Chapter II: Introduction (1)

The weasel's agility and speed take it in and out of retreats, over obstacles and across open places in amazingly rapid fashion and are responsible for the animal's actions being described as "quick as a flash." The common long-tailed weasel of the United States measures approximately a foot and a half in length, of which the tail comprises a third; but the round, slender body is scarcely more than an inch and a half in diameter. Brown above and whitish below in summer dress, the animal is sleek as well as lithe and graceful. It is easy to understand, therefore, why the Bavarian name _Schönthierlein_ (pretty little creature) and the Italian name _donnola_ (little lady) were bestowed upon it. The Spanish name is _comadreja_ (godmother).

In the winter, in temperate and northern regions, the coat becomes pure white except for the black tail-tip. In this dress the correct name for the animal is ermine, a mammal whose fur is known to all and justly esteemed, especially for its luster in artificial light, where it is scarcely excelled in enhancing the beauty of gems and their feminine wearers.

In relation to its weight, the weasel is thought to be unsurpassed, and perhaps it is unequalled among mammals, in the effectiveness with which it exercises its carnivorous heritage; it kills with speed and strength a wide variety of animals including many much larger than itself; and it has been known to attack even man himself when he stood between the weasel and its intended prey. In structure and temperament it is so highly specialized for offense that, when opportunity affords, it sometimes kills, for storage in its larder, far more than enough to meet its immediate needs. After speaking of this tendency, Elliott Coues (1877:129) has said:

"A glance at the physiognomy of the weasels would suffice to betray their character. The teeth are almost of the highest known raptorial character; the jaws are worked by enormous masses of muscles covering all the side of the skull. The forehead is low and the nose is sharp; the eyes are small, penetrating, cunning, and glitter with an angry green light. There is something peculiar, moreover, in the way that this fierce face surmounts a body extraordinarily wiry, lithe, and muscular. It ends in a remarkable long and slender neck in such a way that it may be held at right angle with the axis of the latter. When the creature is glancing around, with the neck stretched up, and flat triangular head bent forward, swaying from one side to the other, we catch the likeness in a moment--it is the image of a serpent." Although Coues' colorful description more closely links the weasel with the symbol of evil than pleases me, his description does emphasize the raptorial character of the weasel.

Even though most weasels are intractable as pets, they have a value to man, as, for instance, when he is plagued by mice. In a field where mice and other small rodents are so abundant as to damage cultivated crops, the weasel is the farmer's best friend. A weasel may inhabit one den until the rodents thereabouts are almost exterminated in an area two or three hundred yards across; in this way the weasel acts as a control, locally, as well as a check more widely, on the increase in size of populations of kinds of rodents upon which it preys. The smaller species are mousers of remarkable efficiency and can, if necessary, follow a mouse to the end of the mouse's burrow. The slender body allows the weasel to pass through any burrow or hole into which it can thrust its head. This ability in an organism as highly specialized for killing other animals as is the weasel, has earned for it a bad name in connection with poultry yards. Authentic instances are recorded in which a weasel, gaining entrance through a knot-hole to a coop of young chickens, killed several dozen of the fowls. In other instances, however, weasels have lived under buildings close by a poultry yard without even molesting the birds in the slightest; in the latter instances the weasels probably were present because there was an abundant supply of rats and mice. At least three poultry raisers (see page 214) have encouraged weasels to live in their poultry yards feeling that the good they do by destroying rats outweighs the damage caused by the occasional weasel which turns to the fowls; the idea is that the individual weasel can be eliminated if he becomes destructive.

Although tending to be nocturnal, weasels are almost as active by day as by night. Their young, numbering 4 to 9, are born in a nest in a burrow and as with other members of the Order Carnivora, are blind, and incapable of looking after themselves at the time of birth. In _Mustela frenata_ of Montana, breeding occurs in July and August, and the young are born in the following April and May. Wright (1948A:342) showed that the gestation period could not have been less than 337 days in one individual and that it averaged 279 (205-337) days in 18 instances. Findings of the same author (1942B:109) showed that the embryos are implanted only 21 to 28 days before the young are born. In the preceding part of the "long gestation period, the embryos lie dormant in the uterus as un-implanted blastocysts. The young female weasel [of _M. frenata_] mates when 3 or 4 months old." Consequently, in the spring, all females of this species may produce young (Wright, 1942A:348). The circumboreal species _Mustela erminea_ likewise has been shown to have a delayed implantation of the ova. Each of these two species, _M. frenata_ and _M. erminea_, has only one litter per year; but the weasel, _Mustela nivalis_, of the Old World seems to lack the delayed implantation, in this respect resembling the ferret (subgenus _Putorius_) as it does also in its ability to have more than one litter per year (see Deanesly, 1944). The manner of reproduction in the South American species _M. africana_ and the circumboreal species _M. rixosa_ at this writing is unknown.

