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.
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 figures [11-15], were obtained.
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 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, 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.
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 in M. f. leucoparia of 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 in M. f. costaricensis and at 35° N at approximately the southern limits of range of M. f. arizonensis and M. 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 species Mustela 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. e and h of pl. 24 and figs. e and f of pl. 27.)
One extreme of this variation in bulla is shown in Mustela frenata neomexicana (fig. e of 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 in Mustela frenata panamensis (fig. e of 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 subspecies M. f. neomexicana and M. 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 in Mustela frenata, and more restrictedly in those subspecies of it which occur in the temperate region. Subspecies of M. frenata in 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. i of pl. 39 and fig. f of pl. 40). The bullae are less inflated even than in the mink, subgenus Lutreola. In M. africana the 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. Using Mustela frenata of 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 that Mustela erminea has less inflated bullae. The bullae are less inflated in southern than in far northern (arctic) populations of Mustela erminea. In erminea the 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. erminea is 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 of Mustela frenata of 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 of erminea. M. africana probably 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 that africana separated from the stem form earlier than M. frenata did. During the time that elapsed since the separation of M. frenata from the stem form, the tympanic bullae of M. erminea probably increased slightly in size, as probably also did the brain but without shoving the auditory complex forward from its former position.
[DISTRIBUTION AND SPECIATION]
Weasels of the subgenus Mustela are known from the Pleistocene but not from deposits laid down at an earlier time (see page [10]). The Pleistocene weasels from Rancho La Brea of southern California and from Potter Creek Cave and Samwel Cave, both of northern California, are subspecifically indistinguishable from the weasels living in those same localities today. The other notable occurrence of weasels in the Pleistocene is in the Conard Fissure of Arkansas. Brown (1908:181, 182, pl. 17) names two kinds from the Fissure. One is an extinct subspecies (Mustela frenata gracilis) possibly of the species which occurs in the same region today and the other, Mustela erminea? angustidens, is an extinct subspecies of a species which occurs only farther north today. M. erminea came south, probably in front of one of the ice sheets, as did several other species of American mammals, now of more northern distribution, that left their remains in Conard Fissure. Mustela rixosa is not recorded as a fossil in America although it is known from the "Diluvial" deposits of the Old World; see Woldrich (1884:1000), who employs the name "Foetorius minutus n. sp.," and see also Zimmerman (1943:295-296).
The ermine, Mustela erminea, is the most generalized of the full species. For example, the number of teeth is as large as in any other species and greater than in certain species. The teeth are sharp-pointed, uncrowded, and individually less specialized than in any other American weasel. M1 has the inner half, or lobe, of approximately the same size as the outer lobe instead of much larger than the outer lobe (the outer lobe is the larger in several other species). The tympanic bullae are less inflated and less protruded from the braincase. The skull is rounded, and has no marked crests and ridges whereas the skulls of the other species are more pronouncedly modeled and sculptured. Therefore, it is possible to think of these other species as derived from M. erminea. A derivation in the reverse direction would be more difficult. From the foot soles of an ermine, or a weasel closely resembling an ermine, the more complex soles of Mustela africana could have been derived by a decrease in hairiness, although it would be necessary to suppose that the thenar pad has been retained in africana and has been lost in the living erminea. The alternate possibility, namely, that the thenar pad was a relatively recent acquisition in the africana line seems less probable. The tail of erminea is of "average" length and in size of entire animal erminea is intermediate between the other American weasels. Structurally, Mustela erminea appears to be nearest the stem form from which all of the living weasels ascended. Its present holarctic distribution is in harmony with the view that it is a direct descendant from the stem form because the stem forms of most of the known kinds of mustelids appear to have lived in the holarctic region. To be sure, Mustela erminea is regarded as having undergone some progressive change in structure, but less than the other weasels, in the period of time when the weasels were evolving from the stem form.
