change from brown to white can be accomplished _only_ by molt; in the
present state of knowledge it would be unscientific to deny that the
change were possible of accomplishment by other means. Also, it is true
that the fifteen specimens before me of _Mustela frenata_, subspecies
included, in process of change from brown to white, with the exception
of the one from Rapid City, South Dakota, if taken individually, do
not, in macroscopic examination, show definite molt lines or other
absolutely convincing evidence of molt. However, these same specimens,
insofar as examined microscopically, do show overhairs all white, or
overhairs pigmented throughout. The lighter color of the proximal parts
of the overhairs in itself should not be accepted as evidence of color
change, for in the fresh summer pelage, the same condition exists.
Also, careful macroscopic examination suffices to show that in the
transitional pelage of autumn, the brown overhairs generally are longer
than the intermixed white overhairs.
Whether the underfur behaves in exactly the same way as the overhair, I
have not myself definitely ascertained, but I assume that the underfur
is molted twice each year, at least in the northern populations of
_Mustela frenata_ and in the other species of more northern
distribution. Schwalbe's (1893) work, including sectioning of the skin
and study of the hair follicles, led him to conclude that the underfur
was molted twice each year in _Mustela erminea_.
In _Mustela frenata noveboracensis_, _M. f. nevadensis_, and _M. f.
nigriauris_, measurements taken on adult males show the overhairs to be
longer in the winter pelage than in the summer pelage of specimens from
the same locality. For example, in _M. f. nigriauris_ from Berkeley,
California, the overhairs of the summer coat (July and August) average
8 millimeters in length on the hinder back and 7 mm. on the belly, but
average 9.5 mm. and 8 mm. respectively in January-taken specimens
possessing the full winter coat. At Ann Arbor, Michigan, in the summer
coat, the longest hairs on the hinder back average approximately 12
mm., and those on the belly, 9.5 mm., against 13 mm. and 9.5 mm.
respectively in winter. Although general observations initially led me
to believe that the black, terminal hairs of the tip of the tail are
longer in the winter pelage than in the summer pelage, actual
measurements fail to show a difference in length.
The change from one coat to the other in the long-tailed weasel has
been described among others by Miller (1930, 1931A), Hamilton (1933)
and Glover (1942) on the basis of captive specimens. In a general way,
the progress of the molt in their specimens agrees with that which I
have been able to make out from examination of skins taken in the wild.
There is, however, this difference: Their specimens show a more spotted
pattern when in process of hair-change than do specimens taken in the
wild. Probably the more or less unnatural conditions under which these
captive animals lived modified the normal progress of molt.
In wild-taken specimens of the species _Mustela frenata_, subspecies
included, the spring molt begins on the mid-dorsal line and proceeds
laterally, producing, at almost any given time, a relatively sharp
molt line separating the white winter hair from the incoming brown
summer coat. However, in autumn the change takes place first on the
belly, then on the sides, and finally makes its appearance over all the
upper parts at about the same time, with the result that the upper
parts have a salt-and-pepper appearance without at this time any
sharply defined molt lines. In general, the molt pattern can be said to
be reversed in the two seasons; in spring, it begins on the back and in
autumn, on the belly. The difference in spring and autumn color pattern
is better illustrated on plate 39 than by additional description.
Swanson and Fryklund (1935:123) have observed that the "spring molt
proceeds differently" than the fall one in _Mustela rixosa_, and
Barrett-Hamilton (1903:309) in commenting on the European hare (and the
stoat?) remarks, "In spring the moult, and with it the brown colour,
progresses in exactly the opposite order . . ." as compared with the
white color of autumn, which that particular writer thought resulted
from removal of pigment from the hairs rather than from molt.
The tail, excepting the black tip, lags in the molt in many instances,
with the result that, especially in spring, it may retain a few white
hairs as late as does the belly. In autumn it is less tardy and so far
as I have observed, becomes white at about the same time that the
general area of the back changes color. On the tail, the black tip
itself, as clearly shown in more than a score of specimens, is molted
at approximately the same time in autumn as is the pelage of the body.
