Do gorilla females join males to avoid infanticide? A quantitative model

Gorilla, Gorilla gorilla, females associate permanently with males. What benefit do the females obtain from the association? Where males are larger than females, as in the gorilla, protection from predation is the long-standing answer. However, protection from infanticidal nonfather males is increasingly suggested as a better hypothesis. Given that a female's alternative strategy is to range alone and mate with many males, a hitherto ignored problem of the anti-infanticide hypothesis is that a female's joining a single male necessarily maximizes the proportion of males in the population who can use the infanticidal strategy. To ask whether lone gorilla females could mate with enough males to decrease the probability of infanticide below that observed in the wild, we use novel modifications of a gas molecule equation to model encounter rates of a lone female with males. The modifications include separation of encounter rates in fertile periods from those in nursing periods and, importantly, a decline function that converts infanticidal males to noninfanticidal, on mating. Parameters include density of males, speed of travel, and duration of fertile and nursing periods. In two other great apes (chimpanzee Pan, orang-utan Pongo), females are more or less solitary, and infanticide rates are very low (less than or equal to5%). With these species' values in the model, equivalently low infanticide rates result. Thus, the model appears valid. For the gorilla, the model indicates that under most realistic conditions, solitary females mate with so few males that infanticide rates are more than three times the observed rate of 14% of births. Association with a male thus decreases the probability of infanticide. The anti-infanticide hypothesis for grouping is thus supported. But the antipredation hypothesis is not thereby negated, and remains a powerful hypothesis, given that predators are a danger to the female, as well as her infant. (C) 2001 The Association for the Study of Animal Behaviour.