Transcribers Notes:
Italics words are denoted by _underscores_. Bold words are denoted by =equals=.
Whole and fractional parts are displayed as 7-3/4.
Greek text has been transliterated and are denoted by ~tildes~.
Male and Female symbols are represented by [M] and [F] respectively.
UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Vol. 4, pp. 1-466, plates 1-41, 31 figures in text
December 27, 1951
AMERICAN WEASELS
BY
E. RAYMOND HALL
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, plates 1-41, 31 figures in text
December 27, 1951
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1951
23-3758
[Illustration: 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. 2 and 3) 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. 3 to 7) 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, and
_vice 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 page 51.]
[Illustration: FIG. 1. Map showing localities of capture of specimens
depicted in plate 1.]
American Weasels
BY
E. RAYMOND HALL
CONTENTS
PAGE
INTRODUCTION 7
PALEONTOLOGICAL HISTORY 10
SKELETON AND DENTITION 12
DISPARITY IN NUMBERS OF MALES AND FEMALES 19
MATERIALS, ACKNOWLEDGMENTS AND METHODS 21
VARIATION 24
Variation with Age 24
Secondary Sexual Variation 26
Individual Variation 28
Seasonal Variation 30
Variation in Coloration and Molt 30
Variations of Taxonomic Worth 44
DISTRIBUTION AND SPECIATION 54
HISTORY OF CLASSIFICATION 69
Chronological List (annotated) of Specific and Subspecific
Names Applied to American Weasels 71
CHECK-LIST OF AMERICAN SPECIES AND SUBSPECIES OF THE GENUS
MUSTELA 81
ARTIFICIAL KEY TO AMERICAN SPECIES OF THE GENUS MUSTELA 83
DIAGNOSIS OF THE GENUS 83
EXPLANATION OF SYSTEMATIC TREATMENT 84
SYSTEMATIC ACCOUNTS OF SPECIES AND SUBSPECIES 87
_Mustela erminea_ 87
_Mustela rixosa_ 168
_Mustela frenata_ 193
_Mustela africana_ 406
EXPLANATION OF CRANIAL MEASUREMENTS 417
TABLE OF CRANIAL MEASUREMENTS 418
LITERATURE CITED 442
INDEX 461
American Weasels
By E. RAYMOND HALL
INTRODUCTION
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.
[Illustration: FIGS. 2-9. Views of permanent and deciduous teeth of
_Mustela frenata nigriauris_. Incisors not shown. In each instance
teeth are of the left side.
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.