The genus _Mustela_ includes the true weasels, the ferrets and minks. The ferrets commonly are treated as a subgenus, _Putorius_, along with the Old World polecat. The minks usually are accorded subgeneric distinction under the name _Lutreola_, and the true weasels comprise the subgenus _Mustela_, the three subgenera together, along with some other subgenera which are mostly monotypic, comprising the genus _Mustela_. Considered in this way, the group of true weasels, subgenus _Mustela_, has a geographic range roughly coextensive with that of the genus _Mustela_. This range includes Asia and Europe, Northern Africa, North America and northern South America. Java has its weasel. Australia and nearly all the oceanic islands lack weasels, and the animals are absent from roughly the southern half of Africa and the southern half of South America. Other small mustelids, weasellike in shape and with corresponding habits and dentition, take the place of true _Mustela_ in the southern half of Africa and in the corresponding part of South America.

In America the subgenus _Mustela_ occurs from the northernmost land in Arctic America southward to Lake Titicaca in the Andes of South America, a distance of approximately 6900 miles. _Felis_, I think, is the only other genus of land mammals in the western hemisphere that has a geographic range as extensive from north to south. _Felis_ does not range so far north but does range farther south. The one species, _Mustela frenata_, ranges from Lake Titicaca northward to about 57° N in British Columbia or for approximately 5000 miles in a north to south direction and from within the Alpine Arctic Life-zone through the Tropical Life-zone. In North America, weasels occur in almost every type of habitat, being absent only in the extremely desert terrain of western Arizona and western Sonora and in adjoining parts of California and Baja California. Even this area, along the Colorado River, may support some weasels; evidence suggesting that it does so is given in the account of _Mustela frenata neomexicana_.

PALEONTOLOGICAL HISTORY

The paleontological record fails to show the precise ancestry of _Mustela_. The genus has been found in deposits of Pleistocene age, but, so far as I can ascertain, not in deposits of earlier times. The Pleistocene remains are not specifically distinct from Recent (living) species, and in only a few instances (see _M. f. latirostra_ and _M. e. angustidens_) are they even subspecifically distinct from the Recent weasel living in the same area today. It is true that fossil remains from deposits of several stages of the Tertiary beds have in the past been identified in the literature as _Mustela_, but most of these identifications were made many years ago when the generic name _Mustela_ was used in a far broader and more inclusive sense than it is today and much of the fossil material was so fragmentary that the generic identity could not be ascertained, at least at that time. Because the generic identity could not be ascertained, the fossil material was tentatively assigned to the genus _Mustela_, the "typical" genus of the family Mustelidae instead of to some other more specialized or less well-known genus of the family. To satisfy my curiosity about these species of "_Mustela_" of a geological age earlier than the Pleistocene I have personally studied nearly all of the original specimens from North America and have found each to be of some genus other than _Mustela_. Also, such study as I have been able to make of the Old World fossils themselves that have been referred to the genus _Mustela_ up to 1938, and my study of the illustrations and descriptions of the others from there lead to the same conclusion; that is to say, none that is true _Mustela_ is known up to now from deposits older than the Pleistocene.

When, in 1930 (pp. 146-147), I wrote about the taxonomic position of three American genera of fossils (known only from lower jaws), each of which had been previously referred to the genus _Mustela_, I said that they pertained "to that section of the weasel family (Mustelidae) which comprises the polecats, true weasels, ferrets, minks and martens. The fossil specimens . . . are smaller than any other later Tertiary members of the group yet described, and are more primitive than any of the above mentioned Recent relatives. Of the three extinct genera . . . _Miomustela_ [Lower Pliocene or Upper Miocene of the Lower Madison Valley, Montana] is the most primitive and _Martinogale_ [Pliocene, 18 mi. SE Goodland, Sherman County, Kansas] is the most advanced. This view rests largely on the character of M_{=1} which in _Miomustela_ has a deeply basined, short, narrow talonid with a thick, high metaconid situated partly posterior to the protoconid. In _Martinogale_ the talonid is incipiently trenchant, long, broad, and it has a lesser developed metaconid which is situated more anterior [ly]. _Pliogale_ [Lower Pliocene, Humboldt County, Nevada] is intermediate in this respect.

