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.
In skulls of weasels, the individual variation in size is more than it
is in relative proportions. Hensel (_op. cit._) has stressed that
weasels, like other carnivores, produced "dwarfed" individuals more
than do herbivorous mammals. I cannot vouch for the accuracy of this
view, but can say that individual variation is not greater than in some
other fissiped carnivores. Impressions to the contrary probably result
largely from failure to recognize age-variation. When skulls of a large
series from any one locality are arranged first by sex, and under each
sex according to probable age on the basis of extension anteriorly of
the sagittal crest and of degree of postorbital constriction,
individual variation is seen to be less than a cursory examination,
even of only one sex, would suggest.
Study of a large series of one age of one sex of one species from one
locality shows that some parts, of the skull for example, vary more
than other parts. In illustration, among 22 male topotypes of _Mustela
frenata washingtoni_ the least interorbital breadth varied 25 per cent
(9.0 mm. to 12 mm.) whereas the length of the tooth-rows varied only
13.3 per cent (15.6 mm. to 18.0 mm.). In color the individual variation
definitely is more in areas of intergradation between subspecies than
in other areas. Details of one such instance of intergradation are
given in the account of _Mustela frenata spadix_.
Statements to the effect that there is much individual variation in the
color of weasels, were made mostly fifty years or so ago by writers who
had but few specimens from widely separated localities. Where marked
climatic differences exist between localities only a few miles apart,
marked differences occur in coloration of the weasels from the
different localities. Much of what formerly was mistaken for individual
variation now proves to be geographic variation. Individual variation
actually is of slight amount in comparison with that in mammals
generally. Differences in size and relative proportions of parts
usually are correlated with geographic differences in color. The color
does fade slightly in the period between molts. Also as a result of the
seasonal color change, in autumn along the upper margin of the Austral
Life-zone, some individuals become white whereas others become white on
only the underparts, the upper parts changing only to lighter brown.
Probably it would be correct to say that this variation was a
combination of seasonal and individual variation rather than either one
alone.
As might be supposed, individual variation is not the same in all
species or subspecies. For example, p2 is always absent in _Mustela
africana_ and always present in certain subspecies of _M. frenata_. In
some other subspecies of _M. frenata_, p2 is absent approximately as
often as present. In the writer's experience, when only a few specimens
are available for comparison, individual variation is more difficult to
distinguish from specific and subspecific (geographic) variation than
is age-variation or secondary sexual variation.
Among the larger series of specimens examined, only one instance of
what might be called a mutation in the old sense of a large, sudden
change, was detected. That was the loss of the second lower molar in
many (less than a third) of the specimens from Newfoundland. The six
instances of abnormal coloration described on pages 41 to 43, might be
regarded as mutations of large magnitude but no evidence was found of
repetition of an abnormality in any one population. Otherwise, in
every instance where plotted, the manifestations of a variation
arranged themselves about the mean in such a way as to form a smooth,
unimodal curve.
Seasonal Variation
When subspecific and specific variations are the objectives of study,
seasonal variation must be understood, in order to be excluded from
consideration, in the same way that variations ascribable to age, sex
and individualism must be understood in order to be excluded from
consideration. In weasels, change in color of the pelage is the
seasonal variation most important for the systematist to understand.
Other seasonal variations in the pelage are hairiness versus nakedness
of the pads of the feet, length of the pelage on the body, and possibly
the density of the pelage on the body. In the northern half of North
America, roughly speaking, seasonal change in color is so pronounced
(white in winter and brown in summer) as to be easily recognized. South
of this area, in the Austral and Sonoran life-zones, the color of the
winter pelage differs only slightly from that of the summer pelage. In
these more southern latitudes the winter pelage in almost all
subspecies is of lighter color than the summer pelage and has a smoky
suffusion. With material of the two seasons in hand for comparison,
close attention to the variation will permit the systematist to
recognize the difference in shade of brown as seasonal variation and
not geographic or specific variation. Farther south still, in the
Tropical Life-zone, seasonal difference in color was not detected in
the material studied. Seasonal change in color is discussed in the
section immediately following.
Variation in Coloration and Molt
In all American weasels (subgenus _Mustela_) the color, at least in
summer, is brown with more or less white or whitish on the underparts.
In one species, _Mustela africana_, there is a longitudinal stripe of
brown on the middle of the light-colored underparts; this stripe is
absent in each of the other three American species. Two species, _M.
erminea_ and _M. frenata_, always have a black tip on the tail. Of the
other two species, _M. africana_ lacks the black tip and _M. rixosa_
may or may not have a few black hairs in the tip of its tail. White or
light yellowish facial markings occur in subspecies of _M. frenata_
from the southwestern United Stated to Central America. Subspecies
having the most extensive light-colored facial markings have the
remainder of the upper part of the head black. In weasels without light
facial markings the upper parts of the head all are brown. In the two
species, _M. erminea_ and _M. frenata_, the extent to which the light
color of the underparts extends down the insides of the legs and out on
the underside of the tail, or the absence of light color on these
parts, is a matter of geographic variation. The same can be said for
_M. rixosa_ except that first its tail is unicolored and second
individual variation as well as geographic variation accounts for the
color pattern on the underparts and legs in animals from the
southeastern part of the range of the species.
