Infanticide and infant defence by males--modelling the conditions in primate multi-male groups.

Infanticide by primate males was considered rare if groups contain more than one adult male because, owing to lower paternity certainty, a male should be less likely to benefit from infanticide. Guided by recent evidence for strong variation of infanticide in primate multi-male groups, we modelled the conditions for when infanticide should occur for a group with a resident and an immigrant male. Setting the parameters (e.g. infant mortality, reduction of interbirth interval, life-time reproductive success, genetic representation) to fit the conditions most commonly found in nature, we develop a game-theoretic model to explore the influence of age and dominance on the occurrence of infanticide and infant defence. Male age strongly impacts the likelihood of an attack which is modified by the father's defence. If the new male is dominant he is likely to attack under most circumstances whereas a subordinate male will only attack if the father does not defend. These model scenarios fit the conditions under which infanticide is known to occur in primate multi-male groups and offer an explanation why infanticide is common in some multi-male groups and rare in others. Overall, the benefits for infanticidal males are strongly governed by a reduced interbirth interval while advantages via improved genetic representation in the gene pool contribute but a minor fraction.