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.
Cortisol excretion in males of group living species is often associated with social rank and competition for oestrous females. Rank-related patterns of cortisol levels can be used to study mechanisms of rank maintenance and costs associated with mate competition. Bonobos (Pan paniscus) are interesting because males form a linear dominance hierarchy but are not dominant over females and therefore aggressive male–male competition over access to females alone is not considered to be a successful reproductive strategy. In this study on social correlates of urinary cortisol in wild male bonobos, we investigated the relationship between cortisol levels and several aspects of mate competition, including male rank, aggression rates, and association time with oestrous females. We found that cortisol levels correlated positively with dominance rank when oestrous females were present, but not when they were absent. This result is consistent with the idea that aggressive behaviour plays a minor role in maintenance of high rank. While aggression received from males and females explained within-individual variation in cortisol levels, it was the time spent in association with oestrous females that best explained between-individual variation in male cortisol levels. The observed increase in male cortisol may be associated with spatial proximity to oestrous females and could result from anticipated aggression from other group members, reduced feeding time in the males, or a combination of both.
In primates, age, sex, and social status can strongly influence access to food resources. In Pan, these criteria are assumed to influence access to vertebrate meat. However, the significance of meat in terms of its role in the nutrition of Pan is still debated. Here we present a study using stable carbon and nitrogen isotope ratios in hair samples from habituated, wild bonobos (Pan paniscus) to explore these issues. Over a period of 5 mo hair samples were collected from fresh bonobo nests at LuiKotale, Democratic Republic of Congo. Hair samples were assigned to known individuals and were of sufficient length to allow the evaluation of isotopic variation over several months. Samples of plant foods and sympatric fauna were also analyzed. The δ13C and δ15N results of the bonobo hair were remarkably homogeneous over time and for the group as a whole. There are no differences in diet between the sexes. Within the group of males, however, there was a positive correlation between dominance status and δ15N. The isotopic data indicate that the contribution of fauna to bonobo diet is marginal and that plant food is the dietary protein source. In only some cases did elevated δ15N hair values correlate with observed faunivory and not correspond to the δ15N measured in the dominant plant foods. Given the large variation in hunting and meat eating of Pan across the African continent, the detection of seasonal changes in faunivory by elevated δ15N values in sectioned ape hair is a promising approach.
Variation in male mating success is often related to rank differences. Males who are unable to monopolize oestrous females alone may engage in coalitions, thus enhancing their mating success. While studies on chimpanzees and dolphins suggest that coalitions are independent of kinship, information from female philopatric species shows the importance of kin support, especially from mothers, on the reproductive success of females. Therefore, one might expect a similar effect on sons in male philopatric species. We evaluate mating success determinants in male bonobos using data from nine male individuals from a wild population. Results reveal a steep, linear male dominance hierarchy and a positive correlation between dominance status and mating success. In addition to rank, the presence of mothers enhances the mating success of sons and reduces the proportion of matings by the highest ranking male. Mothers and sons have high association rates and mothers provide agonistic aid to sons in conflicts with other males. As bonobos are male-philopatric and adult females occupy high dominance status, maternal support extends into adulthood and females have the leverage to intervene in male conflicts. The absence of female support to unrelated males suggests that mothers gain indirect fitness benefits by supporting their sons.