"These three forms are of special interest as possible ancestors of the subgenus _Mustela_, true weasels. No members of this subgenus, nor related forms which can with any degree of certainty be regarded as directly ancestral to them, have yet been described from Miocene or Pliocene deposits. _Palaeogale_ of the Old World and _Bunaelurus_ of North America, each of Oligocene age, have been placed by Schlosser (1888, p. 116) and Matthew (1902, p. 137) as members of the primitive group of mustelids ancestral to _Mustela_. This course seems logical; and with no truly intermediate links between these forms of the Oligocene on the one hand, and _Mustela_ which first appears in the Pleistocene, on the other, more definite statements about ancestral positions of the small Oligocene forms can hardly be made. The deciding considerations for authors who placed _Palaeogale_ and _Bunaelurus_ as ancestral to _Mustela_ were the absence of a metaconid on M_{1} and the trenchant talonid of that tooth. These characters are found also in _Mustela_. On the other hand certain structures in the basicranial region of _Palaeogale_ and more especially of _Bunaelurus_ indicate that these genera possibly are not close to the ancestral form of Mustela . . . _Martinogale_ may stand near the ancestral form of _Mustela_ and . . . _Pliogale_ may be ancestral to _Martinogale_. _Pliogale_, in turn, may have had an ancestor similar to _Miomustela_. If this should prove to be the case, _Palaeogale_ and _Bunaelurus_ might be regarded as an independent branch which displays merely a parallelism to _Mustela_ in the loss of the metaconid on M_{1} and the development of a trenchant talonid on that tooth. The writer would make it clear that he does not hold such to be the case. The ancestral relation of _Martinogale_ to _Mustela_ is presented merely to show the possibility, and not the special probability, of such an origin for _Mustela_. Knowledge of the tympanic bullae and other structures of the basicranial region would go far toward answering the question and until these structures are known [in mustelids of the Later Tertiary,] some uncertainty will remain."

At the present writing I can add to the above statement only a few facts. The discovery of better material of _Bunaelurus_ than was available to previous workers led Simpson (1946), correctly I think, to synonymize _Bunaelurus_ with _Palaeogale_. Simpson figures the cranial foramina in _Palaeogale_. The differences, between _Palaeogale_ and _Mustela_, in cranial foramina, possibly are only the result of the elongation of the tympanic bullae. The bullae of the subgenus _Mustela_ are seen to be much elongated posteriorly if comparison is made with the bullae of earlier mustelids. Consequently, it might be concluded that there is nothing in the arrangement of the cranial foramina which would preclude the derivation of _Mustela_ from _Palaeogale_. However, the anterior situation of the carotid foramen--well forward along the medial margin of the tympanic bulla--is a character typical of other mustelids and the posterior location of this foramen in _Palaeogale_ might indicate that it was not ancestral to _Mustela_.

SKELETON AND DENTITION

The outstanding features of a weasel's skeleton are its length and slenderness. Whereas the length of the vertebral column measured from the atlas (the first cervical vertebra) to the last sacral vertebra is 175 per cent of the length of the hind leg (as measured from the head of the femur to the tip of the longest claw), the corresponding percentage is only 116 in the raccoon. Stated in another way, the vertebral column and the hind leg are of approximately equal length in a raccoon, but in a weasel the vertebral column is one and three-fourths times as long as the hind leg.

VERTEBRAE

The vertebral column consists of 7 cervicals, and ordinarily 14 thoracics, 6 lumbars, 3 sacrals and, depending on the species, 11 to 23 caudals. For the three species of which skeletons were examined, variations from the normal number of vertebrae are noted in the following table:

TABLE I

Data on vertebrae in three species of the subgenus Mustela (Numerals in parentheses indicate number of specimens)

