1936] MAMMALS OF OREGON 203 



caught 36 of the mice alive in his hands in a few minutes by follow- 

 ing the buck rake, while the men were stacking. Usually 2 or 3 and 

 sometimes a dozen would run from their nests as a haycock was slid 

 from over them and in the confusion they were easily picked up and 

 dropped into a tin can until more than enough were obtained for 

 study of habits in captivity. Hundreds more could have been 

 caught just as readily. Out in the uncut grass and tules their tiny 

 roadways were easily found and traps set across them yielded all the 

 specimens needed, while cut grass and fragments of food showed 

 where and on what they had been feeding. In places they were more 

 numerous than others, and on one spot in a dry meadow where the 

 hay had been cut the writer counted 169 open burrows on a rectangle 

 of 10 by 12 feet. This was in September, the season when the mice 

 were most abundant, and in such places there were certainly 100 mice 

 to an acre. In other places there were probably not more than 1 to 

 an acre and for the whole 40,000 acres, roughly estimated, of marsh 

 and grassland in the Malheur Valley 10 mice to an acre should be a 

 fair estimate of abundance. 



They are good swimmers and like to live along the edges of water 

 but also occupy dry meadows, where no water is available except 

 from rain or dew and what they get in their green food. They are 

 active all the year and about equally so in the daytime or at night, 

 are sociable and friendly with one another and with many other 

 species of native mice, but will fight savagely anything from their 

 own size up to a hayrake, biting severely with their keen incisor teeth. 

 One placed in a cage with a grasshopper mouse of about his own size 

 fought off this predacious species all the evening, but in the morning 

 there was only a bit of skin and bones left to show that he had been 

 unequal to his antagonist. 



Breeding habits. The females have 8 mammae arranged in 2 pairs 

 of inguinal and 2 pairs of pectoral, 2 on each of 4 distinct mammary 

 glands. The young usually number 4 to 8, 8 being the full normal 

 complement for fully adult females. The number of litters in a 

 season or a year is not known and probably varies greatly with 

 abundance and nature of food and congenial environment, but nor- 

 mally there evidently are several litters during the summer. There 

 are records of embryos in specimens collected in every month from 

 March to October, and on August 17 young of 5 different ages 

 were found under a few adjacent haycocks. Apparently they^do not 

 breed during the winter months, as a large number of females 

 examined during November in Nevada showed no signs of embryos. 



Food habits. Grass is the principal food of these mice, but with 

 it are included the sedges, tules, and a great variety of meadow 

 plants, including the edible parts of the green leaves, stems, roots, 

 and seeds. These are obtained by cutting off the base of the stem 

 or plant and drawing it down, often repeating the operation until 

 the seeds are reached and a little pile of sections 1% to 2 inches long 

 lie in a heap. Usually not half of the plant cut for food is eaten, 

 and sometimes all but the seeds or a very small portion is wasted. 

 Also the roots, bulbs, and running rootstalks are dug up and eaten, 

 and the stomachs examined usually show a portion of well-masticated 

 root pulp and seeds with the green tissue. Those kept in captivity 

 are especially fond of rolled oats or any grain or seeds, grass, tule, 



