The Project Gutenberg eBook ofAmerican WeaselsThis ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.Title: American WeaselsAuthor: E. Raymond HallRelease date: July 21, 2013 [eBook #43272]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Richard Tonsing, Joseph Cooperand the Online Distributed Proofreading Team athttp://www.pgdp.net*** START OF THE PROJECT GUTENBERG EBOOK AMERICAN WEASELS ***
This ebook is for the use of anyone anywhere in the United States and most other parts of the world at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this ebook or online atwww.gutenberg.org. If you are not located in the United States, you will have to check the laws of the country where you are located before using this eBook.
Title: American WeaselsAuthor: E. Raymond HallRelease date: July 21, 2013 [eBook #43272]Most recently updated: October 23, 2024Language: EnglishCredits: Produced by Chris Curnow, Richard Tonsing, Joseph Cooperand the Online Distributed Proofreading Team athttp://www.pgdp.net
Title: American Weasels
Author: E. Raymond Hall
Author: E. Raymond Hall
Release date: July 21, 2013 [eBook #43272]Most recently updated: October 23, 2024
Language: English
Credits: Produced by Chris Curnow, Richard Tonsing, Joseph Cooperand the Online Distributed Proofreading Team athttp://www.pgdp.net
*** START OF THE PROJECT GUTENBERG EBOOK AMERICAN WEASELS ***
University of Kansas Publications
Museum of Natural History
Vol. 4, pp. 1-466, plates1-41, 31 figures in text
December 27, 1951
BY
University of Kansas
Lawrence
1951
University of Kansas Publications, Museum of Natural History
Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Edward H. Taylor, Robert W. Wilson
Vol. 4, pp. 1-466, plates1-41, 31 figures in text
December 27, 1951
University of Kansas
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1951
23-3758
Plate 1.Coloration of head and foreparts in ten subspecies of long-tailed weasel,Mustela frenata. All figures are of males, approximately × 1/2.In regions of heavy rainfall (see figs.2and3) there is an increase in pigmentation and extent of blackish color backward over the neck and a decrease in extent of the white facial markings. In regions progressively more arid (see figs.3to7) there is a decrease in pigmentation and extent of blackish color and an increase in extent of the white facial markings.As shown by rearing mammals from humid regions in arid regions, andvice versa, the color is not visibly altered in one or a few generations; the color is an hereditary character. Beginning with the southernmost subspecies (fig.1) and continuing northward to the northern subspecies (fig.10) there is a darkening, next a lightening, and finally a darkening closely conforming to amounts of precipitation in the geographic regions concerned. A fuller discussion of this correlation is given on page51.
Plate 1.Coloration of head and foreparts in ten subspecies of long-tailed weasel,Mustela frenata. All figures are of males, approximately × 1/2.In regions of heavy rainfall (see figs.2and3) there is an increase in pigmentation and extent of blackish color backward over the neck and a decrease in extent of the white facial markings. In regions progressively more arid (see figs.3to7) there is a decrease in pigmentation and extent of blackish color and an increase in extent of the white facial markings.As shown by rearing mammals from humid regions in arid regions, andvice versa, the color is not visibly altered in one or a few generations; the color is an hereditary character. Beginning with the southernmost subspecies (fig.1) and continuing northward to the northern subspecies (fig.10) there is a darkening, next a lightening, and finally a darkening closely conforming to amounts of precipitation in the geographic regions concerned. A fuller discussion of this correlation is given on page51.
Coloration of head and foreparts in ten subspecies of long-tailed weasel,Mustela frenata. All figures are of males, approximately × 1/2.
In regions of heavy rainfall (see figs.2and3) there is an increase in pigmentation and extent of blackish color backward over the neck and a decrease in extent of the white facial markings. In regions progressively more arid (see figs.3to7) there is a decrease in pigmentation and extent of blackish color and an increase in extent of the white facial markings.