===================+=========+=========+=========
|_Mustela_|_Mustela_|_Mustela_
|_erminea_| _rixosa_|_frenata_
-------------------+---------+---------+---------
Number of cervical | (75) | (12) | (65)
vertebrae | 7 | 7 | 7
-------------------+---------+---------+---------
Number of thoracic | (71) | (12) | (54)
vertebrae | 14 | 14 | 14
+---------+---------+---------
| (4) | | (13)
| 15 | | 15
-------------------+---------+---------+---------
The dorsal vertebra| (18) | (12) | (40)
constituting the | 11th | 11th | 11th
anticlinal +---------+---------+---------
| (7) | | (27)
| 12th | | 12th
-------------------+---------+---------+---------
Number of lumbar | (2) | | (11)
vertebrae | 5 | | 5
+---------+---------+---------
| (73) | (12) | (54)
| 6 | 6 | 6
-------------------+---------+---------+---------
Number of sacral | (9) | | (3)
vertebrae | 2 | | 2
+---------+---------+---------
| (65) | (10) | (67)
| 3 | 3 | 3
+---------+---------+---------
| (1) | (2) |
| 4 | 4 |
-------------------+---------+---------+---------
Number of | (73) | (12) | (57)
pseudosacral | 0 | 0 | 0
vertebrae +---------+---------+---------
| (2) | | (6)
| 1 | | 1
-------------------+---------+---------+---------
| | (1) |
| | 11 |
+---------+---------+---------
| | (3) |
| | 14 |
+---------+---------+---------
| (2) | (7) |
| 15 | 15 |
+---------+---------+---------
| (3) | (1) |
| 16 | 16 |
+---------+---------+---------
| (9) | |
| 17 | |
+---------+---------+---------
Number of caudal | (28) | |
vertebrae | 18 | |
+---------+---------+---------
| (11) | | (6)
| 19 | | 19
+---------+---------+---------
| | | (14)
| | | 20
+---------+---------+---------
| | | (14)
| | | 21
+---------+---------+---------
| | | (7)
| | | 22
+---------+---------+---------
| | | (1)
| | | 23
-------------------+---------+---------+---------

Variation according to the species is evident in the number of caudal vertebrae, but in the other categories of vertebrae no consistent difference in number according to species was found in the material examined. Apparently there is also some geographic variation in the number of caudal vertebrae within a species. For example, the one skeleton seen of _Mustela rixosa eskimo_ (no. 219036, U. S. Nat. Mus., from St. Michaels, Alaska) has only 11 caudal vertebrae, whereas in the 11 _Mustela rixosa rixosa_ from Roseau County, Minnesota, the usual number is 15 with extremes of 14 and 16. Similarly specimens of _Mustela frenata_ from Idaho and California almost always have 1 or 2 more caudal vertebrae than do individuals of the shorter-tailed subspecies of the same species from eastern Kansas.

Of the vertebrae, only the cervicals, of which there are 7, were found to be constant in number. In _M. erminea_, two of the seven individuals in which the anticlinal vertebra was the 12th (instead of the 11th) had 15 instead of the customary 14 thoracic vertebrae. In _M. frenata_, seven of the twenty-seven individuals in which the anticlinal vertebra was the 12th (instead of the 11th) had 15 instead of 14 thoracic vertebrae. The one _M. erminea_ with a pseudosacral vertebra had only two instead of the customary 3 sacral vertebrae but the same individual had 15 thoracic vertebrae. Of the six _M. frenata_ with a pseudosacral vertebra, two animals had only two instead of three sacral vertebrae. Conceivably, therefore, the pseudosacral vertebra in each of the three instances mentioned may represent merely an unfused sacral vertebra, instead of a true pseudosacral as occurs in four individuals of _M. frenata_.

TEETH

In American weasels, for example in _Mustela frenata_, the permanent dentition normally is

I 3 C 1 P 3 M 1
-, -, -, -, -, -, -, - or 34 teeth in all. In most respects the
i 3 c 1 p 3 m 2

dentition is typical for post-Tertiary mustelids but in several parts is highly specialized for a diet of flesh, the degree of this specialization being second only to that of the cats, family Felidae. The outstanding specialization is in the first lower molar, in which, as in the cats, the internal cusp (metaconid) is completely suppressed and the heel (talonid) forms an elevated blade for cutting food rather than a basin for crushing it. In one sense the tooth is simplified since it owes its distinctive form to a reduction in number of parts; nevertheless, the distinctive form of the lower molar clearly is correlated with a diet of flesh, and the tooth is correctly to be thought of as the lower blade of a pair of shears; the upper blade is the fourth upper premolar. The reduction in size of the second (last) lower molar and small size of the inner lobe of the one remaining upper molar probably are additional modifications for a diet of flesh.

