Bonobos Are Gentler Than Chimps? Maybe Not.

ARIELLE DUHAIME-ROSS: This is Science Friday. I’m Arielle Duhaime-Ross. Bonobos are a species of great ape, along with gorillas, orangutans, and chimpanzees. Over the years, they’ve gained a reputation as being calmer and more peaceful than other ape species. But recent work published in the journal Current Biology finds male bonobos may be just as aggressive as male chimpanzees, if not more so. Joining me now to talk about that finding is Dr. Maud Mouginot, postdoctoral associate in the Department of Anthropology at Boston University. Welcome to Science Friday.

MAUD MOUGINOT: Thank you. Thank you for the invitation.

ARIELLE DUHAIME-ROSS: Thank you so much for coming on the show. So for listeners who might not have all their great apes straight, who are the bonobos? And where are they found?

MAUD MOUGINOT: Bonobos are one of our closest living relative with chimpanzees. They can be found in the Democratic Republic of Congo. So we say that they’re an endemic species, which means they can only be found there. And there is around 15,000 left.

When we look at a tree, we have humans. And right next to them, we have a branch, which is what we call the pan branch, which group Bonobos and chimpanzees. So they are as close to us as chimpanzees are.

ARIELLE DUHAIME-ROSS: How did they get this reputation for being so peaceful? And how did that reputation persist for so long if it’s not quite right?

MAUD MOUGINOT: That is a great question. I think it started because the first time researchers saw bonobos, there’s a lot of behavior that were not observed in chimpanzees, such as intergroup interaction. So in bonobos, a group of bonobos can encounter […], spend time together, share food, sometimes sleep together in trees, groom together.

And in chimpanzees, group of chimpanzees cannot encounter. They will always avoid each other or probably aggress each other. And also, in bonobos, male do not coerce female, which means they do not push female into mating with them. And we don’t observe infanticide or killings in bonobos compared to chimpanzees. And I think all those behavior gave that perception that bonobos are peaceful.

ARIELLE DUHAIME-ROSS: I see. OK. So what made you want to look at this question in the first place?

MAUD MOUGINOT: There is a different thing that makes me want to look at it. First of all, there is a study that came out in 2017 that looked at reproductive skew, which is inequality in reproductive success among males. So that means you can have higher reproductive skew, where some males are going to sire more offspring than others. And usually, if we have high reproductive skew in a species, that means there is high-contest competition between males. And males are going to fight between each other.

And this study compared bonobos and chimpanzees. And they found that there was higher reproductive skew in bonobos compared to chimpanzees. So the question was, how do bonobo obtain such high reproductive skew if they’re not so aggressive?

So we wanted to look at actually rate of aggression among those two species. And that sounds pretty basic. Like, why hasn’t it been done before? And there is actually only one study that compared directly bonobos and chimpanzees. But they did not use the same sampling method on the field. So for us, we’re like, OK, let’s do this by using the exact same method on the field from the start so we know what we compare.

ARIELLE DUHAIME-ROSS: I was really struck by the literal years of observational data that your team had to gather to reach these conclusions. Can you tell me about that? How did you get these data points?

MAUD MOUGINOT: Yeah, that’s a great question. So if we start with chimpanzees, we are at Gombe National Park. So it’s the Jane Goodall site. So there is– I don’t know– 60 years of data. For bonobos, we started to get good data from 2019, which is a lot less than what we have for chimpanzees.

It’s actually hard to get good male focal follow. So focal follow is when you follow one individual throughout the day. And when you do that type of study, you want as much as possible so you can have a lot of variation. And for this, that might ask to actually get two years or even more of data. So that’s why we have those numbers.

ARIELLE DUHAIME-ROSS: So let me get this straight. You would get up and literally follow around one bonobo all day, just watching what this male did?

MAUD MOUGINOT: Exactly, yes.

ARIELLE DUHAIME-ROSS: How long were your days? And what was that like?

MAUD MOUGINOT: So for me, all the data we have actually come from field assistants mostly, not from my own data, because I haven’t spent two years, unfortunately, on the site. But yeah, usually, we spend– I might spend– I don’t know– 12 hours.

We wake up at 3:30 in the morning. Go start your day at 4:00. Try to join the bonobo around 5:00 because that’s the time they wake up. And then you follow them all day until they go back to bed, so in their nests in trees. And they’re probably going to end up there around 5:00 PM. So that’s a pretty long day at the end.

ARIELLE DUHAIME-ROSS: Wow. All right. So what exactly did your team observe over the observational period?

MAUD MOUGINOT: So they observed a lot of different behavior. Personally, the first thing when I arrived on the field my first week, I would always remember– it was still pitch dark. So I couldn’t recognize, really, the individual. And I saw two ball of fur running in the trees, one after the other. And the other one was screaming, crying. And I was like, what is going on? And the field assistants were not surprised. Like, oh, they’re just fighting. That’s an aggression.

