Chimpanzees and bonobos trade food with others who share with them first, showing similar levels of cooperation and reciprocity as four-year-olds.
Food-for-food exchanges are common in human societies, but such exchanges are relatively rare in animals such as chimpanzees, bonobos and gorillas. Although these monkeys groom and care for others of their own kind, the exchange of food is less common, perhaps because the animals see meals as a source of competition.
The researchers started by testing whether 10 chimpanzees (Pan troglodytes) and 2 bonobos (pan paniscus) would share food with each other uninvited. In the lab, the monkeys were in separate enclosures with reward choices on small plates in the space between their cages, so they could see their mate selection. The researchers gave the monkeys the choice of pulling a plate containing a single treat to their own cage or pulling a mechanism that dispensed a treat to both their own cage and that of another monkey. They found that most monkeys choose to feed themselves.
The team was curious whether chimpanzees and bonobos would be more likely to share food – or withhold it – depending on what their mate chose first. The researchers repeated the test ten times without giving the first monkey a choice, instead faking the initial interaction by letting the monkey reach for only one choice, to deliver food to one or both monkeys.
A similar choice test was given to 48 four-year-olds who could choose between a treat for themselves or for themselves and another child. Each of the trial participants received rewards tailored to their tastes: peanuts for chimpanzees, grapes for bonobos and candy for preschoolers.
When the monkeys saw that another had intentionally shared food with them, they reciprocated about 70% of the time – almost 80% of children made the same choice. When children felt snubbed by another, they were unlikely to share food back, while some monkeys still shared food about half the time, even if their partner was initially “selfish”.
The finding builds on a 2017 study that found chimpanzees were more likely to return favors to others when their partner took a risk to help them, says Sarah Brosnan at Georgia State University, which was not involved in the work.
The results suggest that human ancestors must have reduced their aggressive and dominant tendencies around food to foster cooperation and social bonding.