The absence of the last two upper molars and last molar in the lower jaw would be expected in any mammal as highly specialized for a diet of flesh as is the weasel, but these teeth are absent also in other Quaternary members of the family Mustelidae, many of which are substantially less specialized for a diet of flesh than is the weasel. Therefore, in the weasel, it is reasonable to regard the absence of these teeth more as a heritage than as an indication of a special adaptation. The absence of a first premolar above and below, as in the weasel, is to be expected in any carnivore that has the first lower molar and fourth upper premolar highly specialized for shearing, but the loss of these premolars and the small size of the second premolars may be as much the result of a slight shortening of the face as it is a result of a lengthening of the third and especially the fourth premolars. The lengthening of these more posteriorly-situated teeth would appear to be an adaptation to a diet of flesh. The cause of the lengthening of the mentioned teeth and the reason for the absence of the first premolars probably will be unknown until the fossil record is more complete.

The teeth of American species vary little except in size. The absence of P2 in _Mustela africana_ is the only difference of a qualitative (presence or absence) nature that was detected. Also, the Central American subspecies of _Mustela frenata_ exhibit a tendency to early loss of P2 and thus foreshadow the condition typical of _M. africana_.

As a whole the dentition of the weasel exhibits a high degree of specialization for a diet of flesh and this specialization is fully as evident in the deciduous dentition as in the permanent dentition.

The deciduous, or milk, dentition, of _Mustela frenata_, as known from immature specimens of _Mustela frenata noveboracensis_ and _Mustela frenata frenata_ available for this study, is comprised of canines, one on each side above and below, and 3 cheek teeth on each side above and below. See figures 2-9. The upper cheek teeth from anterior to posterior are: a minute peglike tooth in general similar to the first premolar of the permanent dentition; a shearing tooth in general similar to P4 of the permanent dentition; and an anteroposteriorly compressed tooth in general similar to M1 of the permanent dentition. In the lower jaw, behind the canine, there is first a minute peglike tooth, second a two-rooted tooth similar in general outline to a permanent third premolar, and finally a shearing tooth corresponding in function to m1 of the permanent dentition.

No postnatal specimens which show deciduous incisors have been examined.

Selected, outstanding differences between the permanent teeth and the deciduous teeth are as follows: In the deciduous teeth the canine above has on the posterior face a well-defined ridge extending from the tip to the cingulum. This ridge is absent or at most faintly indicated in the permanent tooth. The lower deciduous canine, in cross section is seen to have a marked indentation on the anteromedial border in the region of the cingulum; this indentation is lacking in the permanent tooth. The anterior one of the deciduous cheek teeth, both above and below, is single rooted and its crown-surface is only about one-fifteenth as much as that of the anterior premolar of the permanent dentition. The second deciduous cheek tooth below has two roots, usually fused, and differs from p4 of the permanent dentition in having the tip of the principal cusp more recurved, in having the anterior basal cusp better developed and the posterior heel less well developed.

The second deciduous cheek tooth above corresponds in function and general plan of construction to P4 of the permanent dentition but differs from that tooth in the more pronounced protostyle, longer tritocone, more posteriorly located deuterocone and as noted by Leche (1915:322) separation of the protocone and tritocone by a notch. The third upper deciduous tooth has a single cusp internally and two cusps laterally. Thus it reverses the relation of parts seen in M1 where the internal moiety is larger than the lateral or buccal moiety. The third deciduous tooth below differs from m1 in very much shorter talonid and separation of the paraconid from the protoconid by a deeper notch.

All the features in which the last two deciduous teeth, both above and below, are described as differing from their functional counterparts in the permanent dentition, are features found in the permanent teeth of primitive fossil mustelids and certain fossil and Recent viverrids. Even so, taking into account Leche's (1915) work, which shows that the milk teeth of some carnivores have structures lacking in the corresponding permanent teeth of the same individual animal and also in the teeth of genera that seem to be ancestral, a person suspects that some of the structural features mentioned above are not inheritances of ancestral conditions but rather specializations of the milk dentition.

Permanent dentition × 3. No. 32421, Mus. Vert. Zoöl., [M], adult; Berkeley, Alameda County, California; obtained October 4, 1921, by D. D. McLean.

Deciduous dentition × 5. No. 132158, U. S. Nat. Mus., [M], juvenile; Stanford University, Santa Clara County, California; obtained May 7, 1898, by W. K. Fisher.

Figs. 2-3. Lateral views of upper teeth, of adult and juvenile respectively.

