erminea_ and _Mustela frenata_ by tail a fourth or less of length of
head and body and without a black tip (at most a few black hairs at
extreme tip in rixosa), and from _M. frenata_ and from _M. erminea_ in
regions where it and _rixosa_ occur together, by basilar length of
skull less than 32.5 in males and less than 31.0 in females.
_Characters of the species._--Size small: Total length less than 250 in
males and 225 in females; tail a fourth or less of length of head and
body, and without a black pencil and at most with a few black hairs at
extreme tip; caudal vertebrae 11 to 16, normally 15 in _M. r. rixosa_,
and 11 in one _M. r. eskimo_ examined; skull with long braincase and
short precranial portion, thus essentially same shape as in _M.
erminea_ but the largest males of _M. rixosa_ always with a lesser
basilar length that even the smallest females of _M. erminea_ or _M.
frenata_ of the same geographic area. In fact no specimens of _M.
frenata_ have skulls so small as the largest _M. rixosa_, and skulls of
equal size of _M. erminea_ and _M. rixosa_, for example, _M. erminea
muricus_ of Colorado and _M. rixosa eskimo_ of Alaska, differ in that
when the skulls are viewed from directly above those of _rixosa_ have
the mastoid processes more prominent, or the braincase is higher in
relation to its width or both differences together prevail. Stated in
another way, comparison of skulls of equal size of _rixosa_ and
_erminea_ shows that in the latter the braincase is more nearly flat
and is wider above and in front of the mastoid processes; therefore,
the greatest breadth of the braincase equals or exceeds the mastoid
breadth, whereas the reverse is ordinarily true of _rixosa_.
_Geographic variation._--In the Old World four subspecies are currently
recognized (see Allen, 1933:316) and the same number is here recognized
in North America. Length of the tail, length of head and body and hind
foot, breadth of the rostral part of the skull in relation to its
length, and position on the side of the head of the line of demarcation
between the dark color of the upper parts and the white underparts, are
the features in which geographic variation has been detected. The
general impression is that the amount of geographic variation is much
less than in _Mustela frenata_ and only slightly less than in _Mustela
erminea_ of the same geographic area.
_Nomenclature._--It is exceptional for a species which occurs in both
the Old-and New-World to take its specific name from New World
material, especially if the name was proposed as recently as 1896; most
circumboreal species take their names from descriptions of European
specimens. Although the least weasel, _Mustela rixosa_ (Bangs) 1896,
seems now to be an exception, it may yet turn out that the first
available name was based on European material. Zimmermann (1943) shows
that the least weasel actually was named on the basis of European
material long before 1896 and concludes that the name _Putorius
minutus_ Pomel, 1853, based on a specimen from France, is the first
available name.
Because _Putorius_ nowadays is relegated to subgeneric rank under the
generic name _Mustela_, we have for consideration the name-combination
_Mustela minuta_ (Pomel). Unfortunately for Zimmermann's conclusion,
_Mustela minuta_ Pomel is not available because it is preoccupied by
_Mustela minuta_ Gervais [= _Palaeogale minuta_ (Gervais),
1848-1852--see Simpson, 1946: 2, 12], a name applied to another species
of small mustelid from the Oligocene or lower Miocene deposits of
Europe.
Some other early names thought by Zimmermann (1943:290) to have been
based on the dwarf weasel of Europe are judged to be _nomina nuda_ and
therefore are to be ignored.
The name _Mustela minor_ Nilsson 1820 was thought by Miller (1912:402)
to be a renaming, and hence a synonym, of _Mustela nivalis_ Linnaeus.
If that is the case the name does not apply to the dwarf weasel. If the
name _Mustela minor_ Nilsson was instead based on the dwarf weasel, the
name might still be unavailable, depending on rulings on secondary
homonyms, because the name might be preoccupied by _[Lutra] minor_
Erxleben 1777 which is a synonym of _[Mustela] lutreola_ Linnaeus 1766.
