HARVEST MOUSE 115 



Portal, a few harvest mice were taken in the runways of meadow mice. 

 And at the last-named place some harvest mice were obtained amid rocks 

 on a steep greasewood-covered hillside which was several hundred feet 

 higher than the grasslands bordering the river, and as far removed from 

 any stream. In all instances our specimens were trapped on the ground. 

 Harvest mice are said sometimes to use birds ' nests above ground as founda- 

 tions for their own nests, but we found no evidence on this point. 



The total population of this species must be great. AVe have no means 

 of stating it in relation to any given area occupied, and furthermore the 

 density of the population varies greatly from place to place. But in favor- 

 able situations, especially amid grassy growths, our trap lines produced 

 harvest mice as long as the lines remained in place. It was no uncommon 

 thing to obtain along with other small rodents 5 or 6 individuals of this 

 species in one night, from 40 to 60 traps set over a half-mile of favorable 

 country. 



At Snelling, in January and May, the males numbered 20 and the 

 females 15, in the cases where sex was recorded. A preponderance of males 

 in January, when breeding activity was commencing, might be expected ; 

 for males then range more widely than females and hence are likely to be 

 caught more commonly in traps. The sexes in the Harvest Mouse are in 

 reality probably about equal. 



The breeding season for the Long-tailed Harvest Mouse is a long one. 

 As just indicated, males began to show breeding activity during the first 

 week of January. By May, young of nearly adult size were abroad in 

 small numbers at Pleasant Valley, and females with embryos were common. 

 The number of embryos ranged from 3 to 6, averaging clovse to 4. A female 

 with large embryos was taken at Smith Creek on July 13, 1920. The 

 absence of trapping records from August to October leaves doubt as to 

 how late the breeding season continues, but it seems likely, from informa- 

 tion gained elsewhere, that it continues until some time in the fall. At 

 El Portal in December, where numbers were taken, no breeding individuals 

 were noted. 



A species which produces four young at a birth, and in which the young 

 mature rapidly and probably breed late in the season in which they are 

 bom, has the potentiality of a rapid numerical increase within a single 

 season. Despite this ability to increase its numbers, the Long-tailed Harvest 

 Mouse has never been found to play any economic role, either harmful 

 or otherwise, for it retires before cultivation and occupies marginal areas 

 only. 



