After reviewing infanticide rates in twenty primate populations, Janson and van Schaik concluded that one of their main counterstrategies may be to encourage multimale grouping. Infanticide typically reduces the fitness of the mother, so females are expected to develop counterstrategies to avoid such losses –. Several other hypotheses have been proposed to explain infanticide. The third condition is that the male has an increased probability of siring the mother’s next offspring. The second condition is that the mother will resume reproduction sooner if the infant is killed. The first condition is that the male had little or no probability of siring the infant. According to the sexual selection theory, infanticide can be an adaptive strategy for males when three conditions are fulfilled. Infanticide by males has been observed or suspected in a wide range of taxa including birds, carnivores, ungulates, and primates –. Alternatively, variability in male strength might give some one-male groups a lower infanticide risk than some multimale groups, which could explain why both types of groups remain common.
If the strength of selection is modest for females to avoid group disintegrations, than any preference for multimale groups may be slow to evolve. In addition, we found limited evidence that female dispersal patterns reflect a preference for multimale groups. Infant mortality, the length of interbirth intervals, and the age of first reproduction were not significantly different between one-male versus multimale groups, so we found no significant fitness benefits for females to prefer multimale groups. The overall rates of infanticide were 2–3 times higher in one-male groups than multimale groups, but those differences were not statistically significant. After including infanticide from causes other than group disintegrations, infanticide victims represented up to 5.5% of the offspring born during the study, and they accounted for up to 21% of infant mortality. The rarity of such infanticide mainly reflects a low mortality rate of dominant males in one-male groups, and it does not dispel previous observations that infanticide occurs during group disintegrations.
Using more than 40 years of data from up to 70% of the entire population, we found that only 1.7% of the infants that were born in the study had died from infanticide during group disintegrations. Here we measure the impact of infanticide on the reproductive success of female mountain gorillas, and we examine whether their dispersal patterns reflect a strategy to avoid infanticide. Previous studies have suggested that female mountain gorillas benefit from residing in multimale groups because infanticide occurs when one-male groups disintegrate after the dominant male dies. Infanticide can be a major influence upon the social structure of species in which females maintain long-term associations with males.
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