Western harvest mice that attain adulthood acquire at least three distinct types of pelage in sequence in the course of their development. The first of these, the juvenal pelage, is short, relatively sparse, and characteristically grayish brown. The molt (post-juvenal molt) from juvenal pelage to subadult pelage seemingly occurs at an early age, perhaps frequently before the young leave the nest, as individuals in juvenal pelage are few among specimens studied by us. Judging from study skins alone, the progress of post-juvenal molt in R. megalotis is similar to that described for R. humulis by Layne (1959:69-71). The subadult pelage is thicker, longer and brighter than juvenal pelage and closely resembles the pelage of adults; it differs from adult pelage dorsally in being somewhat duller and in having less contrast between back and sides.
The pelage of adults varies depending on season. In summer the individual hairs are relatively short (5-6 mm. at the middle of the back) and sparse. The over-all color of the dorsum, sides and flanks is brownish to dark brownish, and the venter is grayish. In winter the pelage is dense, long (8-9 mm. at the middle of the back) and lax. The over-all color dorsally in fresh winter pelage in most specimens is paler (more buffy) than summer pelage, the sides are markedly buffy, and the venter is whitish; even the tail is more pilose and more sharply bicolored than in summer. Adults molt, usually completely but occasionally only partially, at least twice a year—once in spring (in May and June in Nebraskan specimens) from winter to summer pelage, and once in autumn (in October and November in Nebraskan specimens) from summer to winter pelage. Of the two molts, the one in spring is most easily discernible because the contrast in color between worn winter pelage and fresh summer pelage is considerably greater than that between worn summer pelage and fresh winter pelage, and because the progress of spring molt is seemingly more regular than that of autumn molt. In spring, molt proceeds posteriorly in a more or less regular line on both dorsum and venter; in most specimens it is completed first on the venter. In autumn, molt is irregular, or at best is coincident over large parts of the body, and frequently is seen only by searching through the pelage with a fine probe or dissecting needle. In both spring and autumn, molt seemingly is delayed in females that are pregnant or lactating.
In both winter pelage and summer pelage, the upper parts have blackish or grayish guard hairs and shorter, more numerous cover hairs. All the cover hairs are gray basally; some have a buffy band terminally and others have a buffy subterminal band with a terminal black tip. The generally darker over-all color of upper parts in summer pelage results (as seen in Nebraskan specimens) from a narrower band of buff on the cover hairs (only approximately one half the width of the band on hairs in winter pelage), a darker buffy band (ochraceous buff rather than pale ochraceous or straw color), and a relative sparseness of the pelage, which allows the gray basal portion of some hairs to show on the surface. The more grayish venter of summer-taken specimens results from much more of the grayish basal portion of the white-tipped hairs showing through than in the longer, denser pelage of winter.
Wear on the pelage seems in general to produce a paler over-all color of upper parts, evidently due mostly to abrasion of the terminal black tip of the cover hairs, but possibly actual fading of the pelage is involved also. Worn winter pelage is especially notable for its paleness; the buffy tones are accentuated and the upper parts, especially posteriorly, may even appear fulvous. The difference in color of upper parts between specimens in worn winter pelage and fresh summer pelage (or for that matter specimens in fresh versus worn winter pelage) from the same locality is greater in our material than the difference between some specimens in comparable pelages from localities more than 500 miles apart.
We have seen no specimens taken in winter in which we could discern that the autumn molt had been incomplete, but three old adult males in summer pelage indicate that spring molt is not always completed. KU 50154, obtained on August 14, 1952, 5 mi. N and 2 mi. W Parks, Dundy Co., Nebraska, has the entire posterior back and sides still in old winter pelage and does not appear to have been actively molting; the entire venter is in summer pelage. KU 50146, obtained on August 22, 1952, 3 mi. E Chadron, Dawes Co., Nebraska, has small patches or tufts of winter pelage remaining on the rump and likewise does not appear to have been actively molting. KU 72085, obtained on October 13, 1956, 4 mi. E Barada, Richardson Co., Nebraska, is in the process of molting from summer to winter pelage, but has tufts of old winter pelage on the rump.
Geographic Variation
Geographic variation, both in color of pelage and in external and cranial dimensions, is less in R. megalotis in the region studied than in most other cricetine species that occur there. Nevertheless, meaningful variation is present. The assumption that variation in R. megalotis paralleled in degree that of other species, Peromyscus maniculatus for example, led to untenable taxonomic conclusions by some previous workers.
Color of Pelage
Color of pelage is remarkably uniform, considering the geographic extent of the area involved, over most of the northern part of the central grasslands. Perhaps this uniformity results partly from the predilection of the western harvest mouse for grassy habitats, for in most areas on the Great Plains the species is restricted to riparian communities, principally along river systems, where soils, cover, and other conditions approximate those of corresponding habitats farther to the east to a much greater degree than do conditions in upland habitats. Differential selective pressure, therefore, theoretically would be less between eastern and western populations of R. megalotis than in an upland-inhabiting species. In any event, specimens from western Nebraska, Wyoming, northern Colorado, and adjacent areas average only slightly paler dorsally than specimens in corresponding pelages from the eastern parts of Nebraska and Kansas, and many individuals from the two areas can be matched almost exactly.
To the southwest, on the other hand, a trend toward paler (pale brownish, less blackish) upper parts is apparent. Specimens from southwestern Kansas and adjacent parts of Colorado and Oklahoma average slightly paler in comparable pelages than specimens from northeastern Kansas and eastern Nebraska, but most specimens from farther southwest, in northern New Mexico and southwestern Colorado, are discernibly, although not markedly, paler than mice from northern and eastern populations.