Evolution of parental favoritism among different-aged offspring

The theories of intrafamilial conflict and parental investment have yet to examine how parents' decisions about resource allocation are influenced by the fact that their offspring may be different in age. Two counteracting effects of offspring growth on parental allocation of resources have deterred the development of a formal model: A parent may favor older offspring due to their greater reproductive value or favor younger offspring due to their higher marginal returns from extra resources. Using an evolutionary invasion analysis in class-structured populations, I present a formal model that explores how a parent should allocate its resources among different-aged offspring from the viewpoint of the parent. The parent's evolutionarily stable strategy is to allocate its resources such that the marginal benefit to each offspring's survival, weighted by the survival probability to the reproductive age, is equal to the marginal cost to the parent's residual survival. Two general situations are considered in which younger offspring obtain higher marginal returns than older offspring. In nearly all circumstances, a parent is expected to bias its resources toward older offspring. The results may account for the widespread yet puzzling phenomenon of parental bias toward older offspring in view of previous theories of intrafamilial conflict.