The least weasel, Mustela rixosa, seems to be an ancient type and to judge from the size and proportions of its parts, was differentiated from the erminea stem at a time earlier than were the other American Recent species of weasels. In size, in reduction of the tail, and in proportions of the skull, M. rixosa is, in each instance, the most aberrant of all the weasels, Mustela nivalis of Europe and western Asia included. This aberrancy results from the retention of certain primitive features, in the teeth and basicranial region, and from specialization in proportions of the skull. The skull is long, deep, and narrow. These proportions probably are adaptations permitting the animal to follow the smaller kinds of mice into their burrows. In most of that part of North America where erminea and rixosa occur together, erminea is a much larger animal and takes as prey almost all kinds of land vertebrates that it is powerful enough to kill. These include varying hares and ptarmigans. The least weasel, rixosa, can hardly manage such large prey and lives on the smaller rodents. Mustela rixosa may eat numbers of insects (see page [176] beyond),—a kind of food which Mustela erminea is not known to eat. Apparently the two species are able to live in the same areas because each eats a somewhat different kind of food than does the other and hence they do not compete to the point where one is crowded out by the other. This is the case in the latitudes where the two species of weasels are of different bodily size, but in the southernmost latitudes where these two species occur, erminea becomes almost as small as rixosa and only one of the species, to the exclusion of the other, occurs in a given area. All through the Rocky Mountains, south of Montana and in the territory west of these mountains all the way to the Pacific Coast, only the small subspecies of erminea is to be found. In the Alleghenies of the eastern United States only rixosa occurs. In New England where erminea approaches the size of rixosa, the latter is unknown. Probably this exclusiveness results from competition for food, although competition for dens, safe breeding places and other requirements of life may be involved.
The species erminea invaded the western United States and in the process of invasion probably developed there the small size appropriate to permit erminea to live in that latitude before it could do the same thing in the Appalachian region. Later than erminea, the least weasel, Mustela rixosa, which was small to begin with, also spread southward from the holarctic region, stopped short in the western United States at the northern boundary of the area in which erminea was of small size, but in the Appalachian region of the eastern United States continued on southward to the limits of temperature tolerant for it because erminea had not yet penetrated into that region and no other small carnivore was there to offer competition.
The long-tailed weasel, Mustela frenata, occurs mostly south of the regions inhabited by the ermine, and mostly south of the region inhabited by the least weasel which appears to live as well with frenata as with erminea. It is true that erminea and frenata occur in the same region, but this is a relatively narrow belt across the United States; and from within it a person cannot go far either north or south without reaching a region in which only one of the two species occurs. Exception has to be made for the Rocky Mountains and the Sierra Nevada, where erminea is of exceptionally small size. In these mountains and in the boreal mountainous parts of the intervening region of the United States, erminea and the large-sized frenata occur together over a wide area. Presumably the two occupy different ecologic niches, much as rixosa and frenata probably do where they occur together.
Most of the geographic range of the long-tailed weasel, M. frenata, is in the temperate region. Structurally, this species is the most advanced of the American weasels. Its dentition is the most highly specialized for cutting. M1 is relatively small and the inner lobe is slightly larger than the outer lobe. The skull, throughout, is more modeled than in the other species; the rostrum, the lower jaws and the teeth—all parts of the offensive equipment—are well developed relative to the corresponding structures in other weasels; the basicranial region exhibits an advanced stage of development in that the tympanic bullae show the maximum degree of inflation. Also, they are thrust far out of the braincase, thereby providing more room for the relatively larger brain which is protected by a more solidly built braincase than in erminea.
Several subspecies of Mustela frenata occur in the tropics, that is to say, south of the Mexican tableland and on the coastal plain to the east of it. Each is structurally more primitive than subspecies of the temperate region. As compared with Mustela frenata frenata of the temperate Mexican tableland the size in these tropical subspecies is smaller; the tail is shorter; the braincase and entire skull are less modeled; the postorbital breadth is more; the teeth are smaller; the deuterocone of P4 is not so far anterior to the protocone; the tympanic bullae are less inflated, are farther removed from the foramen ovale, and a larger proportion of each bulla is contained within the braincase. These features serve to set off from northern races of frenata all those subspecies of frenata which occur from southern México southward to the northern and western limits of the Amazon drainage of South America. The Amazon Basin is inhabited by another species, Mustela africana, having more primitive characters.
