Primates are more likely to groom others that have groomed them — even if they are unrelated. Credit: Filippo Aureli

"I'll scratch your back if you scratch mine" is not a phrase that scientists have always associated with non-human primates in the past. But a new study shows that this kind of give-and-take could explain much of their selfless behaviour.

Primates regularly groom each other to remove parasites and to reduce tension, but at a personal cost: they become distracted from potential predators and mates. The reason for this altruistic behaviour, biologists had believed, was a hard-wired strategy to promote the overall reproductive success of their families.

Evolutionary biologists Filippo Aureli of Liverpool John Moores University in Liverpool, UK, and Gabriele Schino of the Italian National Research Council's Institute of Cognitive Sciences and Technologies (ISTC-CNR) in Rome, have now combed through dozens of previous studies to quantify how often primates groomed relatives and non-relatives, and how often the favour was returned. They found that, contrary to the prevailing view, primates were more likely to groom others that had groomed them, regardless of their relatedness. Publishing their analysis in Ecology Letters1, the researchers report that reciprocity alone explained about 20% of the variability in grooming behaviour in 14 different species of primates, whereas kinship alone explained only 3%.

"It was a strong effect that really confirmed what we felt for quite a while based on our previous work," says Aureli.

Challenging dogma

Altruistic behaviour has been shown in a variety of social animals, from honey bees to chimpanzees. The new findings are "quite surprising because we've had a very strong view that kinship is the most important contributor to grooming relationships", says Joan Silk, who studies social behaviour in primates at the University of California, Los Angeles. "There's a big debate in the literature about whether there is any reciprocal altruism in nature at all," she adds.

Reciprocity plays a more profound role in contributing to fitness than previously thought. Filippo Aureli , Liverpool John Moores University

Primates exchange grooming for other things, such as food, protection and sex. These cooperative exchanges may promote an individual's fitness, or their chance of survival and reproductive success. "Our results suggest that reciprocity plays a more profound role in contributing to fitness than previously thought," Aureli says.

Still, these findings do not negate the role of kinship, says Dawn Kitchen, a physical anthropologist who investigates non-human primates at Ohio State University in Columbus. Previous studies in baboons, which were not included in the latest analysis, showed that kinship plays a strong part in social bonding2,3. "I don't think this is the last word on this topic by any means," says Kitchen.

Socially savvy

One reason for some scientists' scepticism over reciprocity is that they believe it relies on sophisticated cognitive abilities, such as long-term memory, which would allow animals to keep track of who has shown selfless behaviour towards them and whether they have already reciprocated.

"Up until recently, people had thought that if there was any reciprocity, it would have to be short-term, because [the animals] couldn't remember and keep track for longer periods of time," says Silk.

Another possibility is that animals keep track of altruism through an emotional, rather than purely cognitive, type of book-keeping. "On a daily basis, I don't keep track of what I give to a particular individual," Aureli says. "I act much more based on the gut, and I think that's what animals probably do, too."

Future research could delve into the cognitive and emotional abilities of animals that facilitate reciprocal behaviour, and explore the pervasiveness of reciprocal altruism in the animal kingdom, Aureli says. "I'm optimistic that in more species and different behaviours, we'll find similar patterns."