The tail, excepting the black tip, lags in the molt in many instances, with the result that, especially in spring, it may retain a few white hairs as late as does the belly. In autumn it is less tardy and so far as I have observed, becomes white at about the same time that the general area of the back changes color. On the tail, the black tip itself, as clearly shown in more than a score of specimens, is molted at approximately the same time in autumn as is the pelage of the body. However, the long black hairs, which appear in, say, November, appear to increase in length until January. In spring, the long black hairs of the tip of the tail seem not to be shed at the same time as the rest of the winter pelage, but remain approximately six weeks longer and then are replaced by long black hairs of the summer coat. At any rate, this is the picture presented by a half dozen specimens ofM. f. nevadensisandM. f. longicaudawhich do show a spring molt to be in progress on the black tip of the tail. Schwalbe similarly (1893:536-537) has suggested that the black tip of the tail inMustela ermineain spring is not molted until about two months after the pelage on the rest of the body is changed. Schwalbe (loc. cit.) thinks also that inM. ermineastudied by him, the black tip of the tail in autumn is replaced approximately one month in advance of the pelage on the rest of the body. As indicated above, my specimens ofMustela frenata, subspecieslongicaudaandnevadensis, do not show this discrepancy in autumn. I have considered the possibility that the black tip of the tail, in some species ofMustela, is molted only once while the remainder of the coat was undergoing two molts. My inconclusive data lend but little support to this possibility.
The difference in pattern of color between specimens taken in autumn and spring is known to some fur-trappers of my acquaintance who have suggested that molt occurs in spring, whereas the individual hairs change color in autumn. Reference to plate39will show how gross comparisons might lead one to this erroneous explanation of the color change.
As to time of molt: In eight subspecies ofMustela frenata, namely,noveboracensis,occisor,primulina,spadix,longicauda,arizonensis,nevadensisandeffera, material is available to indicate that the autumnal molt begins in October and is completed in November, and that the spring molt occurs in March or April. A condensed list of specimens providing basis for this statement is as follows:
M. f. noveboracensis: 26 specimens in transitional pelage taken in autumn and 14 taken in spring;M. f. occisor: One topotype has acquired one-fifth of the winter pelage on October 22, 1896;M. f. primulina: 2 in November, one in March, and 2 in April are in process of change;M. f. spadix: 6 autumnal specimens and one in April show pelage change;M. f. longicauda: 7 autumnal specimens and one in April show pelage change;M. f. arizonensis: 12 specimens in autumn and 3 in spring are in process of molt;M. f. effera: One November-taken male has acquired four-fifths of the winter coat and another taken on April 21 at Fort Rock, Oregon, is half finished with the spring molt.
M. f. noveboracensis: 26 specimens in transitional pelage taken in autumn and 14 taken in spring;M. f. occisor: One topotype has acquired one-fifth of the winter pelage on October 22, 1896;M. f. primulina: 2 in November, one in March, and 2 in April are in process of change;M. f. spadix: 6 autumnal specimens and one in April show pelage change;M. f. longicauda: 7 autumnal specimens and one in April show pelage change;M. f. arizonensis: 12 specimens in autumn and 3 in spring are in process of molt;M. f. effera: One November-taken male has acquired four-fifths of the winter coat and another taken on April 21 at Fort Rock, Oregon, is half finished with the spring molt.
It may be added that no marked difference in time of either autumnal or spring molt is apparent as between the more northern and more southern localities from which the mentioned specimens come. With more complete material I would expect to find a difference in this regard.
The material of the other, more southern, subspecies ofMustela frenatahas not been adequate to show the time of molting or the number of molts which occur in one year.
Animals in the northern part of the range ofMustela frenataacquire a white winter coat, whereas those in the southern part acquire a brown winter coat, and in an intervening area the winter coat may be either brown or white. By plotting on a map the localities of capture of all specimens examined in the winter coat, it was possible to outline this intervening area as shown in figure10on page37. However, Dearborn (1932:36) shows that in Michigan some animals have a brown coat in winter at places farther north than figure10shows to be the case. Hamilton's (1933-306) map for New York shows the same to be true in that state. Accordingly, the boundaries of the area shown in figure10, in which both brown and white long-tailed weasels occur in winter, are known to be only approximate; with full information available the belt would be represented as wider.
Fig. 10.Map showing the region (in black) where both the brown and white winter pelage is found in the long-tailed weasel,Mustela frenata.
Fig. 10.Map showing the region (in black) where both the brown and white winter pelage is found in the long-tailed weasel,Mustela frenata.
