'Peaceful' bonobos more aggressive than first thought

Bonobos have long had a reputation for being peaceable apes that are more interested  in making love and solving conflicts than starting fights, almost pacifist in nature compared to their aggressive relatives, the chimpanzees. But new research in the journal Current Biology suggests that this prevailing view is wide of the mark, as Boston University’s Maud Mouginot explains...

Maud - Bonobos show behaviours that are very different from chimpanzees. They can encounter between groups. Different individuals from one group can encounter another one. Sometimes it's a little bit aggressive, but mostly they can encounter, they're going to spend time together, they're going to share food, they're going to copulate, they're going to sleep together at night. In chimpanzees, those encounters are aggressive and some individuals might even kill others from other groups. Also in Bonobos there is no female coercion. So male do not push or force females to copulate with them, whereas in chimpanzees male will coerce females into mating with them. And also in Bonobos we do not have infanticide or we don't observe infanticide or killings between individuals. And we do observe infanticide killings in chimpanzees. And I think all those differences actually brought people to think that bonobos are peaceful and might not show any sign of aggression.

Will - I mean from what you're saying, I'm inclined to agree. How did your study change that notion then?

Maud - So we compared the rate of aggression between bonobos and chimpanzees and we observed that male bonobos do act more aggressively than male chimpanzees when acting aggressively against another male. This is not what we'd expect. We would actually expect Bonobos to be less aggressive than chimpanzees.

Will - It's a different kind of aggression then. It's not that they're less aggressive, it's that they channel this aggression in a different way.

Maud - Absolutely. We look at two types of aggression. So we all have aggression. So that could be chasing, biting, pushing, whatever you can imagine aggression could be. And we have also, we also looked only at contact aggression. So it's when one individual touches another individual such as pushing, biting, grabbing, and those contact aggressions are a little bit more intense because it engages like actually touching another individual. And we still observe higher rates of aggression among male bonobos and those rates of aggression are higher among male bonobos than among male chimpanzees.

Will - Why do we think that there is such a difference then between bonobos and chimpanzees and why do we think that bonobos know when to stop before, as you say, it turns into killing one another?

Maud - So that's a really good question. When we found out those results we were kind of surprised. But we also looked at the coalitionary context. So in chimpanzees males form coalitions against other males. Those coalitions can be against males within their group or outside their group. In bonobos, females make coalitions, but males tend to rarely form those coalitions. They have a more individualistic strategy. And so when male bonobos use aggression, then they kind of know the cost of this aggression because they know they're going to face only one male in front of them. In chimpanzees, if a male chimpanzee wants to act aggressively against another male within the group, then it might face a coalitionary retaliation. So several individuals are against him and then the cost of aggression is really high. But also if he has a bunch of partners with him to act aggressively against one individual, then the costs are really low for him. But what we have to keep in mind is when male chimpanzees act aggressively against another male within his group, it's still high risk because it needs this male for a partnership to defend the group territory. So there are always higher risks for chimpanzees and low variable risks for them. Where bonobos risks of or cost of aggression are a bit more predictable.

Will - It shows that aggression is evolutionarily advantageous at certain points for chimpanzees and for Bonobos. And we're by no means not an aggressive species, just look at the state of the world. Did we get this far purely because we knew when to be aggressive and when not to be?

Maud - When I was looking at humans, we have what Richard Wrangham called the goodness paradox, which is that humans can be highly aggressive in warfare, but we're also very tolerant living in societies of millions of inhabitants. And we share goods and services to other societies. And one hypothesis around this was the self domestication hypothesis, which states that humans have a selection against reactive aggression. So there are two types of aggression that have been defined, which is reactive aggression, which will be like, 'I'm stealing your food and you're really mad at me, so you wanna push me or bite me.' And proactive is like war, right? It is like you plan ahead to act aggressively against someone or a group. The self domestication predicts that humans may have selection against this reactive aggression. So we have lower propensity to react and we're more tolerant, but still we are pretty proactive in terms of aggression. And I think looking at proactive and reactive aggression is a good way to try and understand balance for humans.