In the species frenata, the explanation for this abrupt change in characters between the animals of the temperate highlands and those of the tropical lowlands may be this: In the early Pleistocene, after the emergence of much or all of Central America took place, weasels distributed themselves over the Isthmus and into South America. These weasels were more generalized in structure than those now inhabiting the uplands of México. Failure of this stock of weasels often to cross some still-persisting water barrier, or failure of this stock to cross some water barrier that was widened or reformed because of a rise in sea level in some one of the interglacial periods of the Pleistocene cut the frenata stock into two or more parts. After the land connection was established or re-established and when the necessary precedent plants and rodents again had established themselves, the two groups of weasels, one from the northern tableland of México, and the other from the southern area of tropical complexion, met. The weasels of the frenata stock that reinvaded the area from the north probably did so by following along the chain of high volcanic cones and narrow uplifts. If and when a subsequent inundation occurred in some part of Central America, weasels were stranded on the adjacent mountains—converted into islands—only the higher parts of which were above water. Mustela frenata costaricensis and Mustela frenata goldmani may be examples of a northern stock of weasel that pushed southward in the highlands and became stranded for a short time. Following the latest emergence of land to provide a continuous highway between the two continents, weasels from the south and the insular populations, as for example, M. f. costaricensis, were the first to invade the low tropical areas most recently under water. When the Pleistocene history of Central America is better known, the facts will provide a useful means of testing the hypothesis that has been outlined immediately above.
As explained above, fossil specimens of M. frenata from deposits of the last half of Pleistocene time show that no appreciable change occurred in some areas, for example, in the vicinity of Hawver Cave and Samwel Cave of California, and that but slight change occurred in other areas, for example, in southern California (fossils from Rancho La Brea) and probably in the central United States (fossil from Conard Fissure). It is possible to imagine, therefore, that the two groups of weasels, one occurring southward only as far as the highlands of Central America and the other occurring in northern South America, had not differentiated sufficiently in the period of their isolation to prevent crossbreeding when they last came into contact. If the separation of the two groups had been maintained for a longer period, the two groups, tropical weasels and austral weasels, probably would have been so different when the two met as to prevent crossbreeding and they would have constituted two full species instead of only one.
Mustela africana is the most primitive of the American weasels. Some of the most important structural features that mark it as such are in the basicranial region. The tympanic bullae are less inflated than in other weasels, are pointed anteriorly and posteriorly, and do not have the lateral margins carried outward to the outer margins of the braincase. The mastoid sinus is not involved, by inflation or marked modification in the production of the auditory complex. Between the alisphenoid and the squamosal there is a clear demarcation posteriorly from a point directly lateral to the foramen ovale. This demarcation permits a transverse rounding of the alisphenoid to form a longitudinal ridge between the anterior margin of each bulla and the base of the pterygoid of the same side. Nevertheless, there is no such specialization of this primitive, structural feature such as occurs in some African and Asiatic mustelids in which the tympano-pterygoid part of the alisphenoid fuses with the tip of the hamulus of the pterygoid. However, the tympano-pterygoid eminence has not been obliterated in M. africana as it has in the other American weasels. Another primitive feature in the basicranial region of M. africana is the tendency toward separation of the paroccipital processes from the tympanic bullae. The thenar pad of the foot probably is an inheritance from a primitive ancestor since the pad is present in the viverrids and in a majority of mustelids judged to be more primitive than Mustela.
Some specializations are obvious in Mustela africana. One is the reduction in number of premolars; p2 is absent whereas it is normally present in the other weasels; P2 has one instead of two roots; and, in relation to the other teeth, m2 is smaller. The shortness of the preorbital part of the skull in relation to the length of the skull as a whole may reflect the mentioned reduction of the premolars or retention of a primitive shape of skull, or both. Also, certain features which denote immaturity in other weasels are retained in adults of this species, as for example, sutures on the dorsal face of the preorbital region of the skull.
Figs. 17-22. Views of the feet of American weasels (subgenus Mustela) to show differences in number and arrangement of the pads and variation in degree of hairiness of the soles. × 1-1/2. In each figure, left-forefoot on left, and left hind foot on right.
Fig. 17. M. rixosa rixosa, Halifax, N.S.; juv., ♀, 7425 U.S.N.M.
Fig. 18. M. erminea richardsonii, Ft. Chimo; ad. ♀, 14866 U.S.N.M.
Fig. 19. M. frenata noveboracensis, Mich., July 7, 1913; ad. ♂, 44689 M.Z.
