‘Peaceful’ male bonobos may actually be more aggressive than chimps

Have we misjudged our two closest relatives? Chimpanzees are known for lethal violence while bonobos are widely seen as paragons of peaceful coexistence, free love and female empowerment – but a new study suggests that the reality is more complicated.

Maud Mouginot at Boston University in Massachusetts says she has always thought bonobos’ peaceful reputation was “very reductionist”.

To compare the differences in aggression between bonobo and chimpanzee males, she and her colleagues followed 12 males from three bonobo communities at the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo and 14 males from two chimpanzee communities at Gombe National Park in Tanzania.

The researchers tracked each of the primates from when they woke up each morning to when they returned to their nests to sleep at night, recording details of every aggressive incident. Altogether, they logged over 2000 hours following the bonobo males and over 7300 hours tracking chimpanzees.

Aggressive behaviours included contact aggression – such as hitting, pulling, biting or kicking – and non-contact aggression, such as charging and chasing.

The team found that bonobo males had 2.8 times as many aggressive interactions as chimp males in total, and three times as many incidents of contact aggression.

However, chimp aggression was more likely to involve coalitions of males and to be directed towards females, whereas male bonobo aggression towards females was extremely rare.

“I was not expecting to find such rates of aggression among [bonobo] males,” says Mouginot.

Bonobo males that acted more aggressively towards other males were more likely to mate with females while they were fertile.

According to Mouginot, one explanation for why bonobos act more aggressively could be the differences in bonobo and chimpanzee coalitions, which change the costs and benefits of aggression.

“In bonobos, females form coalitions but rarely males,” she says. “In chimpanzees, males form coalitions against within-group males or to defend a territory. Therefore, if one [chimpanzee] male acts aggressively against another one, he might face a coalitionary retaliation.”

But for male bonobos, the risk of provoking a group response is lower, so the consequences of aggression are more predictable and less dangerous, she says.

The study also found that male-female interactions are very different between the two species. In bonobos, males avoid acting aggressively towards females and they form close associations with them.

Mouginot says she doesn’t think that conclusions can be drawn from this about any traits that humans might share with chimpanzees, bonobos or a common ancestor.

“Researchers often refer to chimpanzees, or sometimes bonobos, as the ‘best model’ for our last common ancestor,” she says. “I think none of those species are a good model – they all went through their own evolutionary path. What is interesting is to look at how some strategies evolve in some species and not others.”

Joan Silk at Arizona State University says this data suggests that bonobo males are at least as aggressive as chimpanzee males, which isn’t what we would expect of “peaceful” apes. It will be important to look at other groups of bonobos and chimpanzees to see if the results are replicated, she adds.

However, Gisela Kaplan at the University of New England in Armidale, Australia, says she found the paper extremely frustrating and that the word “aggression” is being misused.

Chimpanzee groups are ruled by one dominant male, whereas bonobos are ruled by females. Competitions for dominance and mating rights in bonobos shouldn’t be confused with aggression, says Kaplan. “There’s more pointless violence in chimpanzees and humans than in other species like bonobos,” she says.