Bonobos are unique compared to most primate species—and across mammals more generally—in being relatively female dominant. I’ve been working on a more comprehensive post about female power structures in mammals, but I figured I’d post something on bonobos in particular first.

Bonobos are moderately sexually dimorphic, with males being larger than the females. They live in multi-male/multi-female groups, tend to mate polygynously, have high male reproductive skew, and have female-biased dispersal, where the females tend to leave their natal groups while the males often stay in the community they were born in. Many of these factors tend to contribute to relatively strong male dominance, but bonobos are more co-dominant, and comparatively female-dominant compared to most other mammals. What explains this?

One popular explanation is the existence of female coalitions which constrain male dominance. The data on violent encounters in captive populations shows females are more likely to group together and attack a male, but many of these cases seem to represent sustained violent bullying of a lower-status male. Primatologist Amy Parish writes that,

Keepers at several institutions report that attacks appear to be provoked by male bluff displays or by male "nervousness," including "annoying" submissive vocalizations. Some attacks have been associated with external tension, as when unfamiliar zoo staff appeared with a hand-reared bonobo infant (Parish and Hamilton 1995). Attacks often occur outside any obvious context, however.

At the LuiKotale field site in the Democratic Republic of the Congo, female coalitions formed most often against individual males who were displaying aggression towards a female’s offspring, which may hint to some evolutionary history of infanticide by males, even though infanticide has not been observed among bonobos.

The biggest question for me is: why don’t males form coalitions? While female coalitions may play some role in inhibiting male coalitions, I think there are two other main factors at work.

First, bonobos seem to have relaxed feeding competition compared to chimpanzees. Anthropologist Volker Sommer and his colleagues write that,

We found that chimpanzee diet is more diverse, whereas bonobos can rely on a few staple species for longer periods of time – which reflects the more seasonal climate at the chimpanzee site. Both species prefer fruit with elevated contents of water, sugar and fat, but chimpanzees have to cope with much higher levels of anti-feedants such as tannins. Moreover, only bonobos have access to a herb with low levels of fibre but high protein. In addition, chimpanzees invest more time and energy in the removal of seeds from fruit and in digestion. The costs of acquisition of high quality food are thus higher in chimpanzees than in bonobos. The greater constraints in terms of food availability and quality are reflected in greater levels of female-female competition as evidenced by consistently lower levels of gregariousness in chimpanzees measured through the size of nest groups.

Bonobo diets seem more likely to rely on abundantly available fallback foods, reducing competition for resources. So, this relaxed feeding competition seems to make it easier for female bonobos to form coalitions than female chimps, who have stronger female-female competition for food. Similarly, the ‘patchier’ distribution of high-quality foods in chimpanzee territories can lead the males to form coalitions and patrol the boundaries of their territory, to monopolize the best fruit patches where the females come to feed, and prevent rival males from accessing it. For bonobos, no such incentive seems to exist.

The second factor is the strong mother-son relationships bonobos form, and the key role this relationship plays in influencing reproductive success. Adult females and their sons spend more time close to each other, and more time grooming each other, than any other bonobo pair. Arguably this is the most important relationship in bonobo society.

Now, as with many other mammal species, bonobo males have linear dominance hierarchies, with the highest ranked male tending to win intrasexual fights with other males. The highest ranked male also tends to have the greatest reproductive success of any male in the group, fathering as much as 62% of the next generation. But importantly, mothers also play a key role in increasing her son’s reproductive success (and by extension her own) by helping her son get greater access to fertile females and have more mating opportunities. This is particularly important for middle and lower status males, who may not be able to successfully fight higher status males to move up in the dominance hierarchy. Primatologist Martin Surbeck and his colleagues write that,

Overall, higher ranking males were more frequently in proximity to oestrous females, but the mean number of oestrous females in proximity to mid- and low-ranking males increased when their mothers were in proximity. Except for the highest ranking male in the community, the mean number of oestrous females in proximity was higher when their mothers were also in proximity.