Reproductive skew and indiscriminate infanticide

In communally breeding animals, there is an evolutionary conflict over the partitioning of reproduction within the group. If dominant group members do not have complete control over subordinate reproduction, this conflict may favour the evolution of infanticidal behaviour (by either subordinates or dominants or both). Elimination of offspring, however, is likely to be cons trained by the difficulty of discriminating between an individual's own pro geny and those of cobreeders. Here, we develop an evolutionarily stable str ategy (ESS) model of reproductive partitioning, which demonstrates that kil ling of young can be favoured, even if such discrimination is not possible. The model predicts that infanticide will typically be associated with elev ated levels of offspring production, and is most likely to prove evolutiona rily stable when the coefficient of relatedness between cobreeders is low, and offspring are cheap to produce. The effect of infanticide is to release subordinates from the reproductive restraint they would otherwise be force d to exercise, leading to reduced reproductive skew. When infanticide is po ssible, addition of numerous young to the joint brood will not lower overal l productivity, because progeny in excess of the most productive brood size are eliminated. Subordinates are thus free to contribute more young to the brood than would otherwise be the case. In addition, we show that the poss ibility of infanticide may influence the pattern of reproduction within a g roup even if no offspring are actually killed at equilibrium. (C) 1999 The Association for the Study of Animal Behaviour.