UBC Theses and Dissertations
Reproductive success and survival of the young in Peromyscus Britton, Mary Martha
The object of this study was to compare the role of changes in reproduction and mortality in regulating population density in the deer mouse, Peromyscus maniculatus, and if possible to discover some of the factors affecting the reproductive rate. Observations were made on natural and experimentally reduced populations on the University Endowment Lands in 1964 and 1965. In both years numbers remained relatively steady during the summer, increased in the fall, when immature animals replaced the adults, and gradually declined over the winter. Animals were about equally abundant at comparable times in both years, fall densities being about 4.72 mice per acre. The stationary state of these populations was associated with a poor reproductive performance on the part of the females, whose breeding success varied between areas, and was greater in 1964 than in 1965. No change in litter size or in prenatal loss was observed during the period of study. The males, in contrast, were sexually active from March to September on all areas in both years. The greatest loss of mice occurred between birth and age at first capture, after which juveniles survived at the same rate as the adults. Survival was poorer during the breeding season than during the winter, and survival of males was poorer than that of females. Populations whose numbers had. been experimentally reduced and whose age structure had been altered, were not significantly different from the natural populations in mean monthly body weights, reproductive performance, or survival. Mean monthly body weights and reproductive performance were lower, and survival of the young from birth to age at first capture was higher in 1965 than in 1964. The proportion of subadults which became fecund was greatest on this area in 1964. The stationary state of these populations was maintained by changes in survival rather than by changes in reproductive rate. The reproductive performance of the females was fairly constant whereas the loss of young from birth to age at first capture varied. Loss of the young is attributed to their death or emigration in response to aggressive interactions within the population.
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