However, the long black hairs, which appear in, say, November, appear
to increase in length until January. In spring, the long black hairs of
the tip of the tail seem not to be shed at the same time as the rest of
the winter pelage, but remain approximately six weeks longer and then
are replaced by long black hairs of the summer coat. At any rate, this
is the picture presented by a half dozen specimens of _M. f.
nevadensis_ and _M. f. longicauda_ which do show a spring molt to be in
progress on the black tip of the tail. Schwalbe similarly
(1893:536-537) has suggested that the black tip of the tail in _Mustela
erminea_ in spring is not molted until about two months after the
pelage on the rest of the body is changed. Schwalbe (_loc. cit._)
thinks also that in _M. erminea_ studied by him, the black tip of the
tail in autumn is replaced approximately one month in advance of the
pelage on the rest of the body. As indicated above, my specimens of
_Mustela frenata_, subspecies _longicauda_ and _nevadensis_, do not
show this discrepancy in autumn. I have considered the possibility that
the black tip of the tail, in some species of _Mustela_, is molted only
once while the remainder of the coat was undergoing two molts. My
inconclusive data lend but little support to this possibility.
The difference in pattern of color between specimens taken in autumn
and spring is known to some fur-trappers of my acquaintance who have
suggested that molt occurs in spring, whereas the individual hairs
change color in autumn. Reference to plate 39 will show how gross
comparisons might lead one to this erroneous explanation of the color
change.
As to time of molt: In eight subspecies of _Mustela frenata_, namely,
_noveboracensis_, _occisor_, _primulina_, _spadix_, _longicauda_,
_arizonensis_, _nevadensis_ and _effera_, material is available to
indicate that the autumnal molt begins in October and is completed in
November, and that the spring molt occurs in March or April. A
condensed list of specimens providing basis for this statement is as
follows:
_M. f. noveboracensis_: 26 specimens in transitional pelage taken
in autumn and 14 taken in spring; _M. f. occisor_: One topotype
has acquired one-fifth of the winter pelage on October 22, 1896;
_M. f. primulina_: 2 in November, one in March, and 2 in April are
in process of change; _M. f. spadix_: 6 autumnal specimens and one
in April show pelage change; _M. f. longicauda_: 7 autumnal
specimens and one in April show pelage change; _M. f.
arizonensis_: 12 specimens in autumn and 3 in spring are in
process of molt; _M. f. effera_: One November-taken male has
acquired four-fifths of the winter coat and another taken on April
21 at Fort Rock, Oregon, is half finished with the spring molt.
It may be added that no marked difference in time of either autumnal or
spring molt is apparent as between the more northern and more southern
localities from which the mentioned specimens come. With more complete
material I would expect to find a difference in this regard.
The material of the other, more southern, subspecies of _Mustela
frenata_ has not been adequate to show the time of molting or the
number of molts which occur in one year.
Animals in the northern part of the range of _Mustela frenata_ acquire
a white winter coat, whereas those in the southern part acquire a brown
winter coat, and in an intervening area the winter coat may be either
brown or white. By plotting on a map the localities of capture of all
specimens examined in the winter coat, it was possible to outline this
intervening area as shown in figure 10 on page 37. However, Dearborn
(1932:36) shows that in Michigan some animals have a brown coat in
winter at places farther north than figure 10 shows to be the case.
Hamilton's (1933-306) map for New York shows the same to be true in
that state. Accordingly, the boundaries of the area shown in figure 10,
in which both brown and white long-tailed weasels occur in winter, are
known to be only approximate; with full information available the belt
would be represented as wider.
[Illustration: FIG. 10. Map showing the region (in black) where both
the brown and white winter pelage is found in the long-tailed weasel,
_Mustela frenata_.]
