Countering infanticide: Chimpanzee mothers are sensitive to the relative risks posed by males on differing rank trajectories

Infanticide by males is common in mammals. According to the sexually-selected infanticide hypothesis, risks increase when males can kill unrelated infants, and when an infanticidal male’s chance of siring the replacement infant is high. Infanticide occurs in chimpanzees (Pan troglodytes), and rank predicts paternity, so infanticidal low-ranking males are unlikely kill their own offspring while males who rise in rank are more likely to father potential future infants than any existing ones. Given that mothers should be selected to reduce infanticide risk, we predicted that they would attempt to adjust the exposure of their infants to potentially-infanticidal males: specifically, that they would reduce association with low-ranking and rank-rising males. We examined data on female association patterns collected from the Budongo Forest, Uganda, during a period encompassing both relative stability in the male hierarchy and a period of instability with a mid-ranking male rising rapidly in rank. Using linear mixed models, we found that mothers reduced their association with mid-low ranking males, and particularly with the rank-rising male, contingent on infant age, during the period of instability. Our results support the sexually-selected hypothesis for infanticide, and demonstrate that female chimpanzees are sensitive to the relative risks posed by adult males.