Male-male competition magnifies inbreeding depression in wild house mice

The detrimental effects of inbreeding on vertebrates are well documented fo r early stages of the life cycle in the laboratory. However, the consequenc es of inbreeding on long-term survival and reproductive success (Darwinian fitness) are uncertain for vertebrates in the wild. Here, we report direct experimental evidence for vertebrates that competition increases the harmfu l effects of inbreeding on offspring survival and reproduction. We compared the fitness of inbred (from furl-sib matings) and outbred wild house mice (Mus domesticus) in large, seminatural enclosures. Inbred males sired only one-fifth as many surviving offspring as outbred mates because of their poo r competitive ability and survivorship. In laboratory conditions, inbreedin g had relatively minor effects on male reproductive success and no effect o n survivorship. Seminatural conditions did not increase inbreeding depressi on for females, probably because females were not competing for any critica l resources. The overall reduction in fitness from inbreeding was 57%, whic h is 4.5 times as great as previous estimates from the laboratory. These re sults have important implications for medicine, conservation, evolutionary biology, and functional genomics.