From a human perspective, chimpanzees and bonobos often represent two sides of our very nature. Chimpanzees are seen as more conflict ready. Bonobos are considered more peaceful, even demonstrating cooperation between groups. Some new research into both great ape species paints a more nuanced picture of both species and their behavior. Bonobos appear to be actually more aggressive than researchers previously thought within their own communities. The findings are described in a study published April 12 in the journal Current Biology.
“Chimpanzees and bonobos use aggression in different ways for specific reasons,” study co-author and Boston University anthropologist Maud Mouginot said in a statement. “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.”
Pushing, biting, and chasing
In the study, the team focused in male aggression, which is often tied to reproduction. They analyzed three bonobo communities at the Kokolopori Bonobo Reserve in the Democratic Republic of Congo and two chimpanzee communities at Gombe National Park in Tanzania. Researchers observed the behavior of 12 bonobos and 14 chimpanzees with a method called focal follows. This involves tracking one individual animal’s behavior for an entire day and noting how often the animal engaged in aggressive interactions, who they acted aggressively with, and whether or not they were physical. In great apes, these physical engagements included pushing, biting, or chasing an adversary.
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“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,” said Mouginot.
They found that the male bonobos aggressive more frequently than chimpanzees. Overall, bonobos engaged in 2.8 times more aggressive interactions and three times as many physical aggressions than chimpanzees.
Bonobo males were also almost exclusively aggressive towards other males, while chimpanzees were more likely to be aggressive towards females. Chimpanzees were also more likely to use “coalitions” of males, with 13.2 percent of chimpanzee aggression and only one percent of bonobo aggressions featuring these groupings.
The altercations involving groups of males can also cause more injuries and community infighting can potentially weaken the group’s ability to fight off different groups of chimpanzees. Bonobos do not appear to have this issue since most of their disputes are one on one. They have never been observed to kill one another and are not believed to be territorial, which leaves their communities more free to fight amongst themselves instead of outsiders.
Male ‘coalitions’
The more aggressive males in both species also had greater mating success. The team was surprised to see this in bonobos because they have a co-dominant social dynamic where females often outrank males and can be more decisive with mates. Chimpanzees have male-dominated hierarchies, where these male coalitions coerce the females into mating.
“Male bonobos that are more aggressive obtain more copulations with females, which is something that we would not expect,” said Mouginot. “It means that females do not necessarily go for nicer males.”
The team notes that female bonobos and chimpanzees are not exactly passive, but that female aggression warrants its own future research.
The self-domestication hypothesis
These new findings of higher rates of male-male aggression in bonobos contradict a prevailing hypothesis in primate behavior called the self-domestication hypothesis. This idea that goes back as far as Charles Darwin posits that evolution has selected against aggression in bonobos and humans, but not chimpanzees.
[Related: Primates have been teasing each other for 13 million years.]
Some of the findings do support some parts of the self-domestication hypothesis, specifically related to aggression towards females. Compared to chimpanzees, male bonobos direct less aggression towards females. According to the team, this aligns with earlier findings that male bonobos rarely use coercive mating strategies, even if they are physically larger.
The team could not assess the severity of aggressive interactions in terms of whether they caused wounds or injuries. They hope to collect this type of data in the future, along with comparing aggressive behavior that varies between communities and subspecies.
“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,” said Mouginot.
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