124 ANIMAL LIFE IX THE YO SEMITE 



As a group the meadow mice are grass feeders, and when green vege- 

 tation is available they will take little or nothing else. (See fig. 21.) Some- 

 times when the animals are unusually plentiful they gnaw the bark of trees. 

 Since the main item of diet is grass, their whole scheme of existence is 

 modeled for obtaining this sort of food. 



In order to be close to their source of food supply and also to be able 

 to escape if danger threatens while they are forag^ing, these mice carry 

 on their whole existence right in the meadows. They dig small shallow 

 burrows or tunnels dowTi through the sod to a depth of several inches and 

 place their nests in enlarged parts of these tunnels. When they venture 

 out to forage, they follow runways which they cut through the vegetation ; 

 along these they can run readily without interference and are to some 

 extent shielded from observation. These surface runways constitute the 

 most obvious indication of the presence of meadow mice and are the things 

 which a naturalist always searches for when he wishes to locate these 

 rodents. Sometimes in making a runway the grass is merely trampled 

 down, but usually the mice carefully cut away the stems so that the floor 

 of the run is at or even slightly below the level of the ground. In the latter 

 case, to the eye of a person, the runway shows a-s of earth color in contrast 

 with the surrounding green vegetation. The floor of the run is kept free of 

 obstructions by the mice, but often the growing grass arches over the top 

 forming a canopy under which the animals may travel for long distances 

 without much danger of being observed by a foraging hawk or owl over- 

 head. The system of surface runways is often extensive, with many inter- 

 sections; as many as five and even six different paths may radiate from a 

 single hole. The direction and extent is probably dictated by the relative 

 abundance and location of forage material. The usual width is from 

 1 to ll^ inches. The average dimensions are slightly greater in the 

 runways of the Mariposa than in those of the Yosemite Meadow Mouse; 

 the former mouse is on the average a slightly larger animal. In late 

 summer on Tuolumne Meadows many small narrow runways were noted 

 which were thought to have been made by the young of the year after they 

 had taken up an independent existence apart from their parents. 



At the end of September (in 1915), on Tuolumne Meadows, the meadow 

 mice were exceedingly active. Extensive surface runways were seen, as 

 well as many holes leading to underground burrows. In some places there 

 were 4 to G holes per square yard of surface. There was much cut grass 

 in the runways and in one place, under an overhanging bank, a mass aggre- 

 gating about two handfuls was noted. Droppings were scattered along all 

 the runways, and occupation of the runs for a considerable period of time 

 was indicated. 