Fig. 20. M. f. frenata, Brownsville, June 1, 1892; yg. ♂, 34043 U.S.N.M.
Fig. 21. M. frenata panamensis, Panamá, February 17, 1911; sad. ♀, type.
Fig. 22. M. a. africana, Pará, Brazil, Sept., 1908; yg. ♂, 37475 A.M.N.H.
Figs. 17, 18 and 19. Drawn from specimens preserved in alcohol.
Figs. 20, 21 and 22. Drawn from relaxed feet of dried skins.
Mustela africana, all characters considered, is the most aberrant of the American weasels. That is to say, greater difference prevails between M. africana and any other American weasel than exists between any other two American weasels. The distinctive cranial and dental characters, excepting the reduction in number of premolars, are of a primitive nature. For example, the relatively wide postorbital region, the large braincase that is inflated anteriorly, and the flattened tympanic bullae are points of resemblance to the holarctic Mustela erminea, the species which is regarded as most closely resembling the stem form. Also, the mentioned characters in adults of M. africana resemble ontogenic stages passed through by other weasels. Consequently, it is thought that M. africana crossed the filter-barrier from North America to South America, remained isolated from the original stock for a length of time sufficient to permit africana to differentiate from North American weasels and vice versa to such a degree that crossbreeding with the frenata stock was prevented when frenata, at a later time, pushed southward over the, then zoölogically less-effective, water barrier, or continental bridge if it was by this time in existence.
Fig. 23. Diagram indicating probable relationships of the species of American weasels.
The four full species of American weasels may well be thought of as having the same stem form of which erminea is the most nearly direct descendant. Geographic and climatic changes may have operated to isolate, and then to foster morphologic differentiation of, first rixosa in Eurasia, next africana, third the tropicalis section of M. frenata, and finally M. frenata itself, leaving M. erminea as a modern version, somewhat altered to be sure, of the stem form. Some of these ideas are expressed in figure [16]. The climate is different in the ranges of the several species and the climate has changed through time in the ranges of at least many subspecies. Natural selection of morphological features best adapted to a particular kind of climate probably has altered some species more than others. M. erminea in almost every one of its characteristics is generalized and potentially progressive whereas africana retains more characters which are truly primitive along with a few which are specializations. M. africana is potentially the least progressive of any of the American weasels. The most specialized weasels are the North American races of Mustela frenata. A progressive series of increasing specialization is comprised in (1) M. africana, (2) the M. tropicalis (Central American, lowland) section of M. frenata, and (3) the races of M. frenata in North America.
Considering now features of the environment which have obviously influenced the distribution and speciation of weasels, water barriers are important. Bering Strait, Carquinez Strait (along with San Francisco Bay) which opens through the Golden Gate, and the channels between the islands of southeastern Alaska, have contributed to the formation of subspecies. The difference is really slight on the two sides of Bering Strait and San Francisco Bay and is slightly more on two sides of each of several of the channels between the islands of southeastern Alaska. The differences between the weasels on the two sides of one of these water barriers supposedly result from the preservation in animals on one side, or on one island, of small mutations, which would be swamped by crossbreeding if the water barrier were not present. The effect of this isolation is easily seen if ermines from the Queen Charlotte Islands are compared with those of the opposite mainland. The degree of morphological difference is great. Isolationwise, the Queen Charlotte Islands are the seaward end of a chain, beginning with Admiralty Island in southeastern Alaska, and are farther from the mainland, zoölogically, than the distance in actual miles across the water channel would suggest. Between any two islands that are geographically consecutive, however, and between the mainland and the first island of the chain, the difference in the ermines is small. In other places, water barriers of equal or greater width have contributed little if anything to the differentiation from one another of weasels on the two sides of the water barrier. The strait between eastern Canada and Newfoundland is an example.
The absence of water, or scarcity of it to a degree that closely approaches absence, in any large area appears to prevent weasels from living there. At any rate, the one sizeable region of North America from which weasels are unknown is the desert of the southwestern United States and adjoining part of northwestern México. More precisely, in western Arizona, the Mohave Desert and the desert of northwestern Sonora, collectors of mammals have repeatedly sought small carnivores without ever finding any weasels.
