#19. How Play Enables Female Bonobos to Prevent Male Dominance

The feminist revolution in our close ape cousins was a matter of play.

As I described in Letter #18, social play can occur only if the players suppress any instinctive tendency they might have to behave aggressively toward one another. That’s because aggressive behavior would lead either to a fight, which ends the play, or to flight by the less dominant individual, which also ends the play. Social play is necessarily cooperative. Much animal social play takes the form of playful fighting, which is the opposite of real fighting in that the goal is to keep the other happy and prolong participation rather than drive the other away. Because of this, signals for play in the animal world have become signals of nonaggression.

Animals whose success in nature depends on cooperation with others of their kind commonly use play to form cooperative social bonds. For them, play is a way of signaling nonaggression and testing one another’s willingness to cooperate, at least for a period of time. As I pointed out in Letter #18, adult play is more common among pack-hunting carnivores, such as wolves, which must cooperate in killing large game, than it is among carnivores that do not hunt cooperatively. Among primates, adult social play often involves individuals who are reuniting after a period of separation or a male and female prior to mating (Pellis & Iwaniuk, 2000).  In such cases, play may establish or reestablish affiliation, so subsequent cooperation can occur.

Some of the most fascinating research on the role of play in primate social life has been conducted with both wild and captured colonies of bonobos, and that is my topic now

Our Close Cousins the Bonobos

It is often said that chimpanzees are our closest cousins in the animal world, but that is only half true. We are related to bonobos just as much as to chimps. The ape branch that led to both chimpanzees and bonobos split off from that which led to us approximately 6 million years ago, and then it branched again, forming separate lines to chimps and bonobos nearly 2 million years ago (Corballis, 1999). They were divided into separate populations by the Congo River and subsequently evolved, in rather different ecological conditions, into two distinct species, which are similar to one another in many ways but different in ways very relevant for our discussion of play.

Bonobos and chimps look quite like one another and their social communities are in some ways similar, but bonobos are far less aggressive toward one another than are chimpanzees, and this allows them to live in larger and more fluid groups than is the case for chimpanzees. Bonobos exhibit more intraspecies tolerance and cooperation than chimpanzees or any other nonhuman apes, and they are also the most playful of all nonhuman apes, especially in adulthood (Palagi, 2008; 2023). Male bonobos do form dominance hierarchies, but these are subtler and involve less threatening and fighting than is the case for male chimpanzees

A number of studies indicate that adult bonobos of both sexes use play to prevent or reduce fighting in stressful situations. In one study of a captive colony, play was most frequent during the pre-feeding period, a time when tension in the group was especially high because of anticipated competition for food (Palagi, Paoli & Tarli, 2006). In another study, play among adults increased significantly when the animals were temporarily restricted to relatively crowded indoor quarters (Tacconi & Palagi, 2009). There is little or no evidence of such tension-reducing play in adult chimpanzees.

The most striking social difference between bonobos and chimpanzees, however, is that female bonobos are generally dominant over males (Parish & de Waal, 2000). Chimpanzees, in contrast, exhibit the far more typical primate pattern of male dominance over females, which is sometimes quite violent (Muller, Kahlenberg, & Wrangham, 2009).

How Female Bonobos Dominate Males

Female bonobos dominate males even though they are smaller and physically weaker than the males. They accomplish this by coming to one another’s aid in aggressive encounters with males (Palagi 2023). If a female is attacked or threatened by a male, other females (her friends) come quickly to her aid, and together they drive off the offending male. Male bonobos, in contrast, do not help one another in encounters with females.

The capacity of female bonobos to form and maintain such cooperative relationships is especially striking given that bonobos practice female exogamy, meaning that the females, not the males, leave their natal group and join a new one upon reaching sexual maturity. Thus, the bonds formed among female adult bonobos are most often among individuals who are not close relatives and were not raised together.

How do female bonobos form these cooperative friendship bonds? According to researcher Elisabetti Palagi (2011; 2023) and others, they do so largely through play. In one study, Palagi (2006) compared the social behaviors of a captive group of bonobos with those of a captive group of chimpanzees, both housed in semi-natural conditions. As expected, she found far more friendly, cooperative interactions and far fewer aggressive interactions among the bonobos than among the chimps. She observed equivalent amounts of play among juveniles for the two species, but far more adult-adult play among bonobos than among chimps. Adult chimps sometimes played with juvenile chimps, but almost never with other adults.

