Caesar and his fellow simians in the blockbuster film may have had a few canines to grind, but new research reveals that chimpanzees are more interested in sharing a meal than starting a revolution. A paper released this week in Proceedings of the National Academy of Sciences by Victoria Horner, J. Devyn Carter, Malini Suchak, and Frans de Waal of the Yerkes National Primate Research Center at Emory University in Atlanta, Georgia, is the first to document what the researchers call "spontaneous prosocial choice by chimpanzees," a species that, until now, was thought to be "indifferent to the welfare" of others.
Drawing by Devyn Carter from a video still.
The researchers made their breakthrough by presenting chimpanzees with a simplified moral test where they had to choose to be selfish or prosocial. Pairs of chimpanzees were brought into the testing room where they faced each other separated only by a wire mesh. On one side was a bucket containing 30 tokens that the chimpanzee could give to an experimenter for a food reward. Half of the tokens were of one color that resulted in