Estimates of Lethal Equivalents and the Cost of Inbreeding in Mammals

Abstract: The costs of inbreeding in natural populations of mammals are unknown despite their theoretical importance in genetic and sociobiological models and practical applications in conservation biology. A major cost of inbreeding is the reduced survival of inbred young. We estimate this cost from the regression of juvenile survival on the inbreeding coefficient using pedigrees of 40 captive mammalian populations belonging to 38 species. The number of lethal equivalents ranged from –1.4 to 30.3, with a mean of 4.6 and a median of 3.1. There was no significant difference between populations founded with wild‐caught individuals, a mixture of wild‐caught and captive‐born individuals, and individuals of unknown origin. The average cost of a parent‐offspring or full sibling mating was 0.33, that is, mortality was 33% higher in offspring of such matings than in offspring of unrelated parents. This is likely to be an underestimate.