Degree of moisture is closely correlated with color in weasels. Humidity and cloudiness as well as actual precipitation seem to be involved. Even if we take into account average annual rainfall alone, the darkest-colored weasels are found in the areas of heaviest rainfall and the lightest-colored weasels in areas of lightest rainfall (extreme type of desert where no weasels occur being excepted). In any large region where there is a geographic gradient in rainfall, the transition from light to dark color almost exactly parallels the increase in amount of rainfall. Within a given species the same color reappears in widely separated areas that have the same amount and seasonal distribution of rainfall. This correlation is repeated so often that one can almost certainly say that heavy rainfall, or the associated phenomena of high humidity and cloudiness, acting separately or together, causes an increase in intensity of color. Relative extent of the color of the upper parts and underparts and presence and absence of light facial markings seem also to be correlated, in a more general way, with differences in rainfall. A fuller discussion of the nature and amount of the variation in color is given on page [51].
Temperature seems not to be an important factor in directly limiting the distribution of weasels, since M. frenata occurs from the hottest to some of the coldest parts of the Americas. Do M. erminea and M. rixosa range no farther south, than they do at present, because high temperatures constitute a barrier? No evidence is known to me which provides an answer, one way or the other, to this question. Granting that temperature is unimportant in limiting the distribution of weasels, it seems to cause geographic variation. Increase in mean annual temperature is correlated with decreased size in M. erminea and with increased size in M. rixosa. Temperature, it seems, causes the hair to vary; the pelage is harsher and sparser in weasels from tropical regions than in those from boreal regions. Difference in number of hairs is especially well shown on the soles of the feet. In the weasels from the far north, the pads are concealed by hair and in the weasels from the tropical regions the soles are mostly bare. Also, the hair on the soles of the feet is longer in northern than in southern weasels. Furthermore there is seasonal change in length of the hair on the soles of the feet; at a given locality in southern Canada the hair of the white winter coat is so long on the soles of the feet as to obscure completely the palmar and plantar pads whereas the hair of the brown summer coat is shorter and leaves these pads boldly exposed to view. This seasonal change, as would be expected, is most marked in animals of northern regions and is not perceptible in those from the tropics; it is correlated with increase in seasonal change as the distance from the equator increases.
Temperature and moisture acting together may cause extensive white facial markings, that neither alone would cause. In Mustela frenata these markings occur where there is heavy rainfall and high mean annual temperature. Where there is heavy rainfall and a low mean annual temperature they do not occur and where there is high mean annual temperature and light rainfall the markings are not pure white but are of the same color as the underparts. [Plate 1] and the description of color on page [51] may be consulted in this connection. Extremely high mean annual temperature together with extremely heavy rainfall may inhibit the development of light facial markings. M. f. meridana, panamensis and costaricensis are cases in point. In either direction, north or south, from the territory inhabited by these three subspecies a similar combination of temperature and rainfall is found and similar light facial markings appear there.
Considering the delicate response of structure to climate, a person naturally questions whether or not natural selection accounts for all of the differences between subspecies. To show that natural selection determines the color of Mustela frenata, it would be necessary to assume that climate, color, and utility of color are positively correlated. Although climate (rainfall) and color are correlated in such a manner that three subspecies of weasel in places as far apart as New England, Perú, and the state of Washington are colored alike, other features of the three environments are unlike. Kinds of animals which the weasel catches for food, and flora in which the weasel finds concealment, are dissimiliar. If natural selection alone determined the color, some difference in color would be expected between the weasel which needed to be obliteratively colored, that is camouflaged, the better to catch a Phyllotis in Perú and the weasel in Washington which needed nature's aid in catching Microtus. Mustela frenata goldmani of the highlands of southern México, which is known to attack the huge pocket gophers, Orthogeomys and Cratogeomys, has a weaker dental armature than Mustela frenata texensis which does not have to overcome prey so formidable as does goldmani. Equally formidable enemies endanger M. f. goldmani and texensis. Examples of this nature could be multiplied. Without actually proving anything concerning selection, these examples give reason for us to suppose that some characters are not determined by natural selection.