Two names seemingly available for weasels, and in use for them today,
which might replace _rixosa_ as the name of the species, are, first,
_Mustela boccamela_ Bechstein, 1801, of Sardinia [= _Mustela nivalis
boccamela_ of Miller, 1912, 405] and second, _Putorius numidicus_
Pucheran, 1855, of Morocco and Algeria [= _Mustela numidica_ of Allen,
G. M., 1939, 183]. As they stand in the current literature, _Mustela
numidica_ is a species distinct from the dwarf weasel and the other
name, _Mustela nivalis boccamela_, is an insular subspecies of the
mouse weasel. Zimmermann (1943:292), however, implies that _M.
numidica_ may belong to the dwarf weasel group when he says "Ob auch
_iberica_ BARR.-HAM. als Unterart zu _minuta_ POM. zu stellen ist, soll
hier nicht untersucht werden, ebensowenig die von CABRERA vermutete
Zugehörigkeit der grossen nordafrikanischen _M. numidica_ PUCH. zur
'_iberica_-Gruppe'." The answer to this problem requires a taxonomic,
rather than a nomenclatural, decision. Whether either _M. numidica_ or
_M. boccamela_ are conspecific with the dwarf weasel I cannot at this
time ascertain for want of adequate specimens. Because these two names,
_M. boccamela_, and _M. numidica_, are assigned to kinds of weasels
which are currently regarded as specifically distinct from the dwarf
weasel, and because all the other names which certainly have been
assigned to Old World populations of the dwarf weasel before 1896, so
far as I know, are _nomina nuda_ or are preoccupied, the next available
name, _Mustela rixosa_ (Bangs, 1896), is here employed.
_Remarks._--This species may have a wider geographic range in
northeastern North America than is now known. Strong (1930:7) writes
that the Naskapi Indians of the interior country of Labrador between
Hamilton Inlet and Ungava Bay "have only one name for weasel,
_mé-tah-kwut_, but they say there are three kinds in their territory, a
large, an intermediate, and a very small weasel. The latter suggests
the least weasel . . . which has not been recorded from northern
Labrador."
In the northern part of the range of the species, the winter pelage is
white and the summer pelage is brown. In the southern part of the
range, that is in the range of the subspecies _allegheniensis_, the
winter pelage is either brown or white and the time of the molt into
winter pelage is irregular; each of eleven individuals from
Pennsylvania, Michigan and Ohio, taken in December, January, February
and March is mostly white but retains some considerable part of the
brown pelage of the previous coat on top of the head and usually also
along the midline of the entire dorsum. These eleven animals include
individuals of each sex. Of each sex, some are adults and some are
subadults. Therefore, the delayed or incomplete fall molt, at present,
cannot be correlated with either sex or with any particular age. No
wild-taken specimens of _M. erminea_ or of _M. frenata_ of the same
region show this delayed or incomplete molt.
Possibly this delay or incompleteness of molt is the result of the same
cause that lies behind the birth of some _M. rixosa_ in midwinter. As
listed below, several litters of young have been found in midwinter. In
fact it appears that in the United States, young may be born in every
month of the year although, according to existing information, more
litters are produced in spring and in winter than in summer and autumn.
Many juveniles and young of _allegheniensis_ examined in study
collections clearly were born in spring but about as many seem to have
been born in midwinter as at any other time (in the light of present
knowledge) and this is in contrast to what we know of the two other
species of American weasels since their young, so far as known, are
born in spring.
One instance is worthy of detailed comment. An adult female, no. 783
Ohio State Museum, taken on January 31, 1931, at Vinton, Meigs County,
Ohio, bears the following notation on the attached label "nest plowed
out of ground. Very small young escaped--marked like parent. [F] was
nursing." The enlarged mammae on the dried skin substantiate the
statement that the female was nursing young. She has a brown mask
continuous from one ear through the eye, across the forehead and
through the other eye to the opposite ear. On each side of the body a
stripe of brown 5 to 10 mm. wide extends from the upper part of the
foreleg back to the thigh and base of the tail, uniting there with its
opposite and covering the tail. There are a few spots of brown on the
shoulders, and rump and one on the middle of the back. Otherwise the
specimen is white. One implication of the statement on the label that
the young which escaped were marked like the parent (presumably this
female parent) is that this female is a partial albino. I am more
inclined, however, to the view that there was an unseasonable activity
of the particular glands of internal secretion the hormones of which
promote embryonic growth and that these glands, or others controlled
by them, were in some way responsible for an abnormal progress of molt,
or for a reversal of molt in that one molt began before the previous
molt had been completed.
