110 ANIMAL LIFE IN TEE YOSEMITE 



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 pi. 25a) ; tail usually slightly longer than 

 head and body; hind foot and ear (fig. 10b) both of moderate size. Head and body 

 314 to 4 inches (87-100 mm.), tail 3% to 41/3 inches (95-110 mm.), hind foot about 

 % inch (21-23 mm.), ear from crown about % inch (17-20 mm.); weight % to 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 

 canons, 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 canon ; 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 



