Richard Collins: Learn about the 'common enemy effect' from bonobos

Bonobo (left) and Chimpanzee (right) Although they may look very similar, bonobos and chimpanzees were identified as two distinct species of great ape in 1929. Bonobos have a more slender, gracile build than chimpanzees, who are considered more robust. From birth, bonobos tend to have dark faces with pink lips and hair that often looks as if it were parted down the middle. Chimpanzees, on the other hand, are usually born with light faces that freckle and darken with age. Image: Ape Initiative 

Unusual chimpanzee specimens were sent from Bolobo, on the Congo River, in the 1920s. Somebody, misspelling the town’s name, wrote ‘Bonobo’ on a shipping crate destined for Europe. German zoologist Eduard Tratz and the Austrian Heinz Heck, of ‘Heck’s cattle’ fame, began calling the strange chimps ‘bonobos’. Like Captain Boycott, the sender had unwittingly added a new noun to the lexicon.

Scientists once thought that chimpanzees, our closest animal relatives, were peace-loving vegetarians. Other great apes, the gorillas, orangutans, and humans, were known to kill during conflicts. Lesser apes, the gibbons, were also occasional killers. Chimps, however, seemed never to resort to violence, until it was discovered that these ‘white-haired boys’ of the ape tribe ambushed monkeys and ate their flesh.

A few million years ago, the Congo River divided the African rainforest in two. Chimps are most reluctant to swim, so the river kept two communities apart. Those living north of the river evolved into ‘common’ chimpanzees, while those south of it became bonobos, with longer arms and darker faces. They part their hair in the middle.

There are also behavioural differences between the two.

A three week old Bonobo baby is ruffled by his mother. In their habitat in central Africa, Bonobos are threatened with extinction, being thought to be the rarest of all the great ape species. Picture: AP Photo/Martin Meissner

Bonobos are entirely non–violent; these gentle souls have never been observed killing each other. Theirs is an ideal species, therefore, in which to investigate the evolutionary origins of the inter-communal violence, which is tearing our human world apart. Researchers from Kyoto University have been investigating.

Creatures facing a new threat, real or perceived, tend to draw closer to their peers. Forming alliances, they become more willing to cooperate with each other. In a paper just published, the Kyoto researchers say: "In a number of species, perceived outgroup threat can promote in-group cohesion."

This ‘Common Enemy Effect’ is observed throughout much of the animal kingdom. It is found even among fish. People who lived through World War II, in my experience, talk most nostalgically about the period. The terrors of that bleak time brought everyone closer together. Suicide rates fell during ‘the War’.

The researchers posed an intriguing question; did apes inherit the Common Enemy Effect from their primate ancestors, or did the various species develop it subsequently? Bonobos, they thought, might provide an answer. If they had inherited the trait from their forebears, but lost it, traces of it might still be present in them today.

The researchers selected eight groups of bonobos, at five locations. Matching an experiment carried out previously on chimpanzees, they played audio recordings of other bonobos to them, simulating the threat of intruders. Hearing the recordings, the bonobos became more vigilant and spent more time grooming each other; like humans, they seemed to be putting themselves on a war footing. The effect, however, was less evident among bonobos than among common chimpanzees. They had evidently inherited the Common Enemy Effect but had subsequently ignored it.

"Bonobos teach us that the way our ancestors treated other groups does not seal our fate. Our species has features of both chimpanzee and bonobo group relations, so it is crucial that we understand how both can, and have, evolved," says lead-author Shinya Yamamoto.

• James Brooks et al. Increased alertness and moderate in-group cohesion in bonobos’ responses to outgroup cues. 2024. PLOS 1.