Hamilton (1933:302) has pointed out that "Where half of the weasels
remain brown, these brown winter specimens are always males." The
results of my own examination of specimens not studied by Hamilton, in
a general way provide confirmatory data. More exactly, my examination
reveals that at the most northern localities where brown specimens
occur, only males are in this coat. In explanation, it may be said that
in plotting on a map localities of capture of specimens in the winter
coat, thirteen places were found where both sexes were represented and
where both brown and white winter coats were found. With the two sexes,
it is theoretically possible to have nine different combinations of
coat color. With males all brown, there might occur females (1) all
brown, (2) all white, or (3) some brown and some white. In addition to
these three combinations, we might have three more by finding the
mentioned types of female coat color repeated where all males are
white, and three more, or nine in all, by substituting a population of
males some of which were brown and some of which were white. Seven of
these possible combinations actually were found. The two combinations
not found were all white males with all brown females, and all white
males with females both brown and white. In the three instances where
the males all were brown and the females all were white, the localities
of capture were in the northern part of the variable area. This
indicates that where the brown winter coat occurs at northern
localities, the brown individuals are all males. Farther south, of
course, the females, too, acquire the brown winter coat.
Stated in another way, there is a broad belt across North America from
the Atlantic to the Pacific in which males of _Mustela frenata_ at any
one locality may be either brown or white in winter. Inside this broad
belt there is a narrower one, approximately half as wide, in which
females at any one locality may be either brown or white.
In support of the idea that color of the winter coat is an hereditary
matter and that it is not dependent on temperature, the following
evidence derived from my transplanting specimens of _Mustela frenata_
supports the idea that color of the winter pelage is dependent on
heredity and not on temperature or snowfall.
A male captured on June 24, 1937, in the brown summer coat in Salt Lake
City, Utah, was received by me at Berkeley, California, five days later
and kept in captivity almost six months. On November 17, 1937, half the
pelage was white and on December 27, 1937, when next examined, the
animal was in the full, white, winter coat as it was on January 25,
1938, when it died. Native weasels all turn white in winter in Salt
Lake City, but in Berkeley native weasels always are brown in winter.
A juvenile or young animal, a male, captured in May, 1936, at
Lafayette, Contra Costa County, California, was kept there until August
13, 1936, when transferred to Calneva at the north end of Lake Tahoe,
California. The weasel was kept at Calneva until its death on December
23, 1937. In both the winter of 1936-'37 and in that of 1937-'38, the
winter coat was brown as in animals from its place of origin (Contra
Costa County) and unlike weasels of the Tahoe region nearly all of
which turn white in winter.
Two females, each approximately two months old, captured on May 1,
1936, at James Landing, 4 miles northwest of San Pablo, Contra Costa
County, California, were kept in Berkeley, California, until August 13,
1936, when they were transferred to the mouth of Blackwood Creek, on
the west side of Lake Tahoe, California. On October 25, 1936, both
weasels escaped. On December 25, 1936, the headless body of one of
these was found approximately 300 yards south of the mouth of Blackwood
Creek. The animal had been dead at most a few days when found and was
in the brown winter coat. At the place of its origin all weasels are
brown in winter but at the mouth of Blackwood Creek only 2 of 60
weasels caught there in the winter coat were brown; the other 58 were
white. The headless weasel was identified, as one of the two formerly
in captivity, by means of certain short toes, the ends of which had
been clipped off when the animal was a captive. No trace of the second
female was found.
A female of unknown age, in white winter pelage, captured 4 miles
southeast of Tahoe City, California, and kept there until April 3,
1937, on which date it was brought to Berkeley, California, molted to
brown in the spring. The first signs of the brown coat were noted on
April 14. On May 24 or 25 she gave birth to 4 young which lived less
than ten days. In the following winter this animal acquired a white
coat. As previously noted, weasels native to the Berkeley area, where
this female was kept, have brown coats in winter.
The weasels were in every instance kept in cages out-of-doors. The
sides of the cages were open to the elements. A nest box in each cage
provided shelter. All were of the species _Mustela frenata_.
The significant results, it seemed to me, were that the winter coat was
the kind found in the area where the weasel originated instead of the
kind found in weasels native to the areas in which the specimens were
held in captivity.