Excepting this one specimen, no. 783 from Vinton, Ohio, all of those in
transitional pelage indicate that the direction of the molt pattern is
the same as in _M. frenata_ and _M. erminea_. That is to say, the
autumnal molt begins on the midventral line and the molt in spring
begins on the mid-dorsal line. Furthermore, the normal progress of each
molt appears to follow the same pattern that has been described above
for _Mustela frenata_.
A possible explanation of unseasonal molt in the southeastern area of
occurrence of the species _Mustela rixosa_, and a possible explanation
of the abnormal molt of the female from Vinton, Ohio, is that the
species has only relatively recently invaded the area, and has had
insufficient time to adjust the physiology of its molting mechanism to
the longer periods of daylight that obtain later in autumn and earlier
in spring than farther north. In the other two species of American
weasels, the change in length of periods of light, it will be recalled,
is known to indirectly control both molt and some changes in the sexual
cycle. Wright (1942B:109) has shown that molt in spring precedes by one
or two months the birth of young in _M. frenata_, that the two
phenomena are correlated in a way that is statistically significant,
and recognizes that progressively longer periods of daylight may be the
causal stimulus. The suggestion made above that _M. rixosa_ does not
live in New England or in the Rocky Mountains of the western United
States because each of the two areas already is inhabited by weasels of
almost equally small size, is in line with the idea that _rixosa_ is a
recent immigrant to America, or more precisely that _rixosa_ arrived
later than _erminea_.
_Natural History._--Habitat and Numbers.--Soper (1946:136) recounts
that near the junction of the Antler and Souris rivers, Manitoba, this
species occurs "both in the river valleys and on the upper prairies,"
and later (1948:55), with reference to the Grand Prairie of the Peace
River region of Alberta, writes that the least weasel "inhabits both
parklands and mixed wood forest environments."
At most times, wherever found, the least weasel is regarded as rare.
Not only mammalogists regard it as rare and as a desirable catch, but
Indians likewise value it, probably because of its rarity. For example,
Osgood (1901:69-70), who caught a female least weasel at Tyonek,
Alaska, writes that: "The natives regard the capture of one of these
rare animals as a piece of great good fortune. One old Indian who
frequently visited our cabin told us that his brother who had caught
one when a small boy had in consequence become a 'big chief'; and he
assured me that since I had caught one I must surely be destined to
become a man of great wealth and power."
Swenk's (1926:313-330) account of the species in Clay County, Nebraska,
shows, however, that the animal was far more abundant in 1916 and 1917
than subsequently and inferentially than it was before 1916. Clearest
proof of multiannual fluctuation is provided by P. O. Fryklund's
(Swanson and Fryklund, 1935:120-126) receipt of weasels from Roseau
County, Minnesota. From 1895 to 1932 he had approximately equal
opportunity to receive least weasels each year. Those which came to his
attention were distributed by years as follows: 1895-1927, 7
individuals in all; winter of 1927-28, 3 individuals; winter of
1928-29, 59 individuals; 1929-1930, 84 individuals; 1930-1935, 3
individuals. "These records indicate a very definite increase in the
abundance of least weasels in the Roseau region [in] the two years from
the autumn of 1928 to the spring of 1930. Mr. Fryklund has handled 166
least weasels in his 40 years in Roseau County, and of these, 143 were
taken in the two years mentioned."
The maximum home range of the least weasel is two acres and a weasel
seldom travels farther than ten rods from its burrow according to
Polderboer (1942:146) who, in the period December 20, 1939, to January
2, 1940, studied four least weasels and one long-tailed weasel on a 144
acre farm in Butler County, Iowa.
Behavior
Of the voice, Llewellyn (1942:441) records that his captive specimen
taken in Virginia uttered a shrill shriek when seizing prey or when
teased. When excessively annoyed the weasel also emitted musk.
The sense of smell is used in hunting as was witnessed by George L.
Fordyce; he observed a least weasel following the scent of a
_Peromyscus_ and saw the least weasel overtake and kill the mouse
(Seton, 1929 (2):637).
