WHITE-FOOTED MICE 111 



to manifest itself among captive mice, namely, propensity toward violent 

 combat between adults, especially during the periods when they are sex- 

 ually 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. 



Big-eared White-footed Mice. Peromyscus truei (Shufeldt)^ 



Field characters. — Size about t\Aice that of House Mouse, and somewhat greater than 

 Common White-footed Mouse; tail about equal to head and body; ears large (20 mm. or 

 over). (See fig. lOd.) Pelage long and dense; tail well haired, with a slight 'pencil' 

 at tip. General coloration above dark brown, sharply set off from pure white of under 

 surface; feet white. 



Occurrence. — Eesident on west slope of Sierra Nevada, chiefly in Upper Sonoran Zone. 

 Eecorded from Pleasant Valley eastward to 6 miles east of Coulterville and to El Portal 

 (subspecies gilberti). Also east of Sierra Nevada on Williams Butte (subspecies truei). ^ 

 Lives about rocks and brush. Nocturnal. 



White-footed mice of the big-eared or truei group are present on both 

 slopes of the Sierra Nevada but never in as large numbers or so widely 

 distributed as the common species {mmiiculatus) . The range of this species 

 on the west slope lies chiefly in the Upper Sonoran Zone; on the east side 

 it was found in the belt of piiion pines. The Gilbert Mice of the west side 

 were found on the ground in brushy places or under pines and other trees 

 near the chaparral, and on one occasion about a deserted building. The 

 True Mice of the east slope were mostly taken in or near rocks, though this 

 species is not necessarily a rock dweller. 



9 Two slightly differing subspecies of this group occur on the opposite slopes of the 

 Sierra Nevada. Their ranges do not touch at any point in the Yosemite region, but they 

 do come together at other localities to the south, and intergradation takes place there. 

 Hence the two are considered as subspecies of one species. 



Gilbert White-footed Mouse, Peromyscus truei gilberti (Allen). The form common 

 through much of California west of the Sierra Nevada. It was found locally from Pleas- 

 ant Valley eastward to Smith Creek (6 miles east of Coulterville) and to El Portal. 



True White-footed Mouse, Peromyscus truei truei (Shufcldt). A slightly paler, 

 more silky haired subspecies, which occupies a wide range of territory east of the Sierra 

 Nevada. It was recorded by us on the south slope of Williams Butte in small numbers. 



Measurements. — Gilberti: head and body Si/o to 4i^ inches (88-110 mm.), tail 

 3% to 41/3 inches (94-111 mm.), hind foot % to 1 inch (23-25 mm.), ear from crown 

 % to 1 inch (20-26 mm.), weight % to 1% ounces (23.5-41 grams). Truei: head and 

 body 3% to 4 inches (91-100 mm.), tail 32/3 to 4 inches (92-100 mm.), hind foot about 

 % inch (22.5-24 mm.), ear from crown % to 1 inch (20-24 mm.), weight %o to 1% 

 ounces (20.6-33.0 grams). 