Another question upon which data obtained from a study of Mustela has some bearing, is this: Where the geographic ranges of two subspecies meet, why does not the swamping effect of crossbreeding cause one subspecies to disappear? Although swamping may have occurred in some instances, it does not occur in the majority of instances. Witness the long-continued existence of the living subspecies Mustela frenata nevadensis of which skulls are available from Pleistocene deposits. Therefore, its distinctive characters, cranially at least, have been maintained for a long time. Furthermore, these characters are maintained over a large geographic region more than a thousand miles across. On the eastern margin of its range, at the eastern base of the Rocky Mountains in Colorado, M. f. nevadensis intergrades in a relatively narrow belt with the lighter-colored, longer-tailed and cranially different Mustela frenata longicauda, which has a geographic range almost equally extensive. M. f. longicauda also is uniform in its characters over a large area but at approximately 400 miles east of the base of the Rocky Mountains, it begins to intergrade with the darker-colored, shorter-tailed and cranially different Mustela frenata primulina and does so over a belt of 100 miles or more in width. At any given locality within this wide belt of intergradation the range of individual variation ordinarily does not exceed that in animals from a given locality well within the geographic range of M. f. longicauda. In the narrow belt of intergradation along the eastern base of the Rocky Mountains, the range of individual variation at several places is greater than in animals from a given locality well within the geographic range of M. f. longicauda or for that matter from well within the geographic range of M. f. nevadensis.
Considering the dominance and recessiveness of genes and the genetic mechanism in general by which characteristics of offspring are inherited from their parents, it would seem that M. f. longicauda and for that matter M. f. nevadensis and M. f. primulina would lose their distinctive characteristics because of the crossbreeding that is every year going on between longicauda and nevadensis on the one hand and between longicauda and primulina on the other hand.
Sumner (1932:84) suggests that homogeneity is prevented by population pressure. Applying his suggestion to the species Mustela frenata we could say that the subspecies longicauda pressing westward meets strong pressure from the subspecies nevadensis pressing eastward and that the width of the zone of intergradation between the two subspecies varies inversely with the strength of the population pressure from the two sides. Sumner recognizes that according to his hypothesis the two contiguous races would remain distinct only so long as there was a preponderance of centrifugal movement from both of the centers of dispersal. Sumner (op. cit.:85) recognizes that an abrupt change of environmental conditions could account in part for the boundaries of the ranges of the two subspecies and finally that his hypothesis does not certainly answer the question of why crossbreeding does not result in homogeneity between two subspecies with contiguous geographic ranges.
The hypothesis of harmoniously stabilized complexes of genes was offered by Timofeeff-Ressovsky (1940:124) to explain why the swamping effect of crossbreeding does not obliterate subspecies. The hypothesis takes into account that any one of several characters of a subspecies may be caused by several genes. Some characters of this kind may be favored by natural selection more than others. In the belt of intergradation between two subspecies, where two of these favored characters meet, a "biological tension" as Huxley (1939:415) terms it "will result, which will produce partial discontinuity between the two groups. Each group will evolve a gene-complex which is not only broadly adapted to the external environment of the central area of its range, but is also harmoniously stabilized, in adaptation to the internal genetic environment, by the selection of modifiers." Crosses, that is to say intergrades, between the two subspecies will lack this stabilization and will therefore be at a selective disadvantage. The zone of intergradation will therefore remain narrow; intermediates are constantly being brought into existence there by crossing but are as constantly being extinguished by selection.
These two hypotheses are the best that geneticists yet have offered. Neither has been tested and both, as originally proposed, would hardly apply everywhere because there are some contradictions.
I can offer no better explanation—in fact no original one as good—but would emphasize that under similar climate, weasels remain constant in character, or at most do not vary beyond certain limits. Crossing at the margins of ranges of two subspecies does not result in homogeneity of weasels. There is, therefore, some stabilizing influence, or influences, that maintain, and even develop, structural characteristics of weasels in opposition to the contrary tendency of crossing.
