BOYLE WHITE-FOOTED MOUSE. Peromyscus boylii boylii (Baird)

Field characters.—Size more than half again that of House Mouse, about one-third larger than Common White-footed Mouse (see pl. 25a); tail usually slightly longer than head and body; hind foot and ear (fig. 10b) both of moderate size. Head and body 3-1/2 to 4 inches (87-100 mm.), tail 3-3/4 to 4-1/3 inches (95-110 mm.), hind foot about 7/8 inch (21-23 mm.), ear from crown about 3/4 inch (17-20 mm.); weight 3/4 to 1-1/4 ounces (22.6-34.4 grams). General coloration above dark brown (bluish gray in juvenal), this color sharply set off along sides from pure white of under surface of body; feet white.

Occurrence.—Common resident on west flank of Sierra Nevada, chiefly in Upper Sonoran and Transition zones, but occasionally at higher stations. Recorded regularly from Pleasant Valley eastward to walls of Yosemite Valley and sparingly at Porcupine Flat and Glen Aulin. Inhabits vicinity of rocks and brush on sides of ravines and cañons, less often grassy places, but as a rule not far from water. Nocturnal.

The Boyle White-footed Mouse is second in point of size, numbers, and extent of range among the four species of white-footed mice in the Yosemite region. It is larger than the Common White-foot but smaller than the Gilbert Mouse, and it also stands between these two in relative numbers as is revealed by our extensive trapping. The range of the Boyle Mouse lies entirely on the western drainage of the Sierras; the species has no counterpart on the eastern side of the mountains in this latitude. The local range embraces much of the territory between the altitudes of 600 and 6000 feet.

A variety of situations is occupied by this mouse, though it is rather more restricted in this respect than is the Common White-foot. Some Boyle mice were captured on brushy and rocky stream banks, others (at El Portal) were in sandy 'second bottom' land under wild grapevines, still others under brush plants on hillsides, and many were obtained at El Portal and in Yosemite Valley, amid rocks on talus slopes covered with golden oaks. In Yosemite Valley this species is a regular inhabitant of the rock heaps along the Valley walls, but it seldom occurs out on the floor of the Valley. One factor which seems to be constant in its requirements is proximity to water, not necessarily very close at hand but where it can be reached during the animal's nightly foraging.

The Boyle Mouse is the best climber among the four local species of white-footed mice. One was trapped on a shelf of rock 10 feet above the bottom of a cañon; on this rock were many droppings indicating that mice had run about on it upon various occasions. Elsewhere Boyle Mice have been seen climbing about in trees.

Among the numerous specimens trapped in Yosemite Valley were many having the ears variously notched and otherwise mutilated; also individuals with tails more or less bobbed. These things point to a certain trait known to manifest itself among captive mice, namely, propensity toward violent combat between adults, especially during the periods when they are sexually active.

Definite information concerning the breeding season of the Boyle Mouse consists of records of embryos in 7 females and the capture of a number of blue-pelaged juvenal animals. The females with embryos were taken from May 19 to June 7 (1915), and held from 2 to 5 embryos, averaging about 3. But blue-pelaged young were taken as early as June 7, indicating birth about a month earlier. Even as late as December 2, blue-coated young were trapped, a fact which indicates that some litters may be born in late October or even early November.