Demographic consequences of incest avoidance in the cooperatively breeding acorn woodpecker

The avoidance of breeding with close relatives is an adaptation to inbreeding depression. Unfortunately, inbreeding depression has proved difficult to document or measure in the wild, despite being frequently observed among animals in captivity. We address this problem indirectly by determining the demographic cost of incest avoidance in the cooperatively breeding, polygynandrous acorn woodpecker, Melanerpes formicivorus, following the death or disappearance of all breeders of one sex within a group (a reproductive vacancy). Groups undergoing female vacancies that also contained female nonbreeding helpers experienced significantly lower reproductive success in each of the subsequent 3 years than those in which either no nonbreeding helpers or only male nonbreeding helpers were present, a decrease attributable to incest avoidance between the helper females and the related breeder males in the group. Using a computer simulation combined with a life-table analysis, we estimated that incest avoidance costs the population 9.2-12.1% in overall reproductive potential (measured in fledglings/female) and decreases the population rate of increase by 1.78-2.33%/year. These results suggest the presence, on average, of at least 1.2-1.8 lethal equivalents per individual, a value of the same magnitude as estimated for several other taxa, including humans. Incest avoidance may compound random demographic and environmental events and significantly facilitate the decline of threatened populations even prior to any detrimental effects of inbreeding depression per se. Copyright 1999 The Association for the Study of Animal Behaviour.