SIBLICIDE - THE EVOLUTIONARY BLACKMAIL

Avian siblicide, whereby some chicks are eliminated from a brood as a direct consequence of sibling aggression, has often been interpreted a s the outcome of a parent-offspring conflict over brood size. This con flict, which arises from the fact that each offspring values itself mo re than it values its siblings, results in offspring favoring smaller families, of higher-quality individuals, than would be optimal for the ir parents to produce. Considered as an evolutionary game, this confli ct is complicated by two factors: before deciding to enforce their pre ferred brood size by eliminating unwanted brood mates, offspring must take into account the readjustments in parental investment that will n ormally ensue from brood reduction. Likewise, optimal parental investm ent will depend on offspring reaction to it. In particular, it is show n that the cost that parents must pay to provide enough resources to p revent siblicide will often be smaller than the benefit they obtain fr om maintaining their preferred brood size. Siblicide can be viewed in these instances as evolutionary blackmail, because its threat is suffi cient to make parents increase brood provisioning levels. As a result of the blackmail, sibling aggression may acquire a signaling role in t he parent-offspring interplay.