Avian Siblicide A Proximate Mechanism For Inclusive Fitness 2
Introduction Siblicide is the act of killing a sibling by a brother or sister as a consequence of extreme sibling aggression. This phenomenon is observed in avian species when siblings are spatially confined in discrete nurseries, typically predatory, and the altricial nestlings are usually large in size. At first, siblicide appears to be maladaptive, due to the reduction of fitness to the parents by the inhibition of genetic flow and for the recipient of the sibling aggression, which will end as a fatality. An ultimate explanation to siblicide is described as selfish behavior, which can be explained by an inversion of Hamilton’s rule, br>c, where b and c represent the costs and benefits of a social act and r represents the coefficient of relatedness of the two individuals. Selfish behavior explains the overall increased inclusive fitness siblicide provides. There are two classifications of siblicide that indicate the occurrence of this behavior. The first type is obligate, where siblicide always occurs regardless of amount of resources available to the offspring. The more dominant, larger, and usually senior offspring will attack the junior offspring until siblicide occurs. The second type is facultative, where siblicide will sometimes occur and is dependent upon extrinsic factors. Some of the issues explored in this review are how an inversion of Hamilton’s rule will select for parents wanting to create an environment that facilitates siblicide for its progeny. Also, it is my belief that obligate and facultative siblicide has manifested as a proximate mechanism in response to the ultimate pressure of increased inclusive fitness for a familial group. The lower limit of Hamilton’s rule, selfish behavior As the inversion of Hamilton’s rule suggests, (Godfrey & Parker 1992) siblicide is a selfish behavior that has evolved because both the condition of the parent’s fitness and the offspring’s fitness outweighs the survival of the marginal offspring. The parent’s fitness is maximized when siblicide satisfies parent-offspring conflict (Trivers 1974) by providing brood reduction to only the nestling with the highest fitness.