Development and Growth of Peromyscus Maniculatus Oreas

Abstract

The present report is a result of a study of the development and growth of Peromyscus maniculatus oreas Bangs, the third race of this species investigated in this connection. Data on the growth of the subspecies artemisiae and bairdii already have been presented (Svihla, 1934, 1935). The deer-mice belonging to the subspecies oreas are distinguishable from other races of maniculatus by their dark, rich coloration and the great length of the tail. The latter, according to Osgood (1909), is longer than in any other subspecies of this species. These mice inhabit the moist mountain and coastal regions of western Washington, and southern British Columbia around Puget Sound. Osgood (1909) stated that they intergrade with artemisiae in central British Columbia, but in the region around Mount Baker, where was secured the original stock for this study, no intergradation occurs. The mice were collected alive in Burt traps set near the Nooksack Power Plant some 10 miles east of the town of Glacier in the Mount Baker National Forest. The forest here is typical of the western coastal region, consisting of such valuable trees as western red cedar, Douglas fir, western hemlock, and silver fir. The undergrowth consists chiefly of tangles of salmonberry, salal and huckleberry. The methods used in breeding these mice and the conditions under which they were raised, were the same as those in the studies previously reported. They were fed the same diet, consisting of ground whole wheat, powdered milk and iodized salt, with the addition of green food twice a week. About 5 grams of raw beef liver also was given each mouse once a week. In a previous paper (Svihla, 1933), I stated that when these mice were brought from the coastal region to Pullman, they failed to reproduce, although they bred normally while in western Washington. The present stock of oreas likewise failed to breed when transferred to Pullman until liver (5 grams once a week) was added to the diet, since which time they have reproduced normally. This confirms the observations of Smith and Seegers (1934), who reported similar beneficial effects from the addition of raw liver to the diet of experimental rats and mice. Newly born young of oreas did not differ in general appearance from those of other subspecies of deer-mice of the maniculatus group. They likewise were pink in color, hairless, with eyes closed and the pinnae of the ears folded down over the aural opening. The vibrissae were well developed. Within 24 hours the backs of the young mice became deeply pigmented from the presence of coloring matter in the developing hairs. On the fourth day the pinnae of the ears were erect, although the ear openings still were closed. The time required for the eyes to open varied from 14 to 18 days but usually took 132