Fig. 10.Map showing the region (in black) where both the brown and white winter pelage is found in the long-tailed weasel,Mustela frenata.
Hamilton (1933:302) has pointed out that "Where half of the weasels remain brown, these brown winter specimens are always males." The results of my own examination of specimens not studied by Hamilton, in a general way provide confirmatory data. More exactly, my examination reveals that at the most northern localities where brown specimens occur, only males are in this coat. In explanation, it may be said that in plotting on a map localities of capture of specimens in the winter coat, thirteen places were found where both sexes were represented and where both brown and white winter coats were found. With the two sexes, it is theoretically possible to have nine different combinations of coat color. With males all brown, there might occur females (1) all brown, (2) all white, or (3) some brown and some white. In addition to these three combinations, we might have three more by finding the mentioned types of female coat color repeated where all males are white, and three more, or nine in all, by substituting a population of males some of which were brown and some of which were white. Seven of these possible combinations actually were found. The two combinations not found were all white males with all brown females, and all white males with females both brown and white. In the three instances where the males all were brown and the females all were white, the localities of capture were in the northern part of the variable area. This indicates that where the brown winter coat occurs at northern localities, the brown individuals are all males. Farther south, of course, the females, too, acquire the brown winter coat.
Stated in another way, there is a broad belt across North America from the Atlantic to the Pacific in which males ofMustela frenataat any one locality may be either brown or white in winter. Inside this broad belt there is a narrower one, approximately half as wide, in which females at any one locality may be either brown or white.
In support of the idea that color of the winter coat is an hereditary matter and that it is not dependent on temperature, the following evidence derived from my transplanting specimens ofMustela frenatasupports the idea that color of the winter pelage is dependent on heredity and not on temperature or snowfall.
A male captured on June 24, 1937, in the brown summer coat in Salt Lake City, Utah, was received by me at Berkeley, California, five days later and kept in captivity almost six months. On November 17, 1937, half the pelage was white and on December 27, 1937, when next examined, the animal was in the full, white, winter coat as it was on January 25, 1938, when it died. Native weasels all turn white in winter in Salt Lake City, but in Berkeley native weasels always are brown in winter.
A juvenile or young animal, a male, captured in May, 1936, at Lafayette, Contra Costa County, California, was kept there until August 13, 1936, when transferred to Calneva at the north end of Lake Tahoe, California. The weasel was kept at Calneva until its death on December 23, 1937. In both the winter of 1936-'37 and in that of 1937-'38, the winter coat was brown as in animals from its place of origin (Contra Costa County) and unlike weasels of the Tahoe region nearly all of which turn white in winter.
Two females, each approximately two months old, captured on May 1, 1936, at James Landing, 4 miles northwest of San Pablo, Contra Costa County, California, were kept in Berkeley, California, until August 13, 1936, when they were transferred to the mouth of Blackwood Creek, on the west side of Lake Tahoe, California. On October 25, 1936, both weasels escaped. On December 25, 1936, the headless body of one of these was found approximately 300 yards south of the mouth of Blackwood Creek. The animal had been dead at most a few days when found and was in the brown winter coat. At the place of its origin all weasels are brown in winter but at the mouth of Blackwood Creek only 2 of 60 weasels caught there in the winter coat were brown; the other 58 were white. The headless weasel was identified, as one of the two formerly in captivity, by means of certain short toes, the ends of which had been clipped off when the animal was a captive. No trace of the second female was found.
A female of unknown age, in white winter pelage, captured 4 miles southeast of Tahoe City, California, and kept there until April 3, 1937, on which date it was brought to Berkeley, California, molted to brown in the spring. The first signs of the brown coat were noted on April 14. On May 24 or 25 she gave birth to 4 young which lived less than ten days. In the following winter this animal acquired a white coat. As previously noted, weasels native to the Berkeley area, where this female was kept, have brown coats in winter.
The weasels were in every instance kept in cages out-of-doors. The sides of the cages were open to the elements. A nest box in each cage provided shelter. All were of the speciesMustela frenata.
The significant results, it seemed to me, were that the winter coat was the kind found in the area where the weasel originated instead of the kind found in weasels native to the areas in which the specimens were held in captivity.
