Sex Roles Are Flexible in Chimpanzees and Bonobos. What Does That Say About Human Evolution?

We often look to the behavior of chimpanzees and bonobos to infer the behavior of our ancestors. For example, male coalitionary aggression in chimpanzees is often taken to indicate violent tendencies in humans. Comparative data on bonobos provide a different picture that emphasizes peace and non-violence. These representations of chimpanzees and bonobos are stereotypes that can be summed up as “Chimps are from Mars, Bonobos are from Venus.” Chimpanzees are described as male-centric and aggressive, whereas bonobos are portrayed as female-centric and peaceful. However, both species have similar social structures. Males reside in the group their entire lives, females typically disperse and must integrate into a new community, and both have highly flexible, fission-fusion dynamics, in which animals join subgroups that frequently change. Although data from wild populations indicate sex-typical patterns of social bonds, understanding the extent to which captive groups exhibit those patterns can illuminate whether those sex differences are innate.

With my colleague, Emily Boeving, I examined whether captive male and female chimpanzees and bonobos have the sex-typical social bonds we would expect from wild populations. In the wild, limited high-quality ripe fruit means that female chimpanzees need to spend more time foraging alone, or in smaller subgroups, in order to get adequate food. This means that female chimpanzees typically have less time and opportunities for socializing, though this varies quite a bit across chimpanzee habitats. Female bonobos, however, spend more time in larger, mixed-sex subgroups, because their habitats have additional food resources. This allows them to socialize easily. But in captivity, where neither species is limited by food resources, would they form the same sort of sex-typical social bonds?

To answer this question, we studied grooming networks in captive bonobos at the Columbus Zoo, and captive chimpanzees at the North Carolina Zoo. We used social network analysis, which uses mathematical modeling to look at group-level patterns of social relationships. We compared the social network position of males versus females and the frequency of same-sex versus opposite-sex grooming bouts. We found that males and females in both species have similar social network positions, and that grooming is distributed equally between same-sex and opposite-sex pairs. This suggests that in the absence of ecological constraints, chimpanzees and bonobos are equally friendly with male and female social partners.

However, we also looked at individual factors that influence social bonds. We compared social network position of wild-born versus captive-born animals, history in the group, and kinship. In the bonobos, wild-born individuals, who were in the group the longest, had the most central social network positions. In chimpanzees, none of these individual factors were associated with social network position. Our findings in bonobos are similar to other studies of chimpanzee social networks, that found wild-born individuals play a larger role in group cohesion. This raises the question about whether our findings from captive groups can be used to answer evolutionary questions, or if the artificial nature of captivity itself is shaping captive animals’ social lives. There are two possible interpretations. One is that wild-born individuals may have greater social skills than captive-born individuals. Captive chimpanzees reared away from their mothers are less extroverted, and less interested in grooming. However, the key variable there is being reared apart from mothers—in the bonobo group, all captive animals were mother-reared. The other possible interpretation is their social position was due to their age and long residence in the group. Since origin, age, and group residence are all conflated in this group, we cannot tease these variables apart. However, residence in the group might be analogous to the position that older females, and all males, hold in wild populations.

Our findings suggest that both chimpanzees and bonobos have a high degree of flexibility in their social bonds, which may be due to their fission-fusion social dynamics. The flexible grouping patterns allow them to change social groups frequently, which allows wild populations to quickly adapt to changing ecological conditions. Such flexibility means that rather than having innate tendencies toward certain grouping patterns, animals have social flexibility that is shaped by their environment. In captivity, such flexibility, combined with more free time, may allow captive chimpanzees and bonobos to invest in grooming relationships equally with other male and female partners. Their choice of friends may be driven by factors such as group residence, as well as other factors we could not account for, such as personality. Our results suggest that, rather than having innate tendencies toward same-sex or opposite-sex friendships, chimpanzees and bonobos make social choices based on individuality. Rather than being from Mars or from Venus, both chimpanzees and bonobos are unique individual Earthlings with complex social lives. Like our closest relatives, humans also evolved within fission-fusion societies with complex social dynamics. Similarly, rather than having rigid patterns based on sex-typical behavior, our own behavioral biology is complicated. In order to understand the range of complexity in our evolutionary cousins’ social lives, perhaps we first need to recognize the range of complexity that exists in our own.

References

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