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 Guatemala, 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.