That the time of molt is determined by the amount of light has clearly been shown by Bissonnette (1944:223) for American weasels of the two speciesMustela ermineaandM. frenata. In his words (op. cit.:246) "Reducing the daily periods of light induced molting and regrowth of new fur. . . . In the Bonaparte weasels [Mustela erminea], white replaced brown. . . . Increasing daily light-periods caused molting and change to dark brown. . . . Incomplete molts in both directions (toward white or toward brown) were produced as a result of early reversal of increase or decrease of daily light-time. . . . That this stimulus is received through the eyes and acts through the anterior pituitary gland is indicated by Bissonnette's [1935:159] studies on ferrets, a nearly related animal. That the thyroids and sex-glands are not essential is at least suggested . . . by Lyman's (1942) study on the varying hare [Lepus americanus]." It can be added that Lyman (1943:451) demonstrated inLepus americanusthat the effect of light is received through the eyes. He demonstrated this by masking the animals. To Wright (1942B:109) who studied the two American weasels,M. ermineaandM. frenata, it seemed likely that the pituitary produced or released gonadotropic hormone at about the time of the spring molt and that this molt and the spring changes in the reproductive tracts of the weasels might be caused by a stimulus from a common source. Later, Wright (1950:130) injected a gonadotropic hormone into long-tailed weasels which had recently acquired their white winter pelage and thereby caused them to lose the white pelage and acquire the brown pelage. It is Lyman (1943:450) who says, in relation toLepus americanus, "When in the physiologically white condition, the melanoblasts of the regenerating guard- and pile-hair follicles contain no melanin-forming enzyme (dopa-oxidase), which may be the reason for the lack of pigment." Schwalbe (1893) by sectioning the skin and microscopically examining the hair-follicles ofM. erminealearned that the basal cells producing hairs lacked pigment granules in autumn when the European ermine (M. erminea) was acquiring its white winter coat and that the cells contained granules of pigment in spring when, as we know, the granules are incorporated in the growing hair and give it its color.
The above material, then, is basis for the account on pages 31 and 32 of what causes the weasel of northern areas to have a white coat in winter. The discerning student will instantly perceive that although some parts of the account on pages 31 and 32 are precisely accurate, other parts are the result of inferences which need to be proved. More careful work of the kind that Schwalbe (1893) and Wright (1942B) did is needed. The account on pages 31 and 32 is merely the best that can be given with the information now available.
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 ofMustela ermineaandMustela 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, ofMustela 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 ofMustela vulgarissince it had no black hairs on the tip of its tail. Flintoff (1935:228, 229) records what may have been an albinoMustela vulgarisfrom Yorkshire and an albinoM. ermineafrom an unstated locality. Jäckel (1873:459) mentions specimens ofMustela ermineaandMustela vulgaris, which were partly "albinistic" or "erythristic." Among the American specimens ofM. ermineaI have not recorded any which appeared to be either partly or wholly erythristic or only partly albinistic. Among the 1550 skins ofM. frenatawhich 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 inM. 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, ofM. 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 ofnoveboracensis, 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 ofM. 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 ofMustela frenatafrom the Pacific Coast of North America obviously have the factor for erythrism operating over a larger part of the body than it does inM. ermineaor than inM. f. noveboracensis, where the underparts sometimes are white. InM. f. latirostraand 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 ofM. 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, ofM. 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 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 speciesMustela frenatahave 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 instanceMustela frenata arthuriof 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 theM. f. arthuri. This same line marks also the eastern margin of the geographic range of the subspeciesMustela 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 ofarthuriand that of the bicolored face offrenata, (2) the long tail offrenataand (3) the convex skull ofarthuri. In the sum of its characters this specimen is exactly intermediate between typicalarthuriand typicalfrenata. 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 ofarthuri. Let us suppose that we obtain a third specimen from the same place and that it has the face marked like that ofarthuribut the tail fully as long, and the skull as lacking in dorsal convexity, as infrenata. Now the score is definitely forfrenata. For convenience of handling, the population is referred tofrenata, providing that the average of specimens from a nearby locality to the westward is not in favor ofarthuri. In event the average of specimens from a locality next adjacent to the westward is in favor ofM. 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 termedintergrades. This implies that their characters are the result of mixed parentage—perhaps a female ofM. f. arthuriand a male ofM. f. frenatabut 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 arthuriandMustela frenata frenata, are identified as subspecies because of the intergradation between them. If at this and all other places where the geographic ranges ofarthuriandfrenatamet 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. richardsoniiandM. 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 asSylvilagus floridanusandS. 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." InMustela 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.
InMustela 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. inM. frenataand from 336 to 228 mm. inM. erminea. There is no cline of sustained slope inM. frenatabut inM. ermineathere 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 inM. frenata, the subspeciesneomexicanahaving the long tail and the two subspeciesarthuriandprimulinahaving short tails. The geographic ranges ofprimulinaandneomexicanaare contiguous. InM. ermineathere is likewise no variation of a clinal nature in length of tail and furthermore the variation is much less than inM. frenata.