Figs. 4-5. Occlusolingual views of upper teeth of adult and juvenile respectively.

Figs. 6-7. Lateral views of lower teeth of adult and juvenile respectively.

Figs. 8-9. Occlusolingual views of lower teeth of adult and juvenile respectively.]

In other deciduous teeth there is clearer evidence of more specialization for a diet of flesh in the deciduous teeth than in the permanent teeth. For example, the upper carnassial of the milk dentition is even more highly sectorial than is the permanent tooth and strikingly like that of some of the cats. The lower tooth that is effective in the shearing action bears no more trace of the metaconid than does the permanent first lower molar. These features of the deciduous dentition suggest that it is more specialized for a diet of flesh than is the permanent dentition. If this be the fact, it may seem especially remarkable because the commonly employed term "milk teeth" suggests that the animal makes but little or no use of these teeth in the short time that they are in place. Accordingly, the student may credit the form of these teeth more to some indirect effects of inheritance than to natural selection acting directly upon the teeth. But, after all, natural selection probably is responsible for the form of these teeth as is indicated by the observations of Hamilton (1933:318-325). He found that these milk teeth are used for eating solid food as soon as the principal shearing teeth are in place. This is three weeks after birth and before all of the deciduous teeth have broken through the gums. These shearing teeth are used for almost two months before being replaced by the permanent teeth and it is, therefore, evident that natural selection could operate to fully as great a degree in determining the form of the deciduous teeth as it may with the permanent teeth.

Hamilton (1933:325-326) found that the permanent dentition was complete at 75 days after birth in captive specimens of _Mustela frenata noveboracensis_. In the same subspecies, he noted 28 days after birth that the canines and carnassial teeth [second deciduous cheek tooth above and third below] had erupted through the gums. Animals 45 days old, Hamilton found, were losing the milk dentition, and had the gums broken through by several of the permanent cheek teeth.

Study of the cleaned skulls available of juveniles indicates that the deciduous teeth which persist longest are, on each side of the mouth, the second cheek tooth above and the third cheek tooth below. These teeth persist until after the permanent P4 and m1 have come into use. These permanent teeth are situated immediately behind their functional counterparts of the milk dentition. P3 and p4 are the teeth of the permanent dentition which ultimately push out the last milk teeth to be lost. Accordingly, in the permanent dentition, P4 and M1 appear before P3 does, and m1 and m2 make their appearance before p4.

DISPARITY IN NUMBERS OF MALES AND FEMALES (IN ZOOLOGICAL COLLECTIONS)

The question has frequently been asked why twice as many male as female weasels are captured. This is the proportion in research collections, as may be seen from table no. 2, and I am convinced that the specimens in these collections are saved in approximately the same proportion as that in which they are caught. Although it might be assumed, upon first consideration, that there are twice as many males as females in nature, selective factors enter into the catch. For example, because a male weasel is approximately twice as heavy as a female, it may be necessary for him, in a given length of time, to travel twice as far as the female to obtain the required amount of food with the result that a given number of traps or snares will catch twice as many males as females. Indeed, Glover (1943B:8) shows that, on the average, in _Mustela frenata noveboracensis_ in Pennsylvania, the male actually does travel slightly more than twice as far as the female (704 feet versus 346 feet). From table no. 2, it may be seen that in most winter months the ratio is 3 males to one female. This ratio is reasonable enough, in view of what has been said, if it is considered also that the lighter weight of the female permits her safely to step on the pans of traps that would be sprung by heavier males.

If in the breeding season, which is April through August in _M. frenata_, the female is passive and if the male is restlessly searching for her, he may thus increase still more his chances of being caught in traps set for weasels.

My own studies of live weasels in nature indicate that in the season when females are attending young which are half grown, or larger, the adult male weasels live singly in dens of their own, separate and apart from the females and their young (Hamilton, 1933:328, records adult males living with the female and her young, but possibly this was when the young were less than half grown). Perhaps these males at that time travel no farther than is necessary to obtain food for themselves. Females, at this time, forage not only to meet their own needs, but for food to supply their young as well. At this time, in May and June, as may be seen from table no. 2, almost as many adult females as adult males _are_ caught. The reason why only relatively more females than in other months, instead of actually more females than males, are caught at this time probably is that the adult males also are extraordinarily active at this time because they are in breeding condition. Perhaps the explanation in part is to be found in the lesser weight of the female (approximately half of the male's weight) which, as indicated above, permits her to step on the pan of a steel trap without springing it whereas the heavier male does spring the trap and as a consequence is caught. Hamilton (1933:299-300), who mentions this selective factor, found an equal number of males and females in the three newly born litters that came under his observation.

