A Primate Puzzle: The Social Power of Sex
For decades, primatologists have drawn stark lines between chimpanzees and bonobos, our closest living relatives. Chimpanzees, we are told, are warlike and hierarchical, their societies built on dominance and aggression. Bonobos, by contrast, are famously amorous, replacing violence with sex as a means of conflict resolution. But what if these differences have been exaggerated? What if, instead of opposing archetypes, these two great apes are variations on the same evolutionary theme?
A new study published in Royal Society Open Science1 by Jake Brooker, Zanna Clay, and colleagues challenges these long-held assumptions. The research presents a direct comparison of sexual behavior in both species, showing that bonobos and chimpanzees actually share more social-sexual strategies than previously thought. And what’s more, these findings may reveal something profound about the origins of human sexuality and cooperation.
Sex Beyond Reproduction: A Shared Ancestral Trait?
In many human societies, sex serves a multitude of functions beyond reproduction. It fosters intimacy, relieves tension, and maintains alliances. This study suggests that such social-sexual behavior is not unique to us—it has deep evolutionary roots, stretching back to a common ancestor we share with chimpanzees and bonobos over six million years ago.
"Bonobos are famous for using sex to navigate social challenges, but much less is known about the role of sex in chimpanzee society," says senior author Professor Zanna Clay. "Against the assumptions of pacifist, sex-mad bonobos and aggressive chimpanzees, we found that both species used sex in similar ways during tense situations, including same-sex pairings."
The researchers observed 53 bonobos and 75 chimpanzees in African sanctuaries, focusing on two critical moments of social tension: post-conflict reconciliation and pre-feeding competition. They found that both species used sexual contact to ease tensions, but in slightly different ways.
Bonobos vs. Chimpanzees: Different Paths, Same Destination
Bonobos, true to their reputation, engaged in sexual contact frequently after fights. Whether it was two females rubbing genitals in a reassuring embrace or a male seeking comfort after a scuffle, sex appeared to play a major role in repairing relationships.
Chimpanzees, on the other hand, relied more on non-sexual methods like grooming and body kissing, but still used sex to a surprising degree.
"Chimpanzees have a wider repertoire of reassurance behaviors," explains lead author Jake Brooker. "But sex still constituted a sizable portion of how they reconciled with each other and managed social stress."
One of the most fascinating discoveries was that in both species, older individuals were more likely to initiate sexual contact in pre-feeding situations. This suggests that these behaviors are learned and passed down through generations, much like human cultural traditions.
Breaking Down the Binary: A More Nuanced View of Ape Society
The findings disrupt the simplistic view of chimpanzees as the aggressive, warlike cousins and bonobos as the peace-loving hippies of the primate world. Instead, they paint a picture of two closely related species using a mix of strategies—sometimes violent, sometimes sexual—to navigate the complex politics of group life.
In bonobos, females are the dominant sex, and sexual behavior helps maintain that social structure. In chimpanzees, where males dominate, sexual interactions still occur in tense moments but are supplemented with a broader toolkit of social bonding behaviors. These variations show how species can develop different solutions to the same evolutionary problems.
Perhaps the biggest takeaway is that these behaviors are likely not unique to either species.
"The fact that both bonobos and chimpanzees use sex in this way provides a fascinating window back in time," says Brooker. "It supports the idea that using sex for social purposes was already present in the last common ancestor they share with humans."
What This Means for Human Evolution
If sex as a social tool existed in our last common ancestor with bonobos and chimpanzees, then it likely played a role in shaping early human societies as well. For years, anthropologists have debated the role of sex in human evolution. Was it purely reproductive? Or did it serve to cement social bonds and ease group tensions?
This study leans heavily toward the latter. The fact that both of our closest relatives employ sex in similar ways suggests that early hominins may have done the same. Our ancestors may have used sexual behavior not just for procreation, but as a fundamental part of social life—something that persists in human societies today in ways both obvious and subtle.
And if that's true, then our evolutionary story is not just one of survival of the fittest, but survival of the most socially adaptable. It suggests that cooperation, alliance-building, and even pleasure were not distractions from the evolutionary game but central to winning it.
Rethinking What It Means to Be Human
This research offers a compelling argument that we need to rethink the rigid categories we impose on both our primate relatives and ourselves. The dichotomy between the hyper-sexual bonobo and the violent chimpanzee is not as clear-cut as once believed. Instead, both species use a blend of behaviors—sometimes aggressive, sometimes affectionate, sometimes sexual—to maintain the fragile equilibrium of their social lives.
And if sex as a tool for social harmony existed before modern humans ever walked the Earth, then it has likely played a role in shaping everything from our closest friendships to our political alliances. Perhaps the ancient idea that "make love, not war" is more than just a slogan—it might be an evolutionary strategy that has been with us for millions of years.
Brooker, J. S., Webb, C. E., van Leeuwen, E. J. C., Kordon, S., de Waal, F. B. M., & Clay, Z. (2025). Bonobos and chimpanzees overlap in sexual behaviour patterns during social tension. Royal Society Open Science, 12(3). https://doi.org/10.1098/rsos.242031
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