Incestuous Mating in a Communal Bird: A Family Affair

Abstract

In the past, observations of animal dispersal has been taken as evidence of inbreeding avoidance. Reduced dispersal by the young of communal breeders makes them ideal for investigating the possibility of inbreeding and/or inbreeding avoidance. Communally breeding pukeko are especially suitable for such a study because they exhibit extreme philopatry, have a polygynandrous mating system, and engage in readily observed copulations. Results from a 7-yr study show that most pukeko remain and breed in their natal territory and incestuous matings are common. In contrast, incidents of dispersal out of the natal territory were infrequent, involving only males. Outbreeding appeared less common than inbreeding. Dispersal by breeding males that had mated previously with their offspring could not be logically linked to incest avoidance, but movements of nonbreeding subadults might have been related to either incest avoidance or intersexual dominance. Except for young nonbreeding males that tend to be subordinate to breeding females, male pukeko dominate females. A few subadult males that were stopped from copulating by the aggressive behavior of their mothers subsequently dispersed, whereas others showed reduced sexual behavior in the presence of their mothers. Both incest avoidance by the mothers (but not the sons) and parental dominance could explain these observations. Parental dominance seems to be a viable explanation because there were no examples of daughters avoiding matings with their fathers or sisters avoiding matings with brothers. Results from other studies of communally breeding birds also show various degrees of incestuous mating. Data from one in-depth study on acorn woodpeckers, from which incest-avoidance mechanisms have been postulated, is considered in detail; the results appear equivocal and do not exclude alternative explanations. Additional factors, such as a level of fecundity markedly higher than that of survival and a long history of communal (and presumably incestuous) habits in pukeko, lead us to question whether inbreeding among pukeko is likely to have deleterious genetic effects. The possibility of nonrandom fusion of gametes would also support this view. Overall, pukeko appear to demonstrate higher levels of incestuous matings than current research and theories on the evolution of vertebrate mating systems would have predicted. We argue that this conflict arises as a product of the unquestioning assumption by many workers that behavior leading to inbreeding has been selected against or that inbreeding-avoidance mechanisms have been selected for in animal populations.