At a nest in a clover stack, in Manitoba, Criddle (1947:69), on
December 27, 1946, found the least weasel "to have been rather remiss
in its sanitary habits as its pile of dung was almost, or quite,
touching the nest and only just to the side of its entrance." There
were 117 voids.
Enemies
The great-horned owl, barn owl and long-tailed weasel are to be counted
as enemies since Nelson (1934:252) found the fur, skull and other
fragments of the skeleton of a least weasel in one of 26 pellets of the
great-horned owl in Wisconsin; Handley (1949:431) found the skull and
other skeletal remains of a least weasel in one of 22 pellets of the
barn owl in Virginia; and Polderboer, Kuhn and Hendrickson (1941), in
Iowa, found the remains of a least weasel in the den and scats of a
_Mustela frenata_. A domestic cat in Michigan killed a least weasel
(Dearborn, 1932B:277).
Food
Mice are killed by the least weasel biting into the back of the head
and neck according to Allen (1940:460) who reported upon the growth of
five young, from Michigan, that he had in captivity. He further states
that a weasel was able to kill a mouse in 30 seconds. One large
_Microtus_ introduced into the cage slept with a weasel for several
days and ate parts of the mice that the weasel killed but then the
weasel killed this mouse! Llewellyn (1942:440-441), in writing of a
captive from Virginia, says: "When a live mouse was placed in the cage,
the weasel sprang upon it almost instantly. Grasping the mouse by the
back of the head, the weasel bit its victim through the skull several
times in rapid succession and held on with its sharp teeth. The sound
of the teeth piercing the bone was distinctly audible at a distance of
several feet. During this interval the weasel hugged the mouse closely
with its fore legs and pressed it firmly to its belly through a kicking
motion of the hind legs. The hold on the back of the head was not
relinquished until the mouse was dead. The killing took only a few
seconds. Upon releasing the mouse the weasel usually came to the front
of the cage and inspected the observer for an interval of several
seconds after which it returned to its prey and began its meal at once.
Sometimes the blood would be licked from the wound in the back of the
head or perhaps an ear would be chewed a bit and the blood licked off,
but never did the weasel 'cut the throat' of its prey and 'suck the
blood.'
"The weasel ate the head and brain first, beginning at the back of the
head and working forward. Just before reaching the nose the process was
reversed and eating then proceeded from the base of the skull toward
the tail of the mouse. The tip of the nose, maxilla with teeth, and the
tail seemed to be the parts least preferred; they were not eaten when
an abundance of food was present. At no time did the weasel place its
front feet on the mouse in an attempt to hold it. A second or third
mouse was killed immediately upon being placed in the cage even though
the first one had not been consumed. The weasel, however, usually
returned to the partially eaten mouse and finished it before starting
on a new one. Upon completing a meal, especially if the meal had been
particularly bloody, the weasel rubbed its chin on the bottom of the
cage, scooting along and appearing more snakelike than normal. Whenever
I attempted to remove a mouse, or partially eaten one, from the cage,
the weasel hung to the mouse tenaciously, and often allowed itself to
be lifted up in this manner.
"In the six days that the weasel was kept in captivity it was fed 10
house mice having a total weight of 118 grams. As no food was given on
one day, the amount of food eaten is probably slightly below the actual
capacity of the animal. Since the weasel weighed only about 32 grams,
the average amount of food eaten a day was slightly in excess of
one-half the weight of the animal."
Polderboer (1942:146-147) found in three dens, in Iowa, bits of
_Reithrodontomys_ (harvest mice) and _Peromyscus maniculatus_ (deer
mouse), and in the digestive tract of one least weasel there was a bone
fragment and a few hairs of a deer mouse. In the account, given beyond,
of a nest, Criddle (1947:69) records the Pennsylvania meadow mouse
(_Microtus pennsylvanicus drummondi_) and the Gapper red-backed vole
(_Clethrionomys gapperi_) as prey at Treesbank, Manitoba. The same
author, concerning the same place, earlier (1926:199-200) wrote that in
1922 the meadow mouse, _Microtus minor_, "went into winter quarters in
great numbers and its homes were well stocked with provisions . . . all
went well until the middle of February, 1923. Then, within a few days,
each was taken possession of by a least weasel (_Mustela rixosa_) and
the inhabitants were quickly destroyed. One dwelling was occupied by
one of these weasels for about two weeks during which time I observed
that it had dragged several mice over the snow to its temporary home.
