THE MAMMALS
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.
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