oregonensis_, no. 47149, Mus. Vert. Zoöl., taken on December 20, 1930,
at Carlotta, Humboldt County, California. This specimen appears to be
white and initially was thought to be merely an individual in the white
winter coat. Closer examination, however, shows that it has a light
wash of ochraceous or faint reddish color. Also, other specimens taken
in winter at Carlotta show that weasels there do not acquire a white
winter coat. The only normally brown area is approximately three
millimeters in diameter at the anterodorsal margin of the pinna of the
right ear. The tip of the tail is black as in a normal specimen. The
specimen in question is actually pure white only on top of the head
from a short distance behind the ears on over the forehead nearly to
the eyes, and on the inside of the ears. In a normally colored animal
this area is the dark area of the head. In this freak, the other parts
of the head, which, in individuals of normal coloration are the white
or light orange facial markings, have the reddish cast of the remainder
of the body, although the color is less intense than on the back. The
collector noted that the specimen had eyes of normal color. A possible
explanation for the coloration of this specimen is that this species
has three factors for color, one for the black tail tip, one for the
reddish color, and a third, missing in the specimen in question, for
the blackish brown.
For some more exact knowledge concerning this erythristic type of
coloration, we are indebted to Pitt (1921:99), who describes a
population of polecats, _Mustela putorius_, in Cardiganshire, England,
in which this erythristic variation is maintained in a state of nature.
In ferrets, _Mustela furo_, Pitt (_op. cit._:114) notes that ". . .
erythrism is certainly dependent on a Mendelian factor, being dominant
to albinism and recessive to the black-brown coloration. Both in the
ferret and polecat, erythrism seems to be correlated with increased
size, and certainly in the ferret is usually accompanied by a quick
temper and general increase in vitality."
Variations of Taxonomic Worth
Variations of taxonomic worth usually are referred to as characters.
For example, shortness of the tympanic bulla is a character, and the
opposite condition, long tympanic bulla, is another character. Specific
variations, that is to say specific characters, are provided by the
color-pattern, length of tail, number of premolar teeth, shape of the
tympanic bullae, and length of the braincase in relation to the length
of the tooth-bearing parts of the skull. Subspecific characters are
provided by color-pattern, color itself, size as measured by weight of
the animal, and its linear measurements, size of the skull, and size
and shape of parts of the skull. The characters distinguishing
subspecies from one another are not of a different nature from those
distinguishing species from one another.
Given any one of the above structural features, say, dorsal outline of
the skull, several characters may be provided by it. For example,
weasels of the species _Mustela frenata_ have the dorsal outline of the
skull convex in southern Louisiana, straight in Missouri and concave in
North Dakota, thus providing three characters. This is geographic
variation. These variations, characters in zoölogical parlance, when
plotted on maps, reveal the geographic occurrence of, say, the convex
shape of the skull. In combination with other characters, for example,
dark color and short tail, basis is provided for recognizing a
subspecies, in this instance _Mustela frenata arthuri_ of Louisiana.
Because the change from convex to flat skull takes place geographically
at about the same place (in eastern Texas) as does the change from
short tail to long tail, and the change from dark color to light color,
it is easy to draw a line there marking the western geographic limit of
occurrence of the _M. f. arthuri_. This same line marks also the
eastern margin of the geographic range of the subspecies _Mustela
frenata frenata_, the subspecies next adjacent to the westward. On this
line and for several miles to either side of it weasels show varying
combinations of these three characters or an intermediate condition as
regards one or more of the characters, or both. For example, from a
locality in eastern Texas a weasel may have (1) a facial pattern
exactly intermediate between that of the unicolored face of _arthuri_
and that of the bicolored face of _frenata_, (2) the long tail of
_frenata_ and (3) the convex skull of _arthuri_. In the sum of its
characters this specimen is exactly intermediate between typical
_arthuri_ and typical _frenata_. Another specimen from the same place
may differ from the first specimen only in having the tail slightly
shorter. The total "score" for the two specimens is, therefore, by a
very slight margin in favor of _arthuri_. Let us suppose that we obtain
a third specimen from the same place and that it has the face marked
like that of _arthuri_ but the tail fully as long, and the skull as
lacking in dorsal convexity, as in _frenata_. Now the score is
definitely for _frenata_. For convenience of handling, the population
is referred to _frenata_, providing that the average of specimens from
a nearby locality to the westward is not in favor of _arthuri_. In
event the average of specimens from a locality next adjacent to the
westward is in favor of _M. f. arthuri_, the total evidence from the
two localities may be weighed together and appropriate decision as to
subspecific status of weasels from the area is made according to what
the average is for the area as a whole.