TABLE 2

Specimens of _Mustela frenata_ (north of the range of _M. f. frenata_) arranged by sex and under each sex by age

KEY:
A: adult [M]
B: [M] ad., % of total adults
C: subadult [M]
D: young [M]
E: juvenal [M]
F: total number of [M]
G: [M] % of total
H: adult [F]
I: [F] ad., % of total adults
J: subadult [F]
K: young [F]
L: juvenal [F]
M: total number of [F]
N: [F], % of total
O: total number of [M] and [F]
P: total number of adults, [M] and [F]

/-----------Male---------\/--------Female--------\
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
| A | B | C | D | E| F | G| H | I| J| K | L| M | N| O | P
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
May | 29| 55| 4| 14| 7| 54|59| 24|45| 1| 9| 3| 37|41| 91| 53
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
June | 42| 53| 14| 40| 8| 97|59| 38|47| 4| 25| 2| 69|41| 166| 80
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
July | 59| 70| 18| 55| 2|130|59| 25|30| 5| 58| 2| 90|41| 220| 84
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
August | 40| 77| 23| 55|..|113|74| 12|23| 2| 25|..| 39|26| 152| 52
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
September| 15| 79| 25| 12| 1| 51|75| 4|21| 4| 9|..| 17|25| 68| 19
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
October | 11| 58| 46| 7|..| 43|66| 8|42|13| 1|..| 22|34| 65| 19
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
November | 41| 70| 48| 1|..| 88|73| 18|30|12| 2| 1| 33|27| 121| 59
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
December | 59| 69| 43| 1|..|108|73| 26|31|15|...|..| 41|27| 149| 85
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
January | 80| 69| 32| 2| 1|126|72| 36|31|14|...|..| 50|28| 176|116
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
February | 45| 66| 19| 5|..| 82|73| 23|34| 4| 3|..| 30|27| 112| 68
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
March | 38| 72| 2|...|..| 57|70| 15|28| 8| 1|..| 24|30| 81| 53
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
April | 30| 67| 2| 4| 3| 39|67| 15|33|..| 2| 2| 19|33| 58| 45
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---
Totals |489| 67|281|196|22|988|68|244|33|82|135|10|471|32|1,459|733
---------+---+---+---+---+--+---+--+---+--+--+---+--+---+--+-----+---

I suppose that in nature there are approximately equal numbers of male and female weasels and further suppose that the selective factors which cause more males than females to be caught are the greater distances traveled by the males and their greater weight.

MATERIALS, ACKNOWLEDGMENTS AND METHODS

At a late stage in the preparation of this manuscript a total of 5,457 specimens had been examined. For the most part these were conventional study-specimens; that is to say, they were stuffed skins with the skulls separate and each was accompanied by the customary data as to locality of capture, date of capture, name of collector, external measurements and sex recorded on the labels by the collectors. Skulls unaccompanied by skins, nevertheless, comprised a large share of the total and a small proportion was made up of skins unaccompanied by skulls, mounted specimens, skeletons, and entire animals preserved in liquid.

It was the recognition of this need for specimens from extensive
areas from which no specimens previously had been collected that
influenced me, approximately a year after the study was begun, to
allot for it a long span of time. The procedure adopted, in
general, was to study the weasels of one species from a given
geographic area in so far as the material warranted, then lay this
aside until additional critical material could be obtained, and
finally, some months or a year later, complete the account. In
this fashion the manuscript of the American weasels received my
attention in each of the past twenty-five years (September, 1926
to date of publication). This is a confession of fact rather than
a recommendation of procedure. This type of procedure unduly
delays the diffusion of knowledge and for a variety of reasons
justifiably annoys other students of the subject. Nevertheless,
many gaps have been filled that otherwise would have remained
open. Although specimens to solve several problems still remain to
be collected and studied, it seems that a point of diminishing
returns has now been reached, which, in fairness to all concerned,
calls for publication of the results so far obtained.

For assistance in the entire undertaking, I am more indebted to
Miss Annie M. Alexander than to any other one person; she provided
the means by which specimens from critical areas were obtained,
made it possible to examine the European collections, and assisted
in other ways. The late Professor Joseph Grinnell and Mr. Charles
D. Bunker, among others, gave truly valuable encouragement and
assistance.