That the time of molt is determined by the amount of light has clearly
been shown by Bissonnette (1944:223) for American weasels of the two
species _Mustela erminea_ and _M. frenata_. In his words (_op.
cit._:246) "Reducing the daily periods of light induced molting and
regrowth of new fur. . . . In the Bonaparte weasels [_Mustela
erminea_], white replaced brown. . . . Increasing daily light-periods
caused molting and change to dark brown. . . . Incomplete molts in both
directions (toward white or toward brown) were produced as a result of
early reversal of increase or decrease of daily light-time. . . . That
this stimulus is received through the eyes and acts through the
anterior pituitary gland is indicated by Bissonnette's [1935:159]
studies on ferrets, a nearly related animal. That the thyroids and
sex-glands are not essential is at least suggested . . . by Lyman's
(1942) study on the varying hare [_Lepus americanus_]." It can be added
that Lyman (1943:451) demonstrated in _Lepus americanus_ that the
effect of light is received through the eyes. He demonstrated this by
masking the animals. To Wright (1942B:109) who studied the two American
weasels, _M. erminea_ and _M. frenata_, it seemed likely that the
pituitary produced or released gonadotropic hormone at about the time
of the spring molt and that this molt and the spring changes in the
reproductive tracts of the weasels might be caused by a stimulus from a
common source. Later, Wright (1950:130) injected a gonadotropic hormone
into long-tailed weasels which had recently acquired their white winter
pelage and thereby caused them to lose the white pelage and acquire the
brown pelage. It is Lyman (1943:450) who says, in relation to _Lepus
americanus_, "When in the physiologically white condition, the
melanoblasts of the regenerating guard-and pile-hair follicles contain
no melanin-forming enzyme (dopa-oxidase), which may be the reason for
the lack of pigment." Schwalbe (1893) by sectioning the skin and
microscopically examining the hair-follicles of _M. erminea_ learned
that the basal cells producing hairs lacked pigment granules in autumn
when the European ermine (_M. erminea_) was acquiring its white winter
coat and that the cells contained granules of pigment in spring when,
as we know, the granules are incorporated in the growing hair and give
it its color.
The above material, then, is basis for the account on pages 31 and 32
of what causes the weasel of northern areas to have a white coat in
winter. The discerning student will instantly perceive that although
some parts of the account on pages 31 and 32 are precisely accurate,
other parts are the result of inferences which need to be proved. More
careful work of the kind that Schwalbe (1893) and Wright (1942B) did is
needed. The account on pages 31 and 32 is merely the best that can be
given with the information now available.
Many writers have commented on the yellowish color, sometimes with a
greenish tinge, found on the fur of weasels in the white winter coat.
The stain is more often found on the tail and hinder-parts of the body
than elsewhere. Possibly, partly on this account, some have ascribed
this color to the smearing of the fur with urine. Still others have
thought it resulted from the smearing of the fur with secretions from
the anal scent glands. Schumacher (1928) takes this point of view, and
while it may be that he has not proved his point, still his conclusions
fit the known facts and seem sound to me. Schumacher points out that
the same soiling of the fur is present in summer as well as in winter,
but that on the summer pelage the stain can be detected only on the
light-colored underparts. It is from this point of view that he
criticizes the systematic worth of white versus yellowish-white
underparts in the summer pelage of geographic races of _Mustela
erminea_ and _Mustela nivalis_. Although in the long-tailed weasels
(_Mustela frenata_) the underparts of all the races are pigmented with
some form of red, orange or yellow, it seems probable to me that the
additional color resulting from the soiling effect of this glandular
secretion explains the greater variation, found at a single locality,
in the color of underparts than of upper parts in the summer pelage.
I have neither seen nor heard of a black weasel in any part of the New
World or of the Old World. I have found only one albino among American
specimens. It is an adult female, no. 121424, American Museum of
Natural History, of _Mustela erminea richardsonii_, taken on August 30,
1935, at Hot Springs, Northwest Territory. This place, I am told by G.