The three individual animals of an intermediate sort are ordinarily
termed _intergrades_. This implies that their characters are the result
of mixed parentage--perhaps a female of _M. f. arthuri_ and a male of
_M. f. frenata_ but probably each parent itself was an intergrade and
the offspring, of which we examined three, owe their characters to
reproductive processes operating in obedience to Mendelian laws of
inheritance.
The two kinds of animals, _Mustela frenata arthuri_ and _Mustela
frenata frenata_, are identified as subspecies because of the
intergradation between them. If at this and all other places where the
geographic ranges of _arthuri_ and _frenata_ met there was no
crossbreeding (no intergrades), the two kinds would be treated as
distinct species. Intergradation, and the lack of it, are accepted as
the criteria of subspecies and species, respectively.
These criteria suffice for animals, in this instance weasels, which
have a continuous geographic distribution. Some kinds of weasels are
confined to islands, as for example the islands off the coast of Alaska
and British Columbia. Because weasels are land animals, crossbreeding
in nature between the weasels of two islands is, of course, impossible.
A modified test (used in the study here reported upon) in deciding on
specific versus subspecific status in these instances can be made as
follows: On the adjacent mainland, ascertain the degree of difference
between two subspecies whose geographic ranges meet (for example, _M.
e. richardsonii_ and _M. e. alascensis_). Next ascertain the degree of
difference between the insular kind of animal and the kind on the
mainland. If the degree of difference is greater when the insular kind
is compared than when only the kinds of the mainland are compared, the
insular kind is to be regarded as a species. If the degree of
difference is no greater between the insular kind and the mainland kind
than it is between the two adjacent mainland kinds, the insular kind is
to be regarded as a subspecies. In short, for insular kinds, the
criterion is degree of difference, with the limitation of geographic
adjacency, rather than intergradation.
The geographic variation (subspecific characters) found could be spoken
of as two kinds: First, there is the variation which is expressed in a
general trend for a long distance, producing, in general, a cline of
even slope; and second, that of inconstant trend in any one direction.
In his "The Rabbits of North America" Nelson (1909:34-35) has commented
on the latter type of variation as follows: "While studying series of
specimens from all parts of the vast range occupied by the geographic
races of such species as _Sylvilagus floridanus_ and _S. auduboni_, I
have been impressed with evidences of fluctuation of both external and
skull characters. These fluctuations are somewhat wavelike in character
and rise to central points of extreme development and then sink away to
intermediate borders beyond which new waves rise. Where the waves of
differentiation are pronounced they mark recognizable geographic races.
Within the area covered by the larger or geographically broader waves
of differentiation (recognized as of subspecific value), smaller waves
of differentiation are included, which may represent local variations
in intensity of characters of the subspecies, or these characters may
diminish and the variation tend in other directions, sometimes even
closely reproducing the characters of another subspecies occupying a
distinct area." In _Mustela frenata_, much of the geographic variation
at first inspection appears to be of this nature. Closer scrutiny,
however, reveals that the repetition, at geographic intervals, of
several features of color and structure are closely correlated with
environmental features which are repeated only at these same places.
In _Mustela erminea_, much of the variation is of the first kind,
namely, that which can be expressed as long clines of relatively even
slope. As several authors have said, zoölogical classification based on
this kind of variation is like dividing the spectrum and depends
largely upon the standards set, for, theoretically, the possibilities
of subdivision are unlimited. Actually, however, none of the clines has
an even slope and the possibilities for subdivision therefore are
limited. Also, when several features are used, instead of only one
feature, the classification is more satisfactory even if the basis is
more complex.
Some features of structure which provide subspecific characters are
mentioned below.