Collections containing weasels which were examined in the study
here reported upon were as follows:

Acad. Nat. Sciences of Philadelphia American Mus. Nat. History Baylor University Berlin Zoological Museum Boston Society of Natural History Brigham Young University British Museum of Natural History California Academy of Sciences Carnegie Museum Charleston Museum Coe College Collection of J. Arnold Collection of Stanley C. Arthur Collection of Rollin H. Baker Collection of William Bebb Collection of R. H. Coleman Collection of Ian McTaggart-Cowan Collection of Stuart Criddle Collection of John Cushing Collection of Walter W. Dalquest Collection of William B. Davis Collection of J. M. Edson Collection of Ralph Ellis Collection of John Fitzgerald, Jr. Collection of Mr. Green Collection of Ross Hardy Collection of Donald V. Hemphill Collection of L. M. Huey Collection of R. W. Jackson Collection of Stanley G. Jewett Collection of E. J. Koestner Collection of J. E. Law Collection of A. H. Miller Collection of Lloye H. Miller Collection of R. D. Moore Collection of J. A. Munro Collection of O. J. Murie Collection of Robert T. Orr Collection of Arthur Peake Collection of Kenneth Racey Collection of William B. Richardson Collection Rocky Mt. Spotted Fever Lab. Collection of Victor B. Scheffer Collection of William T. Shaw Collection of O. P. Silliman Collection of W. E. Snyder Collection of Frank Stephens Collection of T. C. Stephens Collection of D. D. Stone Collection of Myron H. Swenk Collection of Joe and Dean Thiriot Collection of John Tyler Collection of Jack C vonBloeker Collection of Alex Walker Collection of Edward R. Warren Colorado Museum of Natural History Charles R. Conner Museum Cornell University Donald R. Dickey Collection Field Museum of Natural History Florida State Museum Fresno State Junior College Humboldt State Teachers College Illinois Natural History Survey Iowa State College Iowa Wesleyan College Kansas State Agric. College Leland Stanford Junior University Leningrad Academy of Science Los Angeles Mus. Hist. Art and Sci. Louisiana State University Mt. Rainier Nat'l Park Collection Museum of Comparative Zoölogy Mus. Polonais d'Hist. Nat., Warsaw Mus. Vert. Zoöl., Univ. California Museum of Zoölogy, Univ. Michigan National Museum of Canada Naturhistoriska Ricksmuseum, Sweden Neuchatel University Museum New York State Museum Ohio State Museum Oklahoma Agric. and Mech. College Ottawa University, Kansas Paris Museum Provincial Museum of British Columbia Royal Ontario Museum of Zoölogy San Diego Society of Natural History State Hist. and Nat. Hist. Soc. Colo. State Normal School, Cheney, Wash. Texas Cooperative Research Collection United States National Museum University of Arkansas Univ. California Mus. Palaeo. University of Idaho Univ. Kansas Mus. Nat. History University of Minnesota University of Notre Dame University of Oklahoma University of Oregon University of South Dakota University of Utah Univ. Washington Museum of Zoölogy University of Wisconsin Univ. Zool. Mus., Copenhagen

The largest single collection is in the United States National
Museum, where the specimens of the National Museum proper and the
United States Biological Surveys Collection, together, provide
essential materials including a large share of the holotypes.
Specimens in all of the North American collections including
Canada and México have been made available, by loan, and in 1937
materials were examined in the principal collections of northern
and central Europe. After the materials in North American
collections were assembled, special effort, with considerable
success, was made in each of several winters, to obtain specimens
from areas not previously represented in collections.

To the many persons who were in charge of the collections
consulted, to those who at my request sought critical specimens,
and to those who assisted in various stages of assembling data and
in preparation of the manuscript, I am grateful indeed. Likewise,
I am deeply appreciative of the grants-in-aid received from the
Carnegie Institution of Washington, the University of California
Chapter of Sigma Xi, the John Simon Guggenheim Memorial Foundation
and the Kansas University Endowment Association. I am mindful also
of an obligation to those who appropriated funds, by legislative
action, for research use by The University of California and The
University of Kansas.

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

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

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

VARIATION

Variation with Age

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

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

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

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

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

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

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

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

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

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

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

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

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

Secondary Sexual Variation

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

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

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

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

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

Individual Variation

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