That this influence not only maintains uniform characters over areas of large extent, but also permitted their development over large geographic areas, must logically be supposed, for otherwise, considering the swamping effect of crossing, such variations would not have made their appearance in more than a few individuals. Also, if the races had been formed in response to some kind of physiological differentiation, or other non-climatic cause, the characters of the population in the belt of intergradation probably would disappear in a short time. In any event the close correlation between degree of change in weasels and degree of change in climate, at once makes one suspect that climate has been the deciding factor. Finally, when one recalls that in certain parts of the animal, certain characters invariably appear under similar climates and never under dissimilar climates, the evidence is almost conclusive that, given long enough time, the animals vary in response to climate. The variations (characters) may be induced indirectly, but are no less exactly reproduced than if they can be shown to be induced directly.
In considering how the species and subspecies of American weasels were formed and in attempting to account for some of the individual characters, it is profitable to view the facts in the light of some of the theories of species-formation—theories that are accessory to that of organic descent and that are concerned with the modus operandi of organic descent.
In any group of closely related species some of them, by the laws of chance, are almost certain to be more primitive than others. Mustela is no exception and the more primitive species closely match, in several characters, ontogenetic stages passed through by more advanced species. Jaeckel's (1902) theory of metakinesis, therefore, is to be considered since it postulates that many cases of epistasis occur; that is to say, that many sexually adult animals are arrested in development in early otogenetic stages and undergo no further development. Although this theory is appealing upon initial consideration, it is less so when we recall that in Mustela there is a direct correlation of increasingly primitive structure with decreasing latitude as one proceeds from the steppe of North America southward to the equator. It follows that the conditions seen in Mustela can be explained even better than by metakinesis, by assuming that the several species have differentiated from a stem form at different times, have developed at different rates, have developed in different directions and that ontogeny recapitulates phylogeny.
The theory of Age and Area (see Willis, 1922) holds that the species of widest distribution are, on the average, the oldest, and that the species which are distributed over small areas are, in general, of recent origin. So far as the weasels are concerned, little support is given to this theory. The same can be said of any one of the teological theories, including the orthogenesis of post-Darwinian writers. All of these imply a determinate line of variation controlled by the inherent qualities of the organism. The idea that the several species of Mustela result from mutations of large degree and sudden appearance is contrary to the evidence accumulated. In fact the evidence rather clearly indicates that the mutations which may have occurred were of small degree and in most instances owe their preservation to natural selection.
The data obtained by the study of weasels accords almost exactly with the theory of species-forming embodied in Matthew's (1915) "Climate and Evolution." Although the essential features of this theory were made out from a study of families and orders and therefore would not be expected to apply to members of only a genus or subgenus, the facts known about the present distribution of American Mustela, nevertheless, are strikingly in accord with the ideas advanced by Matthew. In the first place, climate is an important factor in the evolution of the weasels. In the second place, the line of migration seems to have been outward from the holarctic region. In the third place, the geographic changes necessary to explain the present distribution of the species of Mustela are not extensive and do not affect the permanency of oceans as defined by the continental shelf. These three statements are, almost verbatim, those made in the first three of the five points of Matthew's (1915:172-173) thesis. The remaining two points of Matthew's thesis have to do with generalizations based on evidence obtained from sources outside the scope of the present study.
Furthermore, the relative degrees of specialization of the different species and subspecies in relation to their geographic distribution are in accord with the ideas elaborated by Matthew. For instance, the most primitive species is farthest south from the probable center of dispersal, the holarctic region. Also the full species become progressively more primitive as one proceeds southward from the holarctic, or at least from the northern half of the nearctic, region. Although, in view of the known geological changes that have occurred in the Caribbean region, we cannot say that the more primitive species owe their positions entirely to having been pushed farther south from the center of dispersal by actual and continuous contact and competition with the more advanced species, this seems to have been the case in a general way. At any rate the more primitive kinds seem to have been prevented from pushing northward by the more advanced kinds which developed there and the latter have actually pushed southward.
Additionally and in review: There is strong indication that the American species of weasels were formed by gradual and slow change. Much of this change probably is the result of natural selection operating on fortuitous variations of a minor nature, but, also, particular features of the environment, especially climate, and more especially amount of rainfall, seem to compel variations that differentiate subspecies and that characterize full species—compel some of them without the direct operation of natural selection, or at least compel them within limits so wide that natural selection exerts no exact control.
