Demographic consequences of incest avoidance in the cooperatively breedingacorn woodpecker

The avoidance of breeding with close relatives is an adaptation to inbreedi ng depression. Unfortunately, inbreeding depression has proved difficult to document or measure in the wild, despite being frequently observed among a nimals in captivity. We address this problem indirectly by determining the demographic cost of incest avoidance in the cooperatively breeding, polygyn androus acorn woodpecker, Melanerpes formicivorus, following the death or d isappearance of all breeders of one sex within a group (a reproductive vaca ncy). Groups undergoing female vacancies that also contained female nonbree ding helpers experienced significantly lower reproductive success in each o f the subsequent 3 years than those in which either no nonbreeding helpers or only male nonbreeding helpers were present, a decrease attributable to i ncest 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 over all reproductive potential (measured in fledglings/female) and decreases th e 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 individua l, a value of the same magnitude as estimated for several other taxa, inclu ding humans. Incest avoidance may compound random demographic and environme ntal events and significantly facilitate the decline of threatened populati ons even prior to any detrimental effects of inbreeding depression per se. (C) 1999 The Association fbr the Study of Animal Behaviour.