If Bonobos Are Aggressive, Does That Deny the "Self-Domestication Hypothesis"?

                                                Vergil, A Male Bonobo at the Cincinnati Zoo

For many years, I taught a course at Northern Illinois University on "Chimpanzee Politics," which included some comparative study of chimpanzees and bonobos, the two primate species most closely related to human beings, because all three species evolved from a common ancestor about 7 million years ago.  

Every time I taught the course, I took the students on a field trip to the Milwaukee County Zoo, which has the largest captive group of bonobos in the world (over 20 individuals).  I have also been to the Cincinnati Zoo, which has about 12 bonobos.  Of course, it would be better to study the social behavior of bonobos in the wild.  But that is hard to do because they are only found in one place in the world--in dense rainforest areas south of the Congo River in the Democratic Republic of the Congo.  Even those biologists who go there to study them find it hard to observe them as they move through the dense canopy of the rainforest.

Chimpanzees seem to be far more aggressive and violent than are bonobos.  Male chimps attack females and other males.  Sometimes these attacks are lethal.  Male chimps also form coalitions with other males to assert a male dominance hierarchy over females and other males.  These male coalitions also patrol the borders of their community, and they can launch attacks against other communities--even to the point of annihilating the whole community in war.

By contrast, bonobos have never been observed to engage in lethal attacks on other bonobos.  Female bonobos seem to be dominant over males.  And the females form coalitions with one another to attack males and mediate their conflicts.  In contrast to chimps, bonobo communities have never been observed to go to war with one another.  Bonobos from different communities can interact with one another peacefully.  All of this peacemaking depends on mutually pleasurable bisexual lovemaking--rubbing their genitals together--females with other females and with males.  This is why the bonobos have been dubbed the "hippie apes" who "make love not war."

Comparing human beings with these two ape species in working out the evolutionary links between the three species has provoked a debate among evolutionary biologists and social scientists.  The Hobbesian scientists argue that human beings are closer to chimps, which shows that the human state of nature was a state of war.  The Rousseauean scientists argue that human beings are closer to bonobos, which shows that the human state of nature was a state of peace.

In my posts on bonobos and the human state of nature, I have argued that Locke's account of the state of nature is closer to the truth than either Hobbes' or Rousseau's, and that evolved human nature combines the natural propensities of both chimps and bonobos.  As Steven Pinker would say, our human nature has both Inner Demons and Better Angels.  Lockean liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tame the Inner Demons while eliciting the Better Angels to motivate voluntary cooperation and nonviolent relationships.

But in contrasting bonobos and chimpanzees, we should not assume that bonobos are utterly peaceful.  That bonobos are often aggressive in their attacks on one another is made clear in new research by Maud Mouginot and her colleagues that was just published online on Friday (Mouginot et al. 2024).

The message from this study as reported in the press--as in Carl Zimmer's report for the New York Times--is that "male bonobos commit acts of aggression nearly three times as often as male chimpanzees do."  That would seem to deny the common view that chimps are far more aggressive than bonobos.  But if you read the article carefully, you will see that the story is much more complicated than that.

Mouginot and her colleagues employed what scientists studying animal behavior call the "focal-animal sampling" method (Altman 1974).  All occurrences of specified actions of an individual, or specified group of individuals, are recorded for a pre-determined period of time.  

For their study, they had all-day focal follow data for 14 chimpanzee adult males from two communities in the Gombe National Park in Tanzania and 12 bonobo adult males from three communities in the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo.  They had recorded hundreds of aggressive dyadic interactions, including contact aggression (physical contact such as hit, pull, bite, kick, or jump-on) and non-contact aggression (such as charge and chase).  They had also recorded whether the focal-male was the aggressor or the victim and whether he was interacting with another male or with a female. 

They wanted to use this data to test the "self-domestication hypothesis" of Brian Hare and Richard Wrangham.  They suggested that bonobos evolved to be less aggressive than chimpanzees just as dogs evolved to be less aggressive than wolves.  Humans selected less aggressive (or friendlier) wolves to become their companions, and over time, wolves evolved into dogs through domestication by human selection.  Similarly, if female bonobos formed coalitions to punish aggressive males, and if females preferred to mate with less aggressive males, which would tend to produce less aggressive offspring, bonobos could have been self-domesticated for being less aggressive or friendlier to one another (Hare, Wobber, and Wrangham 2012).

Moreover, Hare and Wrangham have also suggested that humans could have undergone a similar process of evolution by self-domestication to be less aggressive or friendlier towards individuals within their community (Hare 2017; Hare and Woods 2020; Wrangham 2019).  I have extended this idea of human self-domestication to explain the evolution of Lockean liberalism and the bourgeois virtues as symbolic niche-construction.

What Mouginot and her colleagues have found does not deny the self-domestication hypothesis of Hare and Wrangham, although it might require some refinement in the theory.  They found that there was a higher rate of male-male contact aggression among bonobos than chimpanzees.  And in both species, the more aggressive males had higher mating success.  But they found no evidence to contradict the observation that bonobos never kill other bonobos, while chimpanzees do kill other chimpanzees in fighting both within and between communities.

One possible explanation for why bonobo males engage in more non-lethal aggression with other males than do chimpanzee males is that since bonobo females prevent males from forming coalitions, bonobo males can attack other males without suffering reprisals from male coalitions.

As Hare told Carl Zimmer, the one dramatic difference in aggressiveness between the two species remains:  "Chimpanzees murder, and bonobos don't."

REFERENCES

Altman, Jeanne. 1974. "Observational Study of Behavior: Sampling Methods." Behaviour 48: 227-65.

Hare, Brian. 2017. "Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality." Annual Review of Psychology 68: 155-86.

Hare, Brian, and Vanessa Woods.  2020.  Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity. New York: Random House.

Hare, Brian, V. Wobber, and Richard Wrangham.  2012.  "The Self-Domestication Hypothesis: Evolution of Bonobo Psychology Is Due to Selection Against Aggression."  Animal Behaviour 83: 573-85.

Wrangham, Richard.  2019.  The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution.  New York: Pantheon Books.

Zimmer, Carl.  2024.  "No 'Hippie Ape':  Bonobos Are Often Aggressive, Study Finds." The New York Times, April 12.