Bonobos can be as violent or more violent than chimpanzees

For decades, a dichotomous image of the two primate species closest to humans has been built: while chimpanzees (

Pan troglodytes

) are very aggressive and violence is at the basis of their social relationships, bonobos (

Pan paniscus

), They are the peaceful cousins, who settle almost all their disagreements with sex and caresses. However, the observation of several communities of both species for thousands of hours shows that things are somewhat more complex: male bonobos triple the attacks carried out by chimpanzees when the victim is another male.

“I remember that at the beginning of my first field season, we were in the jungle and the bonobos had just woken up, everything was still calm when I heard screams and cries. I saw two furballs running through the trees, one bonobo chasing the other. It was an attack, in the morning, so early. I saw another one about 10 or 15 minutes later. “It raised a lot of questions in me about the use of violence by the bonobos,” recalls Boston University researcher Maud Mouginot of her four-month stay in the Kokolopori Bonobo Reserve (Democratic Republic of the Congo) in 2019, which It followed another season the previous year in Gombe National Park, where there are several chimpanzee communities. These two periods observing the two species were the basis for starting a work that now, five years later, has just published its results in

Current Biology

.

During 2,047 hours of observing three communities of Kokolopori bonobos, Mouginot and his research colleagues counted 521 attacks. Most of them were simple pushes and chases. But in 14.8% of cases there was physical damage. Meanwhile, in the 7,309 hours watching two groups of Gombe chimpanzees, they recorded 654 aggressive interactions, 15.1% involving contact. “I was surprised by the results! Bonobos have a reputation for being peaceful, but I always thought that was an overly simplistic way of looking at such a complex species,” says Mouginot. It is one of the keys to this work, which quantifies and classifies different types of violence.

Going into detail is when you go from scenarios where everything was black or white to others full of gray. Although in general the attacks of bonobos are less serious (and in all the scientific literature there has not been a single fatal case described), among males of this species there are 2.8 more attacks than among male chimpanzees and they triple when there is Physical damages. “I did not expect to find such rates of aggression between males,” highlights Mouginot. Only in 16% of the cases was it an attack on a female, a percentage almost identical to that observed in the opposite direction, from a female to a male. Everything changes with chimpanzees.

“The male-female dynamic is very different between the two species. In bonobos, females are co-dominant with males and can form coalitions against them. "Therefore, females can act aggressively against males alone or in coalition, and male bonobos rarely act aggressively against females." The situation is radically different among

P.

troglodytes

. “Among chimpanzees, males form coalitions and outrank all females. They sexually force them to mate with them and therefore act aggressively against them. “These social dynamics change the interactions between males and females and the rates of aggression between the sexes.” Specifically, up to 32% of male attacks are directed at a female, while the opposite situation was observed only 1.8% of the time.

The French researcher gives one of the keys when she talks about coalitions. Almost non-existent among male bonobos, it is a common practice among chimpanzees, both to attack or defend themselves within the community itself, and to unleash real wars against other groups in which they actively seek to kill rivals. The other key is mating. Females of both species have periodic swellings on their genitals, indicating that they are ovulating. Among the chimpanzees, they found that the most aggressive ones had a greater number of copulations with the more tumescent females. Although they also saw this relationship among bonobos, the statistical signal was much smaller.

“The aggression of males against females is much lower in bonobos and they do not seem to use it in the context of mating”

Martin Surbeck, primatologist at the Pan Lab at Harvard University, United States

Martin Surbeck was director of the Kokolopori bonobo reserve and is the principal investigator at the Pan Lab at Harvard University (United States). Regarding the violence of the two species, he recalls some differences confirmed by this new work: “First, there have been no reported cases of lethal aggression between bonobos, either within or between groups. "It is possible that some bonobos die as a result of wounds, but we do not see aggression aimed at killing the opponent, as has been described in the case of chimpanzees." The other big difference has to do with sexual

fitness

. “The aggression of males against females is much lower in bonobos and they do not seem to use it in the context of mating. So the absence of sexual violence is still valid, which makes it even more interesting,” he adds.

For Surbeck, “it seems clear that male aggression rates are at least equally high in bonobos (in this study they were higher, but we have to see if this difference holds including other populations). And he concludes, “they are definitely no longer the stereotypes previously portrayed.”

Primatologist Josep Call, from the University of Saint Andrews (United Kingdom), highlights from this work that "the dichotomy between aggressive chimpanzees and peaceful bonobos is a fallacy." He adds that it was something that “experts have known for many years, but still, there are some who like to continue with this false dichotomy, started by de Waal many years ago.” For Call, what must be taken into account is that “both chimpanzees and bonobos are aggressive, what happens is that their styles and targets

are

different.” But this researcher also remembers that neither species is only aggressive. “Of course they are both also peaceful and reconcile after fighting, for example. But, again, their styles are different, for example, bonobos use more socio-sexual behaviors than chimpanzees." The main limitation of Call's work is one that Surbeck already recognized: they only compare between the chimpanzees of Gombe and those of Kokolopori. “I would have liked to see data from other places because there are big differences between populations.” If they had done so, he is convinced that “it would have revealed the existence of variability within and between populations and species.”

The main author of this research, Mouginot, leaves a final reflection. Many primatologists, including herself as she acknowledges, have investigated violence between these animals looking for answers to the reasons for violence between humans, placing chimpanzees on one side of the coin and bonobos on the other. “Researchers often refer to chimpanzees, or sometimes bonobos, as the best model of our last common ancestor. I think none of those species is a good model, they all followed their own evolutionary path. The interesting thing is to observe how some strategies evolve in some species and not in others.”