Total length, of males, ranges from 598 to 360 mm. in _M. frenata_ and
from 336 to 228 mm. in _M. erminea_. There is no cline of sustained
slope in _M. frenata_ but in _M. erminea_ there is a progressive
decrease in total length from north to south.
Length of tail varies from as little as a half to as much as
seven-tenths of the length of the head and body in _M. frenata_, the
subspecies _neomexicana_ having the long tail and the two subspecies
_arthuri_ and _primulina_ having short tails. The geographic ranges of
_primulina_ and _neomexicana_ are contiguous. In _M. erminea_ there is
likewise no variation of a clinal nature in length of tail and
furthermore the variation is much less than in _M. frenata_.
In length of hind foot, which in males varies from 49 mm. in northern
populations of _M. erminea_ to 28 mm. in southern populations, the same
cline is seen as in the total length of animals of this species. In _M.
frenata_, however, there are several decreases and increases along any
straight line which can be drawn through the geographic range of the
species. The range of variation in males is 41 mm. (_M. f.
arizonensis_) to 59 mm. (_M. f. macrophonius_).
Weight of the entire animal is an excellent measure of size but weights
are unavailable for many subspecies. In _M. frenata_, the two
subspecies _texensis_ and _macrophonius_ probably are the heaviest and
_effera_, _arizonensis_ and _helleri_ probably are the lightest.
Geographically the variation in weight behaves in approximately the
same way as does the measurement of total length. In _M. erminea_ the
variation in weight of males is from 206 grams in northern animals to
58 grams in southernmost populations, there being a relatively constant
gradient geographically.
Degree of hairiness of the foot-soles in _M. frenata_ clearly is linked
with the temperature; in regions of high average temperature the
hairiness is least and in regions of low average temperature it is
most. The decrease in hairiness is accomplished in two ways, namely,
smaller breadth and decreased length of individual hairs and decrease
in number of hairs on a given area of dermal surface. This correlation
holds throughout the entire north to south range of the species.
Corresponding differences are found on the same latitude where
topographic diversity in an east to west direction produces northern
conditions at high altitudes and southern conditions at low altitudes.
The conclusion seems unavoidable that climate, directly or indirectly,
determines the degree of hairiness. Less careful observations were made
on the hairiness of the soles of the feet in other species but it is
clear that the northern species _M. erminea_ has the most hair on the
foot-soles and that _M. africana_, the tropical weasel, has the least.
In this regard, _M. frenata_ is intermediate as it is also in
geographic position.
[Illustration: FIGS. 11-15. Dorsal views of adult skulls of each sex of
five subspecies of the ermine, _Mustela erminea_, to show secondary
sexual variation and geographic variation in size of the skull. Males
on the left and females on the right. All × 1.
Note especially the geographic variation in decreasing size of the
skull from north to south in each sex, and that the secondary sexual
variation in size of skull is less in ermines with small skulls than in
those with large skulls.]
[Illustration: FIG. 16. Map showing the localities where the skulls,
represented in figures 11-15, were obtained.]
The maximum length of facial and carpal vibrissae is attained in _M.
erminea_ in the far north. In weasels from north of the Arctic Circle
the longest facial vibrissae extend posteriorly beyond the posterior
border of the ear. In the tropical weasel, _M. africana_, the facial
vibrissae do not extend posteriorly beyond the ear and the carpal
vibrissae are not so long as the distance between their bases and the
apical pad of the first digit. The correlation of long vibrissae with
low temperature, is mentioned here merely because length and density of
pelage were under consideration.
The most obvious and most exact correlation between change in climate
and change in the animal is furnished by color. This is well shown in
the one species, _Mustela frenata_, to which the following remarks
apply unless indication is given to the contrary. The color of the
upper parts varies from bay (blackish brown) in _M. f. panamensis_ to
buckthorn brown (light brown) in _M. f. neomexicana_. The color of the
head varies from solid brown (white chin excepted) to contrasting black
and white markings.