G. Goodwin who obtained the animal, is on the "Nahanni River where the
rugged mountain ridges rise abruptly from the low mud flat lands,
latitude 61, longitude 125." The shortness and coarseness of the hair
corresponds to that of the summer pelage and not winter pelage. The
pelage is everywhere white, even the tip of the tail. True, all except
the nape and top and sides of the head has a faint yellowish-green
tinge which has been supposed to result from staining by secretion of
the anal scent glands but there is no pigment in the hair as in
erythristic specimens. From the Old World, Farurick (1873:17) has
recorded what he regards as an albino of _Mustela vulgaris_ since it
had no black hairs on the tip of its tail. Flintoff (1935:228, 229)
records what may have been an albino _Mustela vulgaris_ from Yorkshire
and an albino _M. erminea_ from an unstated locality. Jäckel (1873:459)
mentions specimens of _Mustela erminea_ and _Mustela vulgaris_, which
were partly "albinistic" or "erythristic." Among the American specimens
of _M. erminea_ I have not recorded any which appeared to be either
partly or wholly erythristic or only partly albinistic. Among the 1550
skins of _M. frenata_ which were in summer pelage or brown winter
pelage, five, described below, show marked abnormalities in color.
Two of these five are partly albinistic. One is an adult male, no.
223880, U. S. Nat. Mus., from Billy's Island, Okefinokee Swamp,
Georgia, which has the nose as well as the area between the eyes white.
Also there is a tuft of white hairs at the anterodorsal margin of each
ear, scattering white hairs suggesting a postorbital bar on each side
of the head, and a patch of white hairs on the mid-dorsal line behind
the ears. Markings of this kind are not abnormal in _M. f. peninsulae_,
the subspecies adjoining on the south, except for the white nose which
clearly is an instance of partial albinism. The second specimen is a
subadult male, of _M. f. noveboracensis_, no. 177679, U. S. Nat. Mus.,
in process of acquiring the brown winter coat, taken on November 27,
1911, at Gaylordsville, Connecticut. It has white markings on the nose,
on the right side of the neck, on the right hind foot and right
forefoot, and on the tip of the tail. The white area of the nose on the
left side extends back to the eye, but on the right side barely
encircles the nose-pad. On the right side of the neck, all that area
between the foreleg and ear is white from the mid-dorsal line
(including 7 or 8 millimeters to the left of the mid-dorsal line) down
to the throat, which is white as it is also in normal individuals. The
toes of the right hind foot are more extensively white than in normal
specimens of _noveboracensis_, and all of the right forefoot as well as
the wrist is white. The tail is of striking appearance because of its
tricolor pattern. The proximal part is of the normal brown color. The
black terminal part commences proximally at the usual place, but the
distal 11 millimeters of the fleshy part of the tail bear only pure
white hairs producing a terminal white pencil 35 millimeters long.
The three other specimens abnormally colored are erythristic
individuals. An adult male of _M. f. latirostra_, no. 7574, coll. D. R.
Dickey, taken on April 14, 1918, at Covina, Los Angeles County,
California, has the color of the upper parts greatly restricted, and,
in addition, has spots and blotches of the color of the underparts
distributed over the back and rump. A spot of this same color occurs
above each ear. Incidentally, this and other subspecies of _Mustela
frenata_ from the Pacific Coast of North America obviously have the
factor for erythrism operating over a larger part of the body than it
does in _M. erminea_ or than in _M. f. noveboracensis_, where the
underparts sometimes are white. In _M. f. latirostra_ and in other
subspecies from the Pacific Coast the light color of the underparts
always is tinged with this reddish color.
Another erythristic specimen is a young male of _M. f. nevadensis_, no.
23493, U. S. Nat. Mus., taken on August 6, 1890, at Birch Creek, Idaho.
It has all of each foreleg, the axillary regions, and a saddle-shaped
area over the shoulders of the same buff-yellow color as the
underparts.