Mate competition, testosterone and intersexual relationships in bonobos, Pan paniscus

Variation in male testosterone levels across and within species is known to be related to mating systems, male dominance rank and aggression rates. When aggression enhances access to mates, dominance status and androgen levels correlate positively. Deviation from this pattern is expected when access to females is determined by factors other than male dominance or when high androgen levels interfere with nonaggressive forms of male reproductive strategies such as paternal care and pair bonding. Bonobos offer an interesting study species to test the relationship between male dominance, aggression and intersexual relationships. On the one hand, males form dominance hierarchies and compete for access to females and mating success varies with rank. On the other hand, males and females are equally dominant, male rank is not only the result of aggression, and strong intersexual relationships might be crucial to male reproductive success. We used behavioural and physiological data from wild bonobos to test relationships between behavioural correlates of mate competition and androgen levels. Aggression and rank were positively correlated, as were aggression and mating success. In the presence of potentially fertile females, male aggression increased but only low-ranking, less aggressive males showed increases in testosterone levels, which consequently tended to be negatively related to rank. High-ranking males who had lower testosterone levels and were less responsive in their testosterone increase were more often involved in friendly relationships with unrelated females. These results suggest that, in bonobos, amicable relationships between the sexes rather than aggressive interactions mediate males’ physiological reactivity during periods of mate competition.