Reproduction, size, and mortality in the Rocky Mountain muskrat

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The Rocky Mountain muskrat, Ondatra zibethicus osoyoosensis (Lord), was studied on two large marshes, Gray's Lake and Dingle Swamp, in southeastern Idaho. Young muskrats were born from late May to mid-August at Gray's Lake and from early May to late August at Dingle Swamp. Thirty-five complete Gray's Lake litters ranged from 2 to 9 and averaged 7.0 young; 33 Dingle Swamp litters varied from 5 to 10 and averaged 7.3 nestlings. Breeding females at Gray's Lake produced an average of 11 young, approximately 1.6 litters per season, and Dingle Swamp breeders, 19 young, with 2.4 litters per season. Barrenness in females appeared to be unimportant at both localities. A sex ratio of 143 males: 100 females was determined for 4,963 animals. A fall ratio of 386 subudults: 100 adults and 403 subadults: 100 adults was established at Gray's Lake and Dingle Swamp, respectively. The methods for aging by pelt primeness pattern and external examination of genitalia were found valid for fall-trapped muskrats. Subadult mortalities from time of birth to fall were calculated to be 16 and 47 per cent for Gray's Lake and Dingle Swamp. Muskrats of southeastern Idaho appear to be among the smallest in the United States.

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Reference Code: A56REE01IDUS

Full Citation: Reeves, H. M., and R. M. Williams. 1956. Reproduction, size, and mortality in the Rocky Mountain muskrat. Journal of Mammalogy 37(4): 494-500.

Location: ANIMAL EF: MAMMALS