Data on six cases of infanticide, one infanticide attempt, and one suspected infanticide in mountain gorillas (Gorilla gorilla beringei) are presented here for the first time. These and previously reported cases in the same population are analyzed in order to assess the circumstances and consequences of infanticide.
Most infanticides occur when infants' mothers are not accompanied by their group's mature male (usually because he has died). Infants in this situation are almost certain to be killed by unfamiliar males unless they are nearly weaned. Active defense of infants by females is ineffective, and females cannot avoid unfamiliar males for prolonged periods. In contrast, infanticide is rare — yet has been observed — in encounters between mature males. It is not associated with group takeovers and male eviction by extra-group males, unlike the case in many other mammals. Demographic constraints and reproductive competition limit the occurrence of defensive coalitions between males. These factors, plus the high risks associated with male/male aggression, inhibit the occurrence of group takeovers by male coalitions. Infanticide shortens interbirth intervals and results in a high probability that a female will mate with the infanticidal male.
These findings support the sexual-selection hypothesis for the evolution of infanticide and strongly support the argument that intersexual mutualism and intraspecific aggression have been central factors in gorillas' social evolution.