Further research, just with bonobos, revealed striking sex differences in adult play (Palagi & Paoli, 2007). Adult females played with one another much more than did adult males, and female-female play was different in form from male-male play. Females played in rough-and-tumble fashion, with much body contact, while males with other males played in parallel fashion without body contact. When adult males played in rough and tumble contact fashion it was either with juveniles or with adult females, not with other adult males. Apparently, it is the high contact rough and tumble play that enables the formation of cooperative bonds in bonobos. The research also revealed that females who rough and tumbled together also often groomed one another and sat in close physical contact with one another.

Perhaps, even though male bonobos are not as aggressive with one another as are male chimps and have more muted dominance hierarchies, their concern for status is sufficient to prevent them from playing with one another in the high-contact fashion that requires the setting aside of dominance concerns and enables long-term friendship bonds. Females, less concerned with status, play freely with one another, and this is at least part of what enables them to unite in confrontations with males. A male bonobo who attempts to mount an uninterested female or grab food from a hungry one will be attacked by her friends, her playmates, so the males learn to defer. I hear it now: “Women of the world, unite (and play).”

Final Thoughts

It’s tempting, when we see such animal examples, to think about how they may relate to humans.  There is at least some cross-cultural evidence that in societies where women form close bonds with multiple other women men are less dominant over women, for the same reason seen in bonobos. The women will gang up on the abusive man. Men may dominate women in part by keeping them separate from one another. But I’ll save that for a possible future letter.

In my next letter I’ll continue examining adult play in animals, this time exploring the differences between species of macaque monkeys that are highly despotic in their social organization, ruled by the alpha male, and those that are far more egalitarian.

As always, I welcome your thoughts and questions in the comments section below. They will add to the value of this letter. If you aren’t already subscribed to this Substack, please subscribe now, and let others who might be interested know about it. By subscribing, you will receive an email notification of each new letter.

References

This letter draws directly from a chapter, which I wrote for an academic book a few years ago: Peter Gray.  The play theory of hunter-gatherer egalitarianism. In D. Narvaez, K. Valentino, A. Fuentes, J. McKenna, & P. Gray (Eds.), Ancestral landscapes in human evolution: culture, childrearing and social wellbeing (pp. 190-213). New York: Oxford University Press. 2014

Other references:

Corballis, M. C. (1999). Phylogeny from apes to humans. In M. C. Corballis & S. E. G. Lea (Eds.), The descent of mind: Psychological perspectives on hominid evolution. Oxford: Oxford University Press.

Muller, M. N., Kahlenberg S. M., & Wrangham, R. W. (2009). Male aggression against females and sexual coercion in chimpanzees. In M. N. Muller & R. W. Wrangham (Eds.), Sexual coercion in primates and humans, pp. 184-217. London: Harvard University Press.

 CPellis, S. M., & Iwaniuk, A. N. (2000). Adult-adult play in primates: Comparative analysis of its origin, distribution and evolution. Ethology, 106, 1083-11104.

Palagi, E. (2006). Social play in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): Implications for natural social systems and interindividual relationships. American Journal of Physical Anthropology, 129, 415-426.

Palagi, E. (2008). Sharing the motivation to play: The use of signals in adult bonobos.  Animal Behaviour, 75, 887-896.

Palagi, E. (2011). Playing at every age: Modalities and potential functions in non-human primates. In A. D. Pellegrini (Ed.), The Oxford handbook of the development of play. Oxford University Press.

Palagi, E. (2023). Adult play and the evolution of tolerant and cooperative societies Neuroscience and Biobehavioral Reviews 148 (2023) 105124

Palagi, E., Paoli, T., & Tarli, S. B. (2006). Short-term benefits of play behavior and conflict prevention in Pan paniscus.  International Journal of Primatology, 27, 1257-1269.

Parish, A. R., & de Waal, F. B. (2000). The other “closest living relative”: How bonobos (Pan paniscus) challenge traditional assumptions about females, dominance, intra- and intersexual interactions, and hominid evolution. Annals of the New York Academy of Sciences, 907, 96-113.

Tacconi, G., & Palagi, E. (2009) Play behavioural tactics under space reduction: Social challenges in bonobos, pan paniscus.  Animal Behaviour, 78, 469-476.