108 ANIMAL LIFE IN THE YOSEMITE 



were accomplished by swift runs; the rapid patter of small feet would be 

 followed by several seconds of quiet while the mouse took account of its 

 new surrounding's. About this time the moon came up and the mouse 

 could be seen clearly in the bright light. Whenever the observer moved, 

 the mouse would scamper into some hiding place; but its fright was of 

 very short duration and it would soon reappear. 



That the Common White-footed Mouse does on occasion range higher 

 than the ground is indicated by the fact that several individuals were 

 trapped on pantry shelves up to six feet above the floor in a house in 

 Yosemite Valley, and another individual was caught eight feet above the 

 ground on top of a prostrate tree in a windfall at Tuolumne Meadows. 

 Practically all our traps were set on the ground, so we are unable to state 

 the extent to which these mice may climb. The animals sometimes venture 

 well out from shelter ; individuals were taken on open ground as much as 

 20 feet away from cover of any sort. Most of those trapped, however, were 

 obtained close to or under logs, rocks, or brush, where the majority of our 

 traps were set. 



At Snelling an adult and a juvenile mouse were caught together in the 

 same trap, this incident suggesting that young individuals may forage for 

 a time in company with their parents. The species is not colonial, in 

 any definite sense of the term ; although it occurs locally in considerable 

 numbers, the adult individuals are, as a rule, intolerant of one another's 

 presence. 



It is not known with certainty that the Common White-footed Mice 

 hibernate. There is even good evidence to the contrary. In the winter 

 months their tracks are often to be seen in Yosemite Valley, on the surface 

 of the snow. Individuals were trapped in December in dead grass and 

 leaves in sheltered places. Here, it seemed likely, they had been running 

 about among the bases of grass stems beneath the snow mantle. 



The breeding season is of long duration and each female very probably 

 bears more than one litter a year. Females with embryos were taken from 

 May 13 until October 24, and evidence, in the form of blue-pelaged juvenals 

 or sexually active males, suggested that, in the lower altitudes at least, the 

 species was breeding practically throughout the year. The number of 

 young to a litter ranges from 3 to 7, averaging 5. Of 38 sets of embryos 

 examined, in two cases there were 3, in eight cases 4, in thirteen 5, in 

 thirteen others 6, and in two cases there were 7. The young come quickly 

 to maturity and some of them undoubtedly breed during the same season 

 in which they are born. Thus within one favorable season, when all of 

 the offspring would be able to find sufficient food of a suitable nature, the 

 numbers of these mice might increase very greatly. 



