspecies. In short, for insular kinds, the criterion is degree of difference, with the limitation of geographic adjacency, rather than intergradation.

The geographic variation (subspecific characters) found could be spoken of as two kinds: First, there is the variation which is expressed in a general trend for a long distance, producing, in general, a cline of even slope; and second, that of inconstant trend in any one direction. In his "The Rabbits of North America" Nelson (1909:34-35) has commented on the latter type of variation as follows: "While studying series of specimens from all parts of the vast range occupied by the geographic races of such species as _Sylvilagus floridanus_ and _S. auduboni_, I have been impressed with evidences of fluctuation of both external and skull characters. These fluctuations are somewhat wavelike in character and rise to central points of extreme development and then sink away to intermediate borders beyond which new waves rise. Where the waves of differentiation are pronounced they mark recognizable geographic races. Within the area covered by the larger or geographically broader waves of differentiation (recognized as of subspecific value), smaller waves of differentiation are included, which may represent local variations in intensity of characters of the subspecies, or these characters may diminish and the variation tend in other directions, sometimes even closely reproducing the characters of another subspecies occupying a distinct area." In _Mustela frenata_, much of the geographic variation at first inspection appears to be of this nature. Closer scrutiny, however, reveals that the repetition, at geographic intervals, of several features of color and structure are closely correlated with environmental features which are repeated only at these same places.

In _Mustela erminea_, much of the variation is of the first kind, namely, that which can be expressed as long clines of relatively even slope. As several authors have said, zoölogical classification based on this kind of variation is like dividing the spectrum and depends largely upon the standards set, for, theoretically, the possibilities of subdivision are unlimited. Actually, however, none of the clines has an even slope and the possibilities for subdivision therefore are limited. Also, when several features are used, instead of only one feature, the classification is more satisfactory even if the basis is more complex.

Some features of structure which provide subspecific characters are mentioned below.

Total length, of males, ranges from 598 to 360 mm. in _M. frenata_ and from 336 to 228 mm. in _M. erminea_. There is no cline of sustained slope in _M. frenata_ but in _M. erminea_ there is a progressive decrease in total length from north to south.

Length of tail varies from as little as a half to as much as seven-tenths of the length of the head and body in _M. frenata_, the subspecies _neomexicana_ having the long tail and the two subspecies _arthuri_ and _primulina_ having short tails. The geographic ranges of _primulina_ and _neomexicana_ are contiguous. In _M. erminea_ there is likewise no variation of a clinal nature in length of tail and furthermore the variation is much less than in _M. frenata_.

In length of hind foot, which in males varies from 49 mm. in northern populations of _M. erminea_ to 28 mm. in southern populations, the same cline is seen as in the total length of animals of this species. In _M. frenata_, however, there are several decreases and increases along any straight line which can be drawn through the geographic range of the species. The range of variation in males is 41 mm. (_M. f. arizonensis_) to 59 mm. (_M. f. macrophonius_).

Weight of the entire animal is an excellent measure of size but weights are unavailable for many subspecies. In _M. frenata_, the two subspecies _texensis_ and _macrophonius_ probably are the heaviest and _effera_, _arizonensis_ and _helleri_ probably are the lightest. Geographically the variation in weight behaves in approximately the same way as does the measurement of total length. In _M. erminea_ the variation in weight of males is from 206 grams in northern animals to 58 grams in southernmost populations, there being a relatively constant gradient geographically.

Degree of hairiness of the foot-soles in _M. frenata_ clearly is linked with the temperature; in regions of high average temperature the hairiness is least and in regions of low average temperature it is most. The decrease in hairiness is accomplished in two ways, namely, smaller breadth and decreased length of individual hairs and decrease in number of hairs on a given area of dermal surface. This correlation holds throughout the entire north to south range of the species. Corresponding differences are found on the same latitude where topographic diversity in an east to west direction produces northern conditions at high altitudes and southern conditions at low altitudes. The conclusion seems unavoidable that climate, directly or indirectly, determines the degree of hairiness. Less careful observations were made on the hairiness of the soles of the feet in other species but it is clear that the northern species _M. erminea_ has the most hair on the foot-soles and that _M. africana_, the tropical weasel, has the least. In this regard, _M. frenata_ is intermediate as it is also in geographic position.

