Intraspecific Litter Size Variation in Microtus californicus: Variation within Populations

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Abstract
An analysis of the patterns of intrapopulation variation in litter size in Microtus californicus from two regions has demonstrated that a major portion of the variation is temporal. Large seasonal changes in mean litter size were found in this species. There was no significant relationship between litter size and body weight, age, size, age at first reproduction, parity, or the presence or absence of lactation. Although heritability of litter size is low (0.17), much of the variation within populations can be attributed to differences between individual females, and presumably has a genetic basis. The effects of litter size variation on the offspring are seen primarily in the weights of juveniles at weaning and on the survivorship of young to weaning age.