Why do female common shrews produce so many offspring?

Female common shrews (Sorex araneus) produce large numbers of offspring at enormous energetic cost. Moreover, the number of offspring per litter often exceeds the number that are reared to weaning. This is particularly puzzli ng because there is intense competition for survival among young shrews onc e they have left the nest. It appears therefore that females could increase their reproductive success by shifting investment from production of appar ently surplus offspring to increase investment in those they are able to re ar. Here, we suggest that the reason female common shrews invest in apparen t overproduction of offspring may be related to patterns of dispersal and i nbreeding in this species. Matings between genetically similar individuals occur frequently in natural populations, and females appear unable to avoid costs associated with inbreeding by selecting mates or their sperm on the basis of genetic compatibility. Production of more offspring than are weane d, coupled with multiple mating, may therefore be a strategy to promote sib ling competition for maternal investment and hence selection of the most ge netically fit young from mixed paternity litters.