Long known to be the peaceful close relative of humans, scientists have discovered that the endangered bonobo is more complicated than previously believed.

A new study has observed male bonobos acting aggressive more often than male chimpanzees within their own communities. The researchers also found that more aggressive males of both species had more mating opportunities.

“Chimpanzees and bonobos use aggression in different ways for specific reasons,” said lead author of the study Maud Mouginot, a Boston University anthropologist, in a news release from Cell Press. “The idea is not to invalidate the image of bonobos being peaceful — the idea is that there is a lot more complexity in both species.”

Earlier studies had looked at aggression in both bonobos and chimpanzees, but this was the first to use the same methods of comparison for the two species’ behavior in the field.

“I think earlier studies did not really try to quantitatively compare the two species. There are papers on chimpanzee’s aggression, and few on bonobo’s aggression, but the methods used do not allow us to directly compare those studies (they have different methods or it is too unclear),” Mouginot told EcoWatch in an email. “Only one study did this (Surbeck et al. 2017), however, they used different methods [in] the field. For the chimpanzees they used group follow (e.g., following a group the entire day) and all day focal follow (e.g., following one individual and recording all their behaviors), for bonobos they used group follow and 10-min focal follow. Because those methods were different, we decided to use data that used the same method on the two species (all day male focal follow).”

The study, “Differences in expression of male aggression between wild bonobos and chimpanzees,” was published in the journal Current Biology.

The research team examined male aggression rates in three communities of bonobos at Democratic Republic of Congo’s Kokolopori Bonobo Reserve, as well as two communities of chimpanzees at Tanzania’s Gombe National Park.

The team looked at a dozen bonobos and 14 chimpanzees, tracking one individual at a time for a whole day, noting the frequency of their aggressive interactions, who they were with and if they engaged in physical contact such as pushing and biting or chasing their adversary.

“You go to their nests and wait for them to wake up and then you just follow them the entire day — from the moment they wake up to the moment they go to sleep at night — and record everything they do,” Mouginot said in the press release.

Overall, the researchers found male bonobos to be aggressive more often than chimpanzees, engaging in three times more physical aggressions and 2.8 times as many aggressive interactions.

Male bonobos were almost only aggressive toward other males, while chimpanzees were apt to be aggressive toward females. Aggression by chimpanzees was more likely to involve male “coalitions” — 13.2 percent compared with one percent of bonobo aggressions.

“Male chimpanzees form coalitions for within group fights but especially for territory defense (which include border patrols or sometimes, killing raids). When one male fights another male from his group, he is taking the risk of losing an important coalitionary partner which would have helped for territory defense. Therefore, this might affect their ability to efficiently defend their group against other groups,” Mouginot told EcoWatch.

The researchers believe coalitions may be one of the reasons aggression is not as frequent among chimpanzees. When groups of males fight, there is the potential for more injuries. And if the fighting is within their own community, it could weaken the ability of the group to fend off other groups.

This isn’t an issue for bonobos since most of their altercations are one-on-one. They are also not believed to be territorial and have never been known to kill one another.

More aggressive males of both species had greater mating success.

“Male bonobos that are more aggressive obtain more copulations with females, which is something that we would not expect,” Mouginot said in the press release. “It means that females do not necessarily go for nicer males.”

This was surprising for the researchers to see within bonobo communities, since they have a social dynamic where females frequently outrank males.

“I don’t think females are specifically attracted to more aggressive males. First, by acting aggressively against other males, more aggressive males may manage to push away other males and then spend more time with females. Second, males are aggressive toward other males but avoid acting aggressively against females. One study (Surbeck et al. 2012) shows that more aggressive males spend more time affiliating with females,” Mouginot told EcoWatch.

In the future, the research team would like to compare the aggressive behavior of other groups of bonobos and chimpanzees to see if that behavior varies between subspecies and communities.

“I’d love to have the study complemented with comparable data from other field sites so we can get a broader understanding of variation within and between species,” Mouginot said in the press release.

Bonobos are listed on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species as endangered. They are only found in forests located below the Congo River in the Democratic Republic of Congo.

Despite their threatened status, Mouginot said there are things people can do to help these rare primates from disappearing altogether.

“People can help by learning about bonobos, spreading their knowledge and sharing the critical situation in which bonobos are. They can also follow local or international associations such as BCI (Bonobo Conservative Initiative). They can make donations to those associations or to research centers to help pursue research which helps to maintain reserves and protect the bonobos too,” Mouginot told EcoWatch.

Bonobos are threatened by illegal hunting, agriculture and development, but Mouginot remains optimistic.

“The future of bonobos is very uncertain. They are endemic to one country (the Democratic Republic of Congo), which makes our ability to save them harder. They are losing their territory over farming and villages, they are also victims of poaching. The IUCN estimates that there are only 15,000 bonobos left, however, they are so remote in the rainforest that it is hard to get an exact count. Nevertheless, I don’t think we should lose hope,” Mouginot said. “Reserves, such as the Kokolopori Bonobo Reserve and many others, help to protect the species. Research also helps by improving our understanding of bonobos and by helping building reserves.”

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