ARIELLE DUHAIME-ROSS: When we say that male bonobos are just as aggressive as male chimps or even more so, what exactly does that mean? Can you quantify that?

MAUD MOUGINOT: Yeah, so we have some quantity. We observed that male bonobos act 2.8 times more aggressively than male chimpanzees. However, we have to keep in mind that when we compare things like this, it’s a little bit too straightforward. That’s why we have statistical analysis that allows us to control for between individual variation, group variation, number of males also in the group. But even with the statistical analysis, we observed a difference between the two species.

ARIELLE DUHAIME-ROSS: OK, I see. So chimps are notorious for their aggression, right? Both males and females tend to fight with other members of their community. And they do so pretty regularly.

They can also recover from those fights fairly quickly in terms of their relationships with the chimps that they have fought with. They’re known for making up. They reconcile and even console each other. Are bonobos the same? Are they different? What did you observe?

MAUD MOUGINOT: Actually, for reconciliation, studies are still going on right now. There is one study that looked at it. And they found a lot less reconciliation among bonobos than chimpanzees.

ARIELLE DUHAIME-ROSS: Really?

MAUD MOUGINOT: Yeah, at least among males.

ARIELLE DUHAIME-ROSS: That’s interesting.

MAUD MOUGINOT: Yeah, it is. And it kind of also makes sense if we look at how dynamics works in the society of bonobos and chimpanzees. In bonobos, males are very individualistic or more individualistic in their strategy. They do not form coalitions of males. So they tend to fight. And if they act aggressively against each other, they know the costs and the benefits of each aggression.

In chimpanzees, the dynamic is very different because males make coalitions within group fight, but also for between group fight. So the costs and benefits of aggression is different. And then the process of reconciliation are not going to have the same benefits.

ARIELLE DUHAIME-ROSS: OK, right, because there are structural differences between these different species in terms of how their communities are built. Can you tell me about that?

MAUD MOUGINOT: Yes. In chimpanzees, male form coalitions. Female rarely or not do so. And male are dominant over females. So male will form coalition for within group fight and to do border patrols and, from time to time, some killing reds if they see one or two individuals by themselves.

In bonobos, it’s totally different. Male rarely or do not form coalitions. But female form coalitions. And they do so sometimes against males. And both male and females in bonobos are co-dominant. So it’s a totally different dynamic.

ARIELLE DUHAIME-ROSS: Which might explain some of the differences, but it’s interesting that you found similarities. So you were looking at individual actions of a few apes from specific ape communities of bonobos and chimps. Can you extrapolate that out to an entire species?

MAUD MOUGINOT: It’s actually one of the things of this study is that we compare only one side of bonobo with one side of chimpanzees. So the main idea is that it will be really nice to have more sites to understand better the variation. What I can tell from this study is that we think that they use aggression differently because of those different coalitionary patterns.

So because male chimpanzees form those coalitions, when they act aggressively against another male, they might face a coalition or a retaliation, which is very expensive, is very risky. On the other way, if there are a lot of male chimpanzees acting against one male chimpanzees, then the costs are lower.

But in any case, when they act aggressively against a member of their own community, it’s costly because this member might have been a future partner for between group interactions, so defending the group. So aggression, in general, is less predictable and more expensive for chimpanzees compared to bonobos. So that might be why we observed a difference of rate of aggression between the two species.

ARIELLE DUHAIME-ROSS: […] for you to be able to conduct a study on them?

MAUD MOUGINOT: So yes, there are only 15,000 left of them. This is what the IUCN has evaluated. And we don’t really know exactly the number. Because they’re so remote in the rainforest, it’s hard to get an actual, predictable number. And I think research on bonobos really allows us to better understand them and then to better protect them.

For example, at the Kokolopori Bonobo Reserve, it’s a reserve. So it allowing us to really protect this species or group of bonobos, which is really great to protect them from poaching or from environmental destruction. So I think research are really important. I’m happy to be part of such research to help also protect the species.

ARIELLE DUHAIME-ROSS: So what’s next? Where do you go from here with this research?

MAUD MOUGINOT: So from here, my dream research would be to actually get more study, more field, to do the same analysis with their chimpanzee group or bonobo group so we can get a better understanding of the variation between species, but also within species in terms of aggression.

Also, for me, I was really interested in the dynamic between male and female. If we look at the data, they are very opposite in chimpanzees. Male act aggressively against females. But females tend to avoid to act aggressively against males. In bonobos, it’s a total different dynamics, where female act aggressively against males. But males tend to avoid acting aggressively against the female. And I’m really interested in that dynamic.

ARIELLE DUHAIME-ROSS: All right. Well, I hope you get to do that kind of research in the future. We’ll be keeping an eye on that. Thank you. Dr. Maud Mouginot is a postdoctoral associate in the Department of Anthropology at Boston University. Thank you so much for talking with me today.

MAUD MOUGINOT: Thank you for your time.