In length of hind foot, which in males varies from 49 mm. in northern populations ofM. ermineato 28 mm. in southern populations, the same cline is seen as in the total length of animals of this species. InM. 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. InM. frenata, the two subspeciestexensisandmacrophoniusprobably are the heaviest andeffera,arizonensisandhelleriprobably are the lightest. Geographically the variation in weight behaves in approximately the same way as does the measurement of total length. InM. ermineathe 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 inM. frenataclearly 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 speciesM. ermineahas the most hair on the foot-soles and thatM. africana, the tropical weasel, has the least. In this regard,M. frenatais intermediate as it is also in geographic position.
Figs. 11-15.Dorsal views of adult skulls of each sex of five subspecies of the ermine,Mustela erminea, to show secondary sexual variation and geographic variation in size of the skull. Males on the left and females on the right. All × 1.Note especially the geographic variation in decreasing size of the skull from north to south in each sex, and that the secondary sexual variation in size of skull is less in ermines with small skulls than in those with large skulls.
Figs. 11-15.Dorsal views of adult skulls of each sex of five subspecies of the ermine,Mustela erminea, to show secondary sexual variation and geographic variation in size of the skull. Males on the left and females on the right. All × 1.Note especially the geographic variation in decreasing size of the skull from north to south in each sex, and that the secondary sexual variation in size of skull is less in ermines with small skulls than in those with large skulls.
Figs. 11-15.Dorsal views of adult skulls of each sex of five subspecies of the ermine,Mustela erminea, to show secondary sexual variation and geographic variation in size of the skull. Males on the left and females on the right. All × 1.
Note especially the geographic variation in decreasing size of the skull from north to south in each sex, and that the secondary sexual variation in size of skull is less in ermines with small skulls than in those with large skulls.
Fig. 16.Map showing the localities where the skulls, represented in figures11-15, were obtained.
Fig. 16.Map showing the localities where the skulls, represented in figures11-15, were obtained.
Fig. 16.Map showing the localities where the skulls, represented in figures11-15, were obtained.
The maximum length of facial and carpal vibrissae is attained inM. ermineain 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) inM. f. panamensisto buckthorn brown (light brown) inM. 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 ofMustela frenata, that ofM. f. panamensishas the maximum of rainfall. Reference to the colored plate (1) will show that inM. 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 topanamensis(2) as a starting point and proceeding northward to the range ofnicaraguae(3), which also has lesser rainfall, thence another step northward to Guatemala, 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, andM. f. frenata(6) from Tamaulipas are from progressively more northern and also progressively drier regions. InM. f. frenata(6) the dark color extends posteriorly only to the ears and is blackish rather than black. InM. 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 betweenneomexicana(7) andpanamensis(2) illustrates the great range of geographic variation in color which occurs in the one species. Continuing from the geographic range ofneomexicana(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.
This geographic variation, it should be remembered, is all within one species. It is the more significant still when we remember that the same correlation, with never an exception, occurs at hundreds of places within the geographic range of the species. A particular feature of climate, namely rainfall, and possibly therefore humidity, is concerned in this correlation. The same correlation, heavy rainfall and dark color, is shown also in the other species of North American weasels. The conclusion is unavoidable that climate, directly or indirectly, determines or influences the color of weasels.
The light facial markings appear in American weasels in two separate geographic areas. One is the southwestern United States, México and northern Central America. The second area is in the same latitude, in Florida and adjoining parts of Georgia and Alabama. In the western weasels the markings are white south of latitude 32° N. North of this latitude, the facial markings, if at all extensive, usually are of the same yellowish color as the underparts of the body. Weasels of southern California and its interior valley usually have these yellowish instead of white facial markings. The light facial markings, in this instance, white markings, attain their maximum extent inM. f. leucopariaof the southwestern margin of the tableland of México, at latitude 19° N. A gradual decrease in area of the light facial markings occurs both to the north and south; they disappear at 10° N inM. f. costaricensisand at 35° N at approximately the southern limits of range ofM. f. arizonensisandM. f. nevadensis. In the mild climate of California the light (yellowish) facial markings are found at still higher latitudes. These light facial markings crop up as vestiginal remnants, consisting of a few white hairs, in some individuals of nearly all races of weasels.
In certain parts of the skull there are trends, in size and shape, which continue for long distances geographically. In other words, clines can be recognized. Changes in size and shape in some other parts of the skull are wavelike; change toward narrower rostrum, for example, is not progressive in a given geographic direction for any great distance. Length of the upper tooth-rows and zygomatic breadth, when expressed as percentages of the basilar length, and also the actual length of individual teeth vary geographically in the same wavelike fashion as does the width of the rostrum.
