Some aspects of reproduction in the vole Microtus townsendii

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Abstract
Changes in various reproductive parameters have pronounced effects on microtine population fluctuations. This study presents a detailed account of reproduction in Microtus townsendii. Populations in two areas were studied. On Westham Island, litter size, estimated by autopsy, was correlated positively with weight. There was no significant difference in litter size between primiparous and multiparous females. Females pregnant in spring had more embryos than those pregnant in summer or fall. Breeding was concentrated in spring and summer, with most females continuously pregnant.In the University of British Columbia Research Forest, litter size at recruitment was found to be unrelated to parity, size, or LAP genotype of the mother. While the size of consecutive litters (determined by autopsy) was significantly repeatable within an individual mother, recruitment litter size was not. Autopsy litter size, as at Westham Island, was largest in spring, but recruitment litter size was largest in autumn. Entire litters were lost most often during winter and summer. Overnight trapping may be responsible for loss of some litters at or just before parturition. Litters born in traps were usually unsuccessful, but otherwise mothers of totally unsuccessful litters seemed to avoid traps. Neither parity nor LAP genotype was related to the loss of entire litters.

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