Sexually coercive male chimpanzees sire more offspring

Abstract

In sexually reproducing animals, male and female reproductive strategies often conflict. In some species, males use aggression to overcome female choice, but debate persists over the extent to which this strategy is successful. Previous studies of male aggression toward females among wild chimpanzees have yielded contradictory results about the relationship between aggression and mating behavior. Critically, however, copulation frequency in primates is not always predictive of reproductive success. We analyzed a 17-year sample of behavioral and genetic data from the Kasekela chimpanzee (Pan troglodytes schweinfurthii) community in Gombe National Park, Tanzania, to test the hypothesis that male aggression toward females increases male reproductive success. We examined the effect of male aggression toward females during ovarian cycling, including periods when the females were sexually receptive (swollen) and periods when they were not. We found that, after controlling for confounding factors, male aggression during a female's swollen periods was positively correlated with copulation frequency. However, aggression toward swollen females was not predictive of paternity. Instead, aggression by high-ranking males toward females during their nonswollen periods was positively associated with likelihood of paternity. This indicates that long-term patterns of intimidation allow high-ranking males to increase their reproductive success, supporting the sexual coercion hypothesis. To our knowledge, this is the first study to present genetic evidence of sexual coercion as an adaptive strategy in a social mammal.

Figure 1

Predicted Likelihood of Paternity Based on Covariates from the Best Model (A) Effect sizes and 95% confidence intervals of variables from the best model (model 1 in Table 1) on probability of paternity. MRank, male dominance rank (MDS-Z); NSagg, standardized nonswollen aggression rate; Relatedness, Queller and Goodnight’s R (see Experimental Procedures). (B) Lines represent effect of aggression rates toward nonswollen females when all other covariates are at their average values. Solid line, middle-ranking males (MDS-Z = 0); dotted line, high-ranking males (MDS-Z = 1); dashed line, low-ranking males (MDS-Z = −1). (C) Lines represent the effect of male rank (MDS-Z) on likelihood of paternity when other covariates are at their average values. Solid line, average levels of nonswollen aggression (NSagg = 0); dotted line, high levels of nonswollen aggression toward females (NSagg = 1); dashed line, low nonswollen aggression (NSagg = −1). (D) The effect of relatedness on likelihood of paternity among all dyads.

Figure 2

Paternity among Highly Ranked Males by Nonswollen Aggression Rate Observed frequency of paternities among dyads with high-ranking males (MDS-Z > 1) versus nonswollen aggression rate. Low represents standardized aggression rates toward nonswollen females < −1; Med-Low: [−1, 0); Med-High: [0, 1); High: ≥ 1.