Size of the skull, on the other hand, shows a sustained trend for a long distance; it becomes progressively smaller from the southern United States southward to Columbia, South America. This clinal variation can be demonstrated by plotting on a graph, the basilar length, the zygomatic breadth, or the weight of the skull. Beginning at Mérida, Venezuela, and proceeding southward to increasing elevations in the mountains of South America, there is a reversal of the direction of the variation in this cline; weight of skull, for example, increases to the southward from Mérida for a considerable distance. A cline of decreasing width of the postorbital constriction of the skull is evident from Panamá north into Texas.
Variations in the tympanic bullae provide many characters useful in distinguishing weasels from different localities. Most of these characters have to do with degree of inflation of the bullae. Indirectly correlated with degree of inflation is first the extent of removal of the anterior margin of the bulla from the glenoid fossa and foramen ovale, and second the form (convex, flat, or concave) of the part of the squamosal bone between the foramen ovale and the anterior margin of the tympanic bulla. As one proceeds southward from, say, southwestern Kansas through the geographic range of the speciesMustela frenata, there is a progressive deflation of the bulla, an increase in length of the space between its anterior margin and the foramen ovale, and the floor of the braincase in front of the bulla changes from ventrally concave to ventrally convex. (See figs.eandhof pl. 24 and figs.eandfof pl. 27.)
One extreme of this variation in bulla is shown inMustela frenata neomexicana(fig.eof pl. 24), in which the anterior margin of the bulla (viewed from the ventral side) rises vertically from the floor of the braincase to form a 90-degree angle. The other extreme, the uninflated bulla, is inMustela frenata panamensis(fig.eof pl. 27), in which the anterior margin of the bulla is not raised above the floor of the braincase. This variation is remarkable because it occurs within a single species. Otherwise, in the family Mustelidae, differences in the tympanic bullae as great as that between the two subspeciesM. f. neomexicanaandM. f. panamensis, occur only between genera. The need for caution in inferring the limits of variation for a particular structure in one species or genus, on the basis of variation in another group, is therefore obvious.
Speaking now of full species, the most inflated tympanic bullae in American weasels are inMustela frenata, and more restrictedly in those subspecies of it which occur in the temperate region. Subspecies ofM. frenatain Central and South America, as already noted, have less inflated bullae. The tropical weasel,Mustela africana, of the Amazon drainage of South America has the bullae still less inflated (see fig.iof pl. 39 and fig.fof pl. 40). The bullae are less inflated even than in the mink, subgenusLutreola. InM. africanathe cleidomastoideus, omotrachelian, levator scapulae, and rhomboideus profundus muscles take origin from a fossa on the mastoid bone, whereas in the forms with greatly inflated bullae these muscles take origin from a raised ridge or tubercle. UsingMustela frenataof the temperate region as a starting point and proceeding northward, a reduction in inflation of the tympanic bulla is seen also in that direction in thatMustela ermineahas less inflated bullae. The bullae are less inflated in southern than in far northern (arctic) populations ofMustela erminea. Inermineathe lesser inflation is real enough but at the same time there appears to be less inflation than actually exists, for the squamosal floor of the braincase is "pushed down." This places the anterior end of the tympanic bulla farther in the braincase than it otherwise would be. Although the anterior end of the bulla is flattened to the extent that it resembles the sharp edge of a splitting-wedge, inspection of the lateral and medial edges shows that in its central part the bulla is more inflated than it is in the weasels of Central and South America.
For reasons set forth later,M. ermineais judged to resemble the ancestral stem form more closely than does any one of the other three American species of weasels. If this judgment is correct, the shape of the tympanic bullae of the American weasels may be explained as follows: In the subspecies ofMustela frenataof the temperate regions of North America the bullae have most nearly been pushed out of the braincase and at the same time have undergone some enlargement. The subspecies of this same species in Central and South America represent an earlier stage in the evolution of American weasels and retain less inflated bullae—less inflated even than those of the southern subspecies oferminea.M. africanaprobably separated from the stem form at a still earlier time if we may judge by the lesser inflation of its tympanic bullae. There are other reasons for thinking thatafricanaseparated from the stem form earlier thanM. frenatadid. During the time that elapsed since the separation ofM. frenatafrom the stem form, the tympanic bullae ofM. ermineaprobably increased slightly in size, as probably also did the brain but without shoving the auditory complex forward from its former position.