MEADOW MICE 125 



These mice are abroad to a considerable extent during the daytime, 

 although they are perhaps most active in the early hours of the night. The 

 protection afforded by the runways, particularly those arched over by grass, 

 probably promotes daytime activity. Several of the animals were seen 

 running about by members of our party. One individual noted in Sentinel 

 Meadow, Yosemite Valley, on October 8, 1914, darted along 20 feet or 

 more of tunnel with amazing speed considering the turns which had to 

 be negotiated. The sensitive 'whiskers' may be of considerable service 

 in traversing the runs ; the animal literally ' ' feels its way. ' ' Its relatively 

 smaller eye, as compared with that of a free-ranging animal like the white- 

 footed mouse, may be indicative of the meadow mouse 's lessened dependence 

 upon acute sight. 



Meadow mice are active during winter as well as summer even in locali- 

 ties above the snow line. In Yosemite Valley in late December runways 

 were found through the snow at the margin of the Merced River, and open- 

 ings to the surface were found at short distances along the runs. In other 

 parts of the region winter nests were seen which had been built on top 

 of the ground where, of course, they had been fully protected by the blanket 

 of snow. Considerable activity on the part of the mice is to be observed 

 during the fall months, evidently in preparation for the long Sierran 

 winter. This was noted on Tuolumne Meadows in late September of 1915, 

 and in Yosemite Valley in early October of 1914. In the latter instance 

 excavation of underground burrows was being diligently prosecuted. 



Our data relative to the breeding of the Yosemite Meadow Mouse are 

 plentiful, chiefly because we spent much time and did a great deal of 

 trapping within the range of this species. The evidence in the form of 

 females with embryos or recently born young points to a breeding season 

 extending at least from March or April until October. Thus, on May 4, 

 1916, a female meadow mouse taken near Williams Butte contained 6 

 embryos. Another captured October 22, 1915, at Gentrys, held 5 small 

 embryos. Between these inclusive dates 30 females with embryos were 

 obtained. The number of embryos ranged from 4 to 9, with an average 

 of between 6 and 7 (6.3). With the long breeding season indicated, it is 

 probable that each female rears more than one and perhaps several broods. 

 The species thus has the ability to increase rapidly when conditions are 

 favorable. A still further factor greatly enhancing the potentiality for 

 increase is that the females may breed at an extremely early age. On 

 May 22, 1916, a female containing 6 embryos was captured; this animal 

 was still in the juvenal pelage and hardly half-grown, as it weighed only 

 24.5 grams, scarcely half the weight of an adult. Two other females taken 

 May 21 and May 4, weighed 30.0 and 32.5 grams respectively, and these 

 also contained embryos. These latter animals had molted out of the short 



