The Momma's Boy Strategy: Why Bonobo Males Tend Not To Form Coalitions

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/promiscuously, 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.

“Mean number of oestrous females in proximity of focal males in relation to absence (filled squares) or presence (open circles) of the male's mother. For all but the highest ranking male, the number of oestrous females in proximity was higher when the mother was also in proximity.” From ‘Mothers matter! Maternal support, dominance status and mating success in male bonobos (Pan paniscus)’

Notably, there are examples of bonobo mothers helping their sons sexually coerce other females. Primatologist Klaree Boose writes that,

We observed 56 attempts of direct sexual coercion performed by two males with a combined success rate of 71.4%. Of the two males who engaged in direct sexual coercion behaviors, the son of the alpha female (Gander) participated in direct sexual coercion events significantly more than any other male. We also observed that all but two females were targets of direct sexual coercion, where only the alpha female and mother to Gander (Unga) and mother (Susie) of the previous alpha male (Donnie) did not receive any direct sexual coercion… conflicts in which [Gander] was involved were almost always met with coalitionary aggressive support from his mother, the alpha female, who was frequently observed to engage in intense forms of retaliatory aggression including physical contact that would often result in the wounding of targets. All individuals in this group were highly deferent to the alpha female, and she received the least amount of aggression from males during the course of this study. Furthermore, females were found to be most receptive to her son’s solicitations for copulations. Evidence that mothers impact the relative dominance rank and subsequent mating success of their sons has been demonstrated in a wild population of bonobos (Surbeck et al. 2011). It is likely that this male’s relative dominance rank was effectively as high as his mother’s when she was present in the party and, therefore, reduced his likelihood of receiving retaliation for his attempts to directly sexually coerce a female. Our data that the success rate of direct sexual coercion attempts was dependent upon presence of the mother of the aggressor support this hypothesis. We also observed several instances of coalitionary sexual coercion (N=11) where the alpha female supported her son during his attempts to directly sexually coerce three separate females, including two parous adult females (Ana Neema and Lady), neither of whom were close associates of the alpha female.

Putting it all together, here is what I think is going on: there seems to be two fruitful reproductive strategies available to males—either through success in the linear male dominance hierarchy, winning conflicts with other males to increase access to fertile females, or through maintaining a strong bond with their (preferably high ranked) mother, which increases male access to fertile females. You might think that middle status males would join together to overthrow the disproportionately successful alpha, but consider the cost: they’d be reducing the time spent with their mothers, and by extension in the vicinity of oestrous females, to develop a same-sex coalition with no guarantee of success. There is a coordination problem in getting such an initiative started, with the costs of spending reduced time with the mother simply being too great.

For lower status males, here is where I think female coalitions might play a role in inhibiting male dominance, through preventing lower status males—who may not have their mother around, or whose mother is similarly low ranked—from developing effective alliances. I’m speculating based on limited information here, though.

Associated with this, I think the long period of pseudo-estrus found among bonobos, with more consistent non-conceptive sexual behavior, also plays an important role in reducing male dominance over females. Surbeck and his colleagues write that,

Results of the present study indicate that female bonobos derive a source of leverage from the reduced predictability of ovulation, possibly because male reproductive success becomes contingent on proceptive behaviors by estrous females. Bonobo females at LuiKotale were more likely to win a conflict when they were maximally tumescent [swollen].

It seems that the males may defer to fertile females, perhaps to stay in their good graces and have increased sexual access to them.

So, we might again think of the possible fitness strategies available to males: 1) win fights with other males and become the (intrasexual) alpha, or 2) have a higher ranked mother who gets them access to fertile females, or 3) if low ranked, be generally well-liked or inoffensive enough to the females to take advantage of mating opportunities without alienating them. Noting again the lack of feeding competition, none of these scenarios seem to lend themselves well to male coalitions.

The close mother-son bond in particular, in my view, contributes to the (relatively) more peaceful intersexual relationships between males and females among bonobos, even though some sexual coercion does seem to exist.

One final note: there is difficulty in comparing bonobos (or chimps) to humans, because of some of the unique constraints each species faces. For example, bonobos may sustain more peaceful intersexual relationships by virtue of more consistent sex due to prolonged pseudo-estrus, paternal uncertainty, and sperm competition reducing infanticide and direct male-male violent competition for sexual access. In humans, this strategy can have the opposite effect, where paternal uncertainty leads to ubiquitous social institutions that regulate female sexuality, and having multiple sexual relationships can lead to reduced male and kin investment into their highly altricial infants, increase the risk of infanticide, and lead to greater male-male conflict over sex.