This residence was examined in April, and in it were discovered six
dead _Microtus minor_, one _Evotomys_, the head of another, and at
least six or eight remnants of small rodents including _Microtus
drummondi_, these last remains being chiefly indicated by the
hair-lined nest of the weasel.
"The homes of 27 other vole communities examined at this time were all
found to have been entered by weasels, the inhabitants having been
killed and partly eaten. Moreover, the weasels had made the homes
temporary centers from which they raided other rodent habitations in
the vicinity. Thus from being an abundant animal this vole was reduced
to insignificance in the course of a few weeks, while all other kinds
of mice had suffered severely from the same enemy."
An instance of predation on _Peromyscus_, revealing some of the methods
of capturing prey, is recounted by Seton (1929 (2):636-637) who quotes
a letter to him from George L. Fordyce, of Youngstown, Ohio, as
follows: "While out in the field this morning (Dec. 26), walking along
the bank of a ravine at the edge of our golf course, I saw a
Field-mouse run out of the bushes into the rough grass that is just
outside of the fair-green of the course. In another instant, what I
thought at first to be a white Mouse came out at the same place. The
Mouse ran into a wheel track, and disappeared under the grass, coming
out about 6 feet from where it went in. The white animal followed
through the same course, and when it came out, I saw that it was a
small Weasel, very little larger than the Mouse, and that it was
following the trail of the Mouse by scent.
"For a time the Mouse ran in circles, and zigzagged about, often . . .
within 4 or 5 feet of the Weasel; but the latter seemed so intent on
the trail, that it did not notice the Mouse to one side. After a time
the latter started toward the open golf course; and when the Weasel
reached the point where the trail was straight, it sighted the prey,
made a sudden dash forward, and, although 25 feet behind, overtook the
Mouse while it was going 3 or 4 feet.
"For a few seconds, they seemed to fight, until the Weasel got the
Mouse by the throat, and started for the bushes, dragging the body.
When it came to within about three feet of me, I moved a little to see
what it would do. It dropped its victim, and ran into the ravine. The
Mouse had a drop of bright red blood in the center of its white throat.
I waited near by for 15 or 20 minutes, thinking the Weasel might come
back, but it did not show up again; even an hour later, the Mouse had
not been disturbed."
There are two suggestions, but no proof that I know of, in the
literature that the least weasel eats insects. Abbott (1884:27-32--1st
ed., 1884) gives considerable information on the food (some insects
included) of the "little weasel" which he describes (_op. cit._; 27) as
having "a little pointed tail of a uniform brown color." Although this
suggests _Mustela rixosa_, Abbott mentions on the next page (page 28)
that a specimen of the smaller weasel measured six and a half inches
from the tip of the snout to the base of the tail and that the tail
itself measured two and a fourth inches to the tip of the last caudal
vertebra. These measurements indicate that _Mustela erminea_ was
involved. Because of the uncertainty as to the species of _Mustela_
involved, Abbott's interesting data on food, nest and behavior are not
recorded in the present work. Seton (1929 (2):636) says that of several
least weasels brought to D. Nicholson at Morden, Manitoba, most of them
decayed so quickly that they could not be saved as specimens. To Seton
this indicated that insects were an important part of the food of the
weasels.
In summary: Least weasels are known to eat harvest mice, deer mice,
meadow mice and red-backed mice; it is suspected that they eat also
insects.
Reproduction
Polderboer (1948:296) has taken six specimens in "northeastern Iowa
[in] . . . January and December--all males in winter pelage. None of
these males showed signs of sexual activity; in all, the testes were
retracted and diminutive in size. . . . A male least weasel in brown
pelage was taken November 17, 1945, at Marion, Iowa. This specimen had
large testes that had descended into the scrotum. The testes, when
removed, were about the size of medium-sized garden peas. Microscopic
examination of the testes and the vasa deferentia showed mature sperms
to be present. . . ."