The maximum length of facial and carpal vibrissae is attained in _M. erminea_ in the far north. In weasels from north of the Arctic Circle the longest facial vibrissae extend posteriorly beyond the posterior border of the ear. In the tropical weasel, _M. africana_, the facial vibrissae do not extend posteriorly beyond the ear and the carpal vibrissae are not so long as the distance between their bases and the apical pad of the first digit. The correlation of long vibrissae with low temperature, is mentioned here merely because length and density of pelage were under consideration.

The most obvious and most exact correlation between change in climate and change in the animal is furnished by color. This is well shown in the one species, _Mustela frenata_, to which the following remarks apply unless indication is given to the contrary. The color of the upper parts varies from bay (blackish brown) in _M. f. panamensis_ to buckthorn brown (light brown) in _M. f. neomexicana_. The color of the head varies from solid brown (white chin excepted) to contrasting black and white markings.

Dark color of the upper parts is associated with a large area of this color; the enlargement of this area is at the expense of the area of light color on the underparts. In the weasels of darkest color the upper parts occupy four-fifths of the circumference of the body (as measured in the anterior lumbar region) but in the lightest-colored weasels the upper parts comprise only two-thirds of the total circumference. In these light-colored animals the color of the underparts extends onto the underside of the tail and down the insides of the legs and over the feet whereas in the animals with the darkest upper parts the entire tail, feet, and legs below the knees ordinarily are of the same dark color as the upper parts. The length of the black tip on the tail varies inversely with the length of the tail, probably because the lightest-colored weasel has the longest tail. In some subspecies the black brush is almost half as long as the tail-vertebrae but in others is less than a fourth as long as the tail-vertebrae.

The extent of the color of the head, as well as the intensity of the color there, varies markedly and is correlated with climatic conditions. The extent and intensity of this dark color is greater in weasels inhabiting regions of heavy rainfall than in those inhabiting regions of sparse rainfall. Considering the geographic range of each subspecies of _Mustela frenata_, that of _M. f. panamensis_ has the maximum of rainfall. Reference to the colored plate (1) will show that in _M. f. panamensis_ (2) the black of the head is extended over all of the upper parts. _M. f. macrura_ (1) of Perú, to the southward, is from an area of lesser rainfall and is correspondingly lighter colored. Returning to _panamensis_ (2) as a starting point and proceeding northward to the range of _nicaraguae_ (3), which also has lesser rainfall, thence another step northward to Guatamala, which has still less rainfall, the weasel there, _M. f. goldmani_ (4) has the black extending posteriorly only to the shoulders. _M. f. leucoparia_ (5) from Michoacán, and _M. f. frenata_ (6) from Tamaulipas are from progressively more northern and also progressively drier regions. In _M. f. frenata_ (6) the dark color extends posteriorly only to the ears and is blackish rather than black. In _M. f. neomexicana_ (7) of the extremely arid parts of Durango, Arizona, and New Mexico the dark marking of the head is confined to a brown spot on the nose. Its geographic range is the most arid of those of all of the subspecies. The contrast between _neomexicana_ (7) and _panamensis_ (2) illustrates the great range of geographic variation in color which occurs in the one species. Continuing from the geographic range of _neomexicana_ (specimen from Safford, Arizona) northwesterly 480 miles to Riverside, California (see 8, _latirostra_), 430 miles north to Point Reyes, California (see 9, _munda_), and finally 570 miles north to Tillamook, Oregon (see 10, _altifrontalis_), each place with more rainfall than the one farther south, another correlation of increasingly dark coloration with increasing amount of rainfall is illustrated.