As shown by rearing mammals from humid regions in arid regions, andvice versa, the color is not visibly altered in one or a few generations; the color is an hereditary character. Beginning with the southernmost subspecies (fig.1) and continuing northward to the northern subspecies (fig.10) there is a darkening, next a lightening, and finally a darkening closely conforming to amounts of precipitation in the geographic regions concerned. A fuller discussion of this correlation is given on page51.
Fig. 1.Map showing localities of capture of specimens depicted in plate1.
Fig. 1.Map showing localities of capture of specimens depicted in plate1.
Fig. 1.Map showing localities of capture of specimens depicted in plate1.
American Weasels
BY
E. RAYMOND HALL
PAGEIntroduction7Paleontological History10Skeleton and Dentition12Disparity in Numbers of Males and Females19Materials, Acknowledgments and Methods21Variation24Variation with Age24Secondary Sexual Variation26Individual Variation28Seasonal Variation30Variation in Coloration and Molt30Variations of Taxonomic Worth44Distribution and Speciation54History of Classification69Chronological List (annotated) of Specific and Subspecific Names Applied to American Weasels71Check-List of American Species and Subspecies of the Genus Mustela81Artificial Key to American Species of the Genus Mustela83Diagnosis of the Genus83Explanation of Systematic Treatment84Systematic Accounts of Species and Subspecies87Mustela erminea87Mustela rixosa168Mustela frenata193Mustela africana406Explanation of Cranial Measurements417Table of Cranial Measurements418Literature Cited442Index461
American Weasels
ByE. Raymond Hall
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 nameSchönthierlein(pretty little creature) and the Italian namedonnola(little lady) were bestowed upon it. The Spanish name iscomadreja(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 page214) 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. InMustela frenataof 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 [ofM. 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 speciesMustela erminealikewise has been shown to have a delayed implantation of the ova. Each of these two species,M. frenataandM. 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 (subgenusPutorius) 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 speciesM. africanaand the circumboreal speciesM. rixosaat this writing is unknown.
The genusMustelaincludes 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 nameLutreola, and the true weasels comprise the subgenusMustela, the three subgenera together, along with some other subgenera which are mostly monotypic, comprising the genusMustela. Considered in this way, the group of true weasels, subgenusMustela, has a geographic range roughly coextensive with that of the genusMustela. 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 trueMustelain the southern half of Africa and in the corresponding part of South America.
In America the subgenusMustelaoccurs 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.Felisdoes 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 ofMustela frenata neomexicana.
The paleontological record fails to show the precise ancestry ofMustela. 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 (seeM. f. latirostraandM. 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 asMustela, but most of these identifications were made many years ago when the generic nameMustelawas 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 genusMustela, 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 thanMustela. Also, such study as I have been able to make of the Old World fossils themselves that have been referred to the genusMustelaup 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 trueMustelais 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 genusMustela, 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 andMartinogale[Pliocene, 18 mi. SE Goodland, Sherman County, Kansas] is the most advanced. This view rests largely on the character of M=1which inMiomustelahas a deeply basined, short, narrow talonid with a thick, high metaconid situated partly posterior to the protoconid. InMartinogalethe 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 subgenusMustela, 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.Palaeogaleof the Old World andBunaelurusof 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 toMustela. This course seems logical; and with no truly intermediate links between these forms of the Oligocene on the one hand, andMustelawhich 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 placedPalaeogaleandBunaelurusas ancestral toMustelawere the absence of a metaconid on M1and the trenchant talonid of that tooth. These characters are found also inMustela. On the other hand certain structures in the basicranial region ofPalaeogaleand more especially ofBunaelurusindicate that these genera possibly are not close to the ancestral form of Mustela . . .Martinogalemay stand near the ancestral form ofMustelaand . . .Pliogalemay be ancestral toMartinogale.Pliogale, in turn, may have had an ancestor similar toMiomustela. If this should prove to be the case,PalaeogaleandBunaelurusmight be regarded as an independent branch which displays merely a parallelism toMustelain the loss of the metaconid on M1and 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 ofMartinogaletoMustelais presented merely to show the possibility, and not the special probability, of such an origin forMustela. 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 ofBunaelurusthan was available to previous workers led Simpson (1946), correctly I think, to synonymizeBunaeluruswithPalaeogale. Simpson figures the cranial foramina inPalaeogale. The differences, betweenPalaeogaleandMustela, in cranial foramina, possibly are only the result of the elongation of the tympanic bullae. The bullae of the subgenusMustelaare 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 ofMustelafromPalaeogale. 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 inPalaeogalemight indicate that it was not ancestral toMustela.