Dark color of the upper parts is associated with a large area of this
color; the enlargement of this area is at the expense of the area of
light color on the underparts. In the weasels of darkest color the
upper parts occupy four-fifths of the circumference of the body (as
measured in the anterior lumbar region) but in the lightest-colored
weasels the upper parts comprise only two-thirds of the total
circumference. In these light-colored animals the color of the
underparts extends onto the underside of the tail and down the insides
of the legs and over the feet whereas in the animals with the darkest
upper parts the entire tail, feet, and legs below the knees ordinarily
are of the same dark color as the upper parts. The length of the black
tip on the tail varies inversely with the length of the tail, probably
because the lightest-colored weasel has the longest tail. In some
subspecies the black brush is almost half as long as the tail-vertebrae
but in others is less than a fourth as long as the tail-vertebrae.
The extent of the color of the head, as well as the intensity of the
color there, varies markedly and is correlated with climatic
conditions. The extent and intensity of this dark color is greater in
weasels inhabiting regions of heavy rainfall than in those inhabiting
regions of sparse rainfall. Considering the geographic range of each
subspecies of _Mustela frenata_, that of _M. f. panamensis_ has the
maximum of rainfall. Reference to the colored plate (1) will show that
in _M. f. panamensis_ (2) the black of the head is extended over all of
the upper parts. _M. f. macrura_ (1) of Perú, to the southward, is from
an area of lesser rainfall and is correspondingly lighter colored.
Returning to _panamensis_ (2) as a starting point and proceeding
northward to the range of _nicaraguae_ (3), which also has lesser
rainfall, thence another step northward to Guatamala, which has still
less rainfall, the weasel there, _M. f. goldmani_ (4) has the black
extending posteriorly only to the shoulders. _M. f. leucoparia_ (5)
from Michoacán, and _M. f. frenata_ (6) from Tamaulipas are from
progressively more northern and also progressively drier regions. In
_M. f. frenata_ (6) the dark color extends posteriorly only to the ears
and is blackish rather than black. In _M. f. neomexicana_ (7) of the
extremely arid parts of Durango, Arizona, and New Mexico the dark
marking of the head is confined to a brown spot on the nose. Its
geographic range is the most arid of those of all of the subspecies.
The contrast between _neomexicana_ (7) and _panamensis_ (2) illustrates
the great range of geographic variation in color which occurs in the
one species. Continuing from the geographic range of _neomexicana_
(specimen from Safford, Arizona) northwesterly 480 miles to Riverside,
California (see 8, _latirostra_), 430 miles north to Point Reyes,
California (see 9, _munda_), and finally 570 miles north to Tillamook,
Oregon (see 10, _altifrontalis_), each place with more rainfall than
the one farther south, another correlation of increasingly dark
coloration with increasing amount of rainfall is illustrated.
This geographic variation, it should be remembered, is all within one
species. It is the more significant still when we remember that the
same correlation, with never an exception, occurs at hundreds of places
within the geographic range of the species. A particular feature of
climate, namely rainfall, and possibly therefore humidity, is concerned
in this correlation. The same correlation, heavy rainfall and dark
color, is shown also in the other species of North American weasels.
The conclusion is unavoidable that climate, directly or indirectly,
determines or influences the color of weasels.
The light facial markings appear in American weasels in two separate
geographic areas. One is the southwestern United States, México and
northern Central America. The second area is in the same latitude, in
Florida and adjoining parts of Georgia and Alabama. In the western
weasels the markings are white south of latitude 32° N. North of this
latitude, the facial markings, if at all extensive, usually are of the
same yellowish color as the underparts of the body. Weasels of southern
California and its interior valley usually have these yellowish instead
of white facial markings. The light facial markings, in this instance,
white markings, attain their maximum extent in _M. f. leucoparia_ of
the southwestern margin of the tableland of México, at latitude 19° N.
A gradual decrease in area of the light facial markings occurs both to
the north and south; they disappear at 10° N in _M. f. costaricensis_
and at 35° N at approximately the southern limits of range of _M. f.
arizonensis_ and _M. f. nevadensis_. In the mild climate of California
the light (yellowish) facial markings are found at still higher
latitudes. These light facial markings crop up as vestiginal remnants,
consisting of a few white hairs, in some individuals of nearly all
races of weasels.
