PARASITIC WHITE-FOOTED MOUSE Peromyscus californicus californicus (Gambel)

Field characters.—Size more than twice that of House Mouse or of Common White-footed Mouse; ear very large (see fig. 10c); tail longer than head and body. Head and body 4 to 5 inches (99-123 mm.), tail 4-5/8 to 5-1/3 inches (117-136 mm.), hind foot 1 to 1-1/8 inches (25-28 mm.), ear from crown 4/5 to 7/8 inch (21-23 mm.); weight 1-1/2—1-3/4 ounces (41.5-48.4 grams). General coloration dusky brown on upper surface, sharply set off from pure white of under surface; feet white.

Occurrence.—Resident in Upper Sonoran Zone on west flank of Sierra Nevada where recorded at Pleasant Valley and El Portal. Lives on hillsides covered with oaks and chaparral; sometimes about deserted nests of Streator Wood Rat. Solitary.

The Parasitic White-footed Mouse is the largest of our four species of white-footed mice; indeed, in point of size it approaches an immature wood rat. The name 'parasitic' was applied to this mouse because it is often found about nests of the wood rat and for a time was believed to live habitually with that Species. Now it is known that the Parasitic White-footed Mouse, while using deserted wood rat nests to some extent, is also to be found in other sorts of shelter. Its particular niche in the fauna of the foothill oak-chaparral belt is not surely known, though this species does not seem to be greatly different in habits from the Boyle and Gilbert mice.

The present species is the least common of our white-footed mice. Only 6 specimens were obtained in all the trapping which we did within its range, while at the same time the other white-footed mice were obtained literally by the score.

In one instance a trapped Parasitic Mouse was found to have its stomach enormously distended with some finely chewed material that smelled like oak mast. The stomach with contents weighed 9.7 grams, which was one-fifth the total weight of the mouse.