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.
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 1Data on vertebrae in three species of the subgenusMustela(Numerals in parentheses indicate number of specimens)
Mustela ermineaMustela rixosaMustela frenataNumber of cervical vertebrae7(75)7(12)7(65)Number of thoracic vertebrae14(71)14(12)14(54)15(4)15(13)The dorsal vertebraconstituting the anticlinal11th(18)11th(12)11th(40)12th(7)12th(27)Number of lumbar vertebrae5(2)5(11)6(73)6(12)6(54)Number of sacral vertebrae2(9)2(3)3(65)3(10)3(67)4(1)4(2)Number of pseudosacral vertebrae0(73)0(12)0(57)1(2)1(6)11(1)14(3)15(2)15(7)16(3)16(1)17(9)Number of caudal vertebrae18(28)19(11)19(6)20(14)21(14)22(7)23(1)
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 ofMustela rixosa eskimo(no. 219036, U. S. Nat. Mus., from St. Michaels, Alaska) has only 11 caudal vertebrae, whereas in the 11Mustela rixosa rixosafrom Roseau County, Minnesota, the usual number is 15 with extremes of 14 and 16. Similarly specimens ofMustela frenatafrom 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. InM. 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. InM. 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 oneM. ermineawith a pseudosacral vertebra had only two instead of the customary 3 sacral vertebrae but the same individual had 15 thoracic vertebrae. Of the sixM. frenatawith 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 ofM. frenata.
In American weasels, for example inMustela frenata, the permanent dentition normally is
I 3 C 1 P 3 M 1-, -, -, -, -, -, -, -i 3 c 1 p 3 m 2
or 34 teeth in all. In most respects the 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 inMustela africanais the only difference of a qualitative (presence or absence) nature that was detected. Also, the Central American subspecies ofMustela frenataexhibit a tendency to early loss of P2 and thus foreshadow the condition typical ofM. 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, ofMustela frenata, as known from immature specimens ofMustela frenata noveboracensisandMustela frenata frenataavailable 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 figures2-9. 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.
Figs. 2-9.Views of permanent and deciduous teeth ofMustela frenata nigriauris. Incisors not shown. In each instance teeth are of the left side.Permanent dentition × 3. No. 32421, Mus. Vert. Zoöl., ♂, adult; Berkeley, Alameda County, California; obtained October 4, 1921, by D. D. McLean.Deciduous dentition × 5. No. 132158, U. S. Nat. Mus., ♂, 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.
Figs. 2-9.Views of permanent and deciduous teeth ofMustela frenata nigriauris. Incisors not shown. In each instance teeth are of the left side.Permanent dentition × 3. No. 32421, Mus. Vert. Zoöl., ♂, adult; Berkeley, Alameda County, California; obtained October 4, 1921, by D. D. McLean.Deciduous dentition × 5. No. 132158, U. S. Nat. Mus., ♂, 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.
Figs. 2-9.Views of permanent and deciduous teeth ofMustela frenata nigriauris. Incisors not shown. In each instance teeth are of the left side.
Permanent dentition × 3. No. 32421, Mus. Vert. Zoöl., ♂, adult; Berkeley, Alameda County, California; obtained October 4, 1921, by D. D. McLean.
Deciduous dentition × 5. No. 132158, U. S. Nat. Mus., ♂, 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 ofMustela 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.
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, inMustela frenata noveboracensisin 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 inM. 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 malesarecaught. 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.