In certain parts of the skull there are trends, in size and shape,
which continue for long distances geographically. In other words,
clines can be recognized. Changes in size and shape in some other parts
of the skull are wavelike; change toward narrower rostrum, for example,
is not progressive in a given geographic direction for any great
distance. Length of the upper tooth-rows and zygomatic breadth, when
expressed as percentages of the basilar length, and also the actual
length of individual teeth vary geographically in the same wavelike
fashion as does the width of the rostrum.
Size of the skull, on the other hand, shows a sustained trend for a
long distance; it becomes progressively smaller from the southern
United States southward to Columbia, South America. This clinal
variation can be demonstrated by plotting on a graph, the basilar
length, the zygomatic breadth, or the weight of the skull. Beginning at
Mérida, Venezuela, and proceeding southward to increasing elevations in
the mountains of South America, there is a reversal of the direction of
the variation in this cline; weight of skull, for example, increases to
the southward from Mérida for a considerable distance. A cline of
decreasing width of the postorbital constriction of the skull is
evident from Panamá north into Texas.
Variations in the tympanic bullae provide many characters useful in
distinguishing weasels from different localities. Most of these
characters have to do with degree of inflation of the bullae.
Indirectly correlated with degree of inflation is first the extent of
removal of the anterior margin of the bulla from the glenoid fossa and
foramen ovale, and second the form (convex, flat, or concave) of the
part of the squamosal bone between the foramen ovale and the anterior
margin of the tympanic bulla. As one proceeds southward from, say,
southwestern Kansas through the geographic range of the species
_Mustela frenata_, there is a progressive deflation of the bulla, an
increase in length of the space between its anterior margin and the
foramen ovale, and the floor of the braincase in front of the bulla
changes from ventrally concave to ventrally convex. (See figs. _e_ and
_h_ of pl. 24 and figs, _e_ and _f_ of pl. 27.)
One extreme of this variation in bulla is shown in _Mustela frenata
neomexicana_ (fig. _e_ of pl. 24), in which the anterior margin of the
bulla (viewed from the ventral side) rises vertically from the floor of
the braincase to form a 90-degree angle. The other extreme, the
uninflated bulla, is in _Mustela frenata panamensis_ (fig. _e_ of pl.
27), in which the anterior margin of the bulla is not raised above the
floor of the braincase. This variation is remarkable because it occurs
within a single species. Otherwise, in the family Mustelidae,
differences in the tympanic bullae as great as that between the two
subspecies _M. f. neomexicana_ and _M. f. panamensis_, occur only
between genera. The need for caution in inferring the limits of
variation for a particular structure in one species or genus, on the
basis of variation in another group, is therefore obvious.
Speaking now of full species, the most inflated tympanic bullae in
American weasels are in _Mustela frenata_, and more restrictedly in
those subspecies of it which occur in the temperate region. Subspecies
of _M. frenata_ in Central and South America, as already noted, have
less inflated bullae. The tropical weasel, _Mustela africana_, of the
Amazon drainage of South America has the bullae still less inflated
(see fig. _i_ of pl. 39 and fig. _f_ of pl. 40). The bullae are less
inflated even than in the mink, subgenus _Lutreola_. In _M. africana_
the cleidomastoideus, omotrachelian, levator scapulae, and rhomboideus
profundus muscles take origin from a fossa on the mastoid bone, whereas
in the forms with greatly inflated bullae these muscles take origin
from a raised ridge or tubercle. Using _Mustela frenata_ of the
temperate region as a starting point and proceeding northward, a
reduction in inflation of the tympanic bulla is seen also in that
direction in that _Mustela erminea_ has less inflated bullae. The
bullae are less inflated in southern than in far northern (arctic)
populations of _Mustela erminea_. In _erminea_ the lesser inflation is
real enough but at the same time there appears to be less inflation
than actually exists, for the squamosal floor of the braincase is
"pushed down." This places the anterior end of the tympanic bulla
farther in the braincase than it otherwise would be. Although the
anterior end of the bulla is flattened to the extent that it resembles
the sharp edge of a splitting-wedge, inspection of the lateral and
medial edges shows that in its central part the bulla is more inflated
than it is in the weasels of Central and South America.