The Development of Male Chimpanzee Reproductive Tactics

Abstract

Across the animal kingdom, males compete to mate with a limited number of reproductively available females. To ensure females mate with them, males often rely on their size and strength. They may fight their male rivals or use aggression to sexually coerce females. Both tactics are employed by adult male chimpanzees (Pan troglodytes). Yet, immature adolescent male chimpanzees, who cannot compete with adult males or intimidate females, nevertheless father offspring. In this dissertation, I investigate how they do so by testing the hypothesis that adolescent males form affiliative relationships with females. I proceed to determine whether males mate and reproduce with females with whom they form strong relationships. To obtain data to test these hypotheses, I studied 30 adolescent (9-15 y) and young adult males (16-20 y) in a community of 200 wild chimpanzees at Ngogo in Kibale National Park, Uganda, over 2 years. I documented their social interactions with other individuals, including their mothers and unrelated females. At the start of adolescence, males formed their strongest relationships with their mothers and received emotional and agonistic support from them. If their mothers died, males provided similar parental-like support to their own younger, orphaned siblings. Mothers and sons continued to affiliate as sons grew up, but by adulthood males formed their strongest affiliative bonds with individuals who were not their mothers. Some of these bonds were with unrelated females as well as with related and unrelated males. Males and females who formed strong bonds groomed often and equitably, reassured frequently, and kept track of each other during travel more often than other male-female pairs even when controlling for the time they spent together and the female’s reproductive state. Despite the affiliative nature of these relationships, males also targeted their female partners for aggression. Affiliative and aggressive behavior displayed by males to females contributed to male mating success. The impact of aggression on mating success increased with male age and especially when they directed it toward females with whom they shared affiliative bonds. These results may help us to understand the evolution of social bonds between human females and males, which can involve affiliation, parenting by males, and sometimes coercive violence.