Chimpanzees are considered conflict-ready, bonobos are considered peaceful. Now a research team has observed that male bonobos become more aggressive within their group than male chimpanzees. The study published in the journal “Current Biology” also shows that in both species, aggressive behavior leads to greater mating success.
Chimpanzees (Pan troglodytes) and the smaller bonobos (Pan paniscus), also known as pygmy chimpanzees, are humans’ closest living relatives. Researchers are studying both species in order to better understand certain behaviors in humans – for example the evolution of aggression.
Until now, it has often been assumed that chimpanzees and bonobos are fundamentally different: conflict-prone chimpanzees versus peaceful bonobos. Male chimpanzees are known to sexually coerce females and sometimes kill members of their own species, even young animals. This has not yet been observed in male bonobos. Researchers suspected that aggression in male bonobos would lead to lower reproductive success.
Bonobo males are more likely to be aggressive than chimpanzee males
A research team led by anthropologist Maud Mouginot from Boston University now compared the aggression of bonobos and chimpanzees. The researchers examined the behavior of three bonobo groups in the Kokolopori Reserve in the Democratic Republic of Congo and two chimpanzee groups in Gombe National Park in Tanzania. In total, they observed 12 bonobos and 14 chimpanzees by following them throughout the day and noting every aggressive interaction. They also examined who the animals were aggressive towards and whether they became physical or not – for example, whether they pushed and bit other animals or just chased them.
Surprisingly, the team found that male bonobos behaved aggressively more often than male chimpanzees. Conflicts with bonobos were usually more harmless. But overall, the researchers counted around three times as many aggressive interactions and physical attacks in bonobos as in chimpanzees. While male bonobos were primarily aggressive toward other males, male chimpanzees were more aggressive toward females.
Coalitions are more common in chimpanzees than in bonobos
Male chimpanzees also formed coalitions more often to attack other chimpanzees. The researchers observed such alliances in a good 13 percent of all aggressive interactions, and in only 1 percent of cases in bonobo males – so they almost always resolved their conflicts in pairs.
Such coalitions could possibly be a reason why aggressive behavior is less common in chimpanzees, the team speculates. If entire groups of males attack each other, the risk of injury or even death is greater. Such fights could sometimes weaken groups so much that they could no longer defend themselves against chimpanzees from other communities.
Aggression increases reproductive success in both species
“Chimpanzees and bonobos use aggression in different ways for specific reasons,” Mouginot is quoted as saying in a statement from the publisher. Although used differently, aggression increases reproductive success in both primate species. In a bonobo group in the Kokolopori Reserve, 80 percent of the offspring came from just two males – those with the most violent behavior, according to the surprising result.
“We did not expect that more aggressive bonobo males would copulate more often with females,” said Mouginot. While in chimpanzees the males are the highest-ranking animals and force females to mate, in bonobo communities the females dominate. Mouginot concludes: “This means that bonobo females do not necessarily prefer the nicer males.”