The most remarkable feature of the coloration of weasels is the winter
whitening. This occurs in the northern part of North America in each of
the three species of weasels found on that continent. The black tip of
the tail in _M. erminea_ and _M. frenata_ remains black in winter. If
an individual of _M. rixosa_ has black hairs on the tip of its tail in
summer, there are thought to be black hairs there also in winter.
Otherwise the winter pelage is all white in northern areas in each of
the three species. In this white winter coat the animal is known as
ermine.
The underlying cause seems to be protective coloration. At any rate,
weasels are always white in winter if they are from areas where snow
lies on the ground all winter, every winter, or almost every winter;
and they are always brown if from areas where there is never, or
rarely, snow in winter. The changes in color are effected by molt, one
in autumn and one in spring. Animals that are brown in winter undergo
the same two molts as do those that are white in winter. The capacity
to acquire a white coat or a brown coat in winter is an hereditary
matter just as one man grows red hair and another grows black hair. In
the weasels, however, all individuals in the north turn white in winter
and if one that was born there is kept through successive winters in
the warmer south where there is no snow, he will still turn white each
winter. A weasel born in a southern area, where all are brown in
winter, molts into a brown (not white) winter coat even when kept in a
cold, snowy, northern area where native weasels of the same species all
turn white. Obviously, therefore, neither snow nor temperature is an
immediate cause and, as we have said, the color in winter is a matter
of heredity. The time of the molt, we now know, is determined by the
amount of light. When nights grow longer and days shorter, a point is
reached at which the lesser light received through the eyes causes the
pituitary gland to cease producing a gonadotropic hormone. Directly or
indirectly, the lack of this hormone stimulates molt and, probably
enzyme action, or the lack of it, causes the melanoblasts of the cells
in the hair follicle to be without pigment. Hence the hair grown from a
follicle under such conditions lacks pigment (melanin) and is white. In
spring, as the days grow longer and the nights shorter, the increasing
amount of light received day by day through the eyes stimulates the
pituitary gland to produce the gonadotropic hormone which directly or
indirectly, stimulates molt and, probably by enzyme action, the
melanoblasts are caused to be present in cells of the hair follicle and
the melanoblasts provide granules of melanin pigment which are
incorporated in cells of the growing hair. These granules of pigment
give the hair its color.
Evidence in support of this hypothesis is given below.
Along the Pacific Coast from British Columbia southward, _M. erminea_
(see fig. 25 on page 95) is brown in winter. This is an area where snow
rarely falls and the temperature in winter ordinarily is above
freezing. In the remaining part of the American range of this species
the temperature in winter is below freezing much of the time and snow
remains throughout the winter or for long periods. In this colder part
of the animal's range, only white coats occur in winter. _M. frenata_
likewise has a white coat in winter in the part of its geographic range
where snow and freezing temperatures prevail throughout most of the
winter and a brown coat in warmer, snowless areas to the southward and
along the Pacific Coast. The third species, _M. rixosa_, exhibits a
corresponding correlation between coat color and climate. On the
Asiatic continent, several species, including _M. erminea_, provide
parallel correlations and nowhere are there any exceptions for the
subgenus _Mustela_. These data are an important part of the material on
which we have based the induction that the underlying cause of seasonal
change in color is a need for protective coloration.
As regards molt, most naturalists who have written upon the subject
regard it as responsible for the change from the white winter coat to
the brown summer coat. However, the change from brown summer coat to
white winter coat has been thought by several writers to be effected by
change in coloration of the individual hairs. Among those holding this
opinion there may be cited Bell (1874:197) in reference to _Mustela
erminea_, and Coues (1877:123) in reference to American specimens to
which he applied the same name. More lately Hadwen (1929) has taken
this same view, and Gunn (1932) also discusses the possibility of the
hairs changing color. Bachman (1839:228-232), Macgillivary
(1843?:158), Audubon and Bachman (1851 (vol. 2):62), Schwalbe
(1893:538), Pearson _et al._ (1913:447), Miller (1930, 1931A), Hamilton
(1933:300) and Rothschild (1942), among others, have been inclined to
the opinion, or positively affirm, that the color change in autumn is
the result of a molt. The papers cited above contain, in turn,
references to many other printed accounts dealing with this question.
To my mind, it has not so far been demonstrated that the change in
color of weasels in autumn is accomplished without a molt. Also so far
as I am aware, no explanation has been given of how the pigment may
disappear from the hair of weasels. Metchnikoff's (1901:156) idea that
the senile whitening of the hair in man is accomplished by phagocytes
which remove the pigment granules would hardly seem to explain the
relatively sudden and complete autumnal change occurring in weasels.
Anyhow, Danforth (1925:108), and some other students have thought that
the action of these phagocytes was at most a factor of slight
importance in the whitening of hair. Whatever be the complete answer to
the question of how the weasel changes color in autumn, at least one
specimen of long-tailed weasel, which is in process of color change in
autumn, presents clear evidence of molt of the overhairs. This specimen
of _M. f. longicauda_ is no. 188408, U. S. Nat. Mus., taken on November
12, 1897, at Rapid City, South Dakota. Other specimens of _M. erminea_
which were taken in autumn similarly show molt to be in progress. For
these and other reasons, I am inclined to the opinion that the autumnal
change in color, like the one in spring, is effected by molt. During
the period of the autumnal color change, Noback (1935:27) had a captive
_M. f. noveboracensis_ and, each morning, found clumps of brown hair on
the floor of its cage; this was strong indication that molt was
responsible for the color change in this instance.