WHITE-FOOTED MICE 109 



A maximum concentration of Sonora White-footed Mice was encoun- 

 tered at Mono Mills in 1916. A line of 30 traps about one-half mile in 

 length was set on the ground in the sagebrush among Jeffrey pines near 

 the mill. On the night of June 6, 10 Peromyscus m. sonoriensis and 6 other 

 rodents were caught in this one trap-line. During the day of the seventh 

 8 chipmunks and 2 Golden-mantled Ground Squirrels were obtained. The 

 night of June 1, 21 Peromyscus and one pocket mouse were trapped; the 

 night of the eighth, 20 Peromyscus and two pocket mice; and the night 

 of the tenth, 15 Peromyscus and 1 Kangaroo Rat. Then the line was taken 

 up. The collector's own footprints made as he visited his traps at night- 

 fall to bait and re-set them would in places by morning be obliterated by 

 the multitude of tiny tracks made during the night. Many of the mice 

 in the traps were partially eaten, probably by others of their own species. 

 Food in general seemed scarce. 



The suggestion presents itself that an unusually large population had 

 resulted from exceptionally favorable conditions, including abundant food 

 during a preceding period; and because of the discontinuance of these 

 favoring conditions, the mice were on the verge of starvation just at the 

 time the member of our party started trapping. The potential powers for 

 the expansion of mouse population, as based upon the figures for rate of 

 breeding given above, are enormous, possibly twenty-fold in a single year. 

 A sequence of favoring conditions may on occasion bring about the full 

 realization of this potentiality; but eventually there will be a return to 

 normal numbers. • 



Many of the Sonora Whit^-footed Mice trapped at Williams Butte in 

 the fall of 1915 were sorely affiicted with huge rabbit-fly bots on the back 

 or flank. An immature male, trapped September 22, 1915, had one of 

 these maggots imbedded beneath the skin on one flank and opening toward 

 the ankle. The mouse weighed 14 grams, and the fly larva 1.3 grams — - 

 nearly one-tenth the weight of the host ! 



In Yosemite Valley when the melting snows at higher levels cause a 

 rise of water in the Merced River, the Valley meadows are flooded and 

 the non-aquatic animals which live there are forced, at least temporarily, 

 to seek higher ground. The white-footed mice then move up-slope, invad- 

 ing, en route, the gardens and even the houses of the people living in the 

 Valley. One householder told us that on one particular night, during such 

 an invasion, there were fully 20 of these mice running about the rooms in 

 her house. After a few days the white-foots leave the neighborhood of 

 the houses and seek their more natural retreats. 



