Life history and populational aspects of the Eastern harvest mouse

Reithrodontomys humulis in the Oak Ridge, Tennessee, area is found mainly in certain habitats dominated by grasses and other herbaceous plants characteristic of early vegetational succession. Populations seldom are dense, but on a 4 ha, old-field area, trapping success ranged from about 4 to 18 individuals per 100 trap nights for 18 consecutive months. This population on the old-field area disappeared during severely cold weather in February-March of 1960, but a few individuals survived on a radioactive area. More females than males were captured in the periods June-November 1958 and May-November 1959, whereas more males were caught from December 1959 to February 1960. However, the sex ratio was about equal over a period of a year or more. The reproductive season extended from about late spring to late fall, but breeding occurred during winters of some years. Litter sizes ranged from 1-8 (av. 3.4). Cannibalism and abandonment of litters were characteristic of newly caught females. Average weights for females during reproductive periods were consistently greater than average weights of males, but even during nonreproductive periods females weighed as much or more than males. Weights of trapped individuals ranged from 3-17 g. Mortality of nestlings appeared to be severe, but survival after leaving the nests seemed to be relatively long. Huddling was common during cold weather. Territoriality seems poorly developed. Some individuals occupied relatively restricted ranges for periods of several months, but other mice shifted their activities from one part of an area to another. Ranges of the mice were less during the colder months than during the warner months. Only heterosexual pairs of mice were ever trapped together, and these captures usually occurred during breeding periods. Erythrocyte count and mean corpuscular volume in the harvest mouse were not significantly different from the measurements for larger mice, but hemoglobin concentration and oxygen consumption were greater than in the larger mice.