[HISTORY OF CLASSIFICATION]
In the earlier accounts of American weasels, from the time of Linnaeus and before, up until 1890, names then in use for European weasels frequently were applied also to those in North America. For the next 50 years, and almost without exception after 1896, the American weasels were regarded as specifically distinct from those in the Old World. In this 50-year period many new names were proposed, usually as full species, although now that material from more localities has been brought together and studied, geographic intergradation is evident between many of the named kinds and most of these names now therefore take only subspecific rank. In 1933 Glover M. Allen showed that Mustela rixosa occurred also in the Old World, and in 1943 I emphasized that a second American species, Mustela erminea, was circumpolar in distribution. In neither rixosa, nor erminea, however, were the subspecies the same in the two continents. To this general outline of the nomenclature, exception must be made for weasels of the southwestern United States, México and Central America, and South America, because as early as 1813 a distinctive name was given to one of these and weasels from the three areas mentioned were, so far as I know, never given names of Old World kinds.
The first paper that could be regarded as revisionary in nature was "Remarks on the species of the genus Mustela" by the zoölogist and world-traveler, Charles L. Bonaparte, in Charlesworth's Magazine of Natural History, for 1838. In that paper three new names, Mustela cicognanii, M. richardsonii and M. longicauda, all still valid, were proposed for American weasels.
Audubon and Bachman in their "Quadrupeds of North America," which appeared in parts from 1845 to 1853, recognized 5 species. Actually they were dealing with only 3 taxonomically valid kinds. For one of these, Mustela frenata noveboracensis, they were misled by the difference in size between males and females, and in the males by the presence of a brown coat in some and a white coat in others. The male that was white in winter they regarded as Putorius ermineus of the Old World; the male that was brown in winter they designated by their earlier proposed name P. fuscus, and the female they named P. agilis. The ermine, subspecies M. erminea cicognanii, they called P. pusillus. Their fifth name, P. frenatus, included at least some animals that today are assigned to the subspecies M. frenata frenata. Each of three and perhaps four of the five names employed by Audubon and Bachman embraced individuals of more than one species and in that sense the names were composite.
Only five years later, in 1858, Professor Spencer Fullerton Baird's great work, "The Mammals of North America," made it clear that no American weasel was identical (in the modern subspecific sense) with any Old World weasel, and he applied most of his names in a correct zoölogical sense. It is true that he thought that the female weasel of the eastern United States was specifically different from the male, misapplied to it the name richardsonii, and did not correctly allocate every one of the few poor specimens available to him of the little ermine (M. e. streatori) of the Pacific Coast; but he did recognize that the least weasel was a distinct kind and his treatment in general was excellent.
After Baird came a period of great confusion in which most writers did no better than had Audubon and Bachman, ordinarily confusing the two sexes as different species, and, in 1877 in his "Fur-bearing Animals," Elliot Coues went rather to the other extreme and allowed only 4 kinds to all of the Americas, regarding two of these, for purposes of zoölogical nomenclature, as identical with the European species.
But, in 1896 Outram Bangs published "A Review of the Weasels of Eastern North America" in which he correctly recognized eight kinds. Although some of these were treated by him as full species, whereas the material accumulated since 1896 has shown that subspecific status is in order, his names, still in use, were correctly applied in every instance, save probably one. This was his use of Putorius richardsonii for the animal now known as M. e. arctica. Unlike the earlier, excellent treatment by Baird, this accurate one by Bangs was heeded and followed by subsequent writers. For example, Dr. C. Hart Merriam in the same year, 1896, accepted Bangs' conclusions except for correcting the application of the name richardsonii. The principal contributions of Merriam's paper "Synopsis of the Weasels of North America" were first, the wider geographic scope and second, the naming as new of several kinds outside the geographic area studied by Bangs. Otherwise the work was not up to Dr. Merriam's usual standard and the internal evidence of haste in its preparation and the superficial study of some of the material at his disposal explain why the weasels of North America since that time have been but little better understood than in 1896. Baird and Bangs, then, unquestionably did the best systematic work on the American weasels.
In 1916 Dr. Joseph A. Allen published a valuable paper on the South American weasels. The material available to him was inadequate and prevented a thoroughly satisfactory treatment. There are too few specimens even today to permit of a thorough treatment of the South American weasels in the present paper; nevertheless the material today is more nearly adequate than it was in 1916 and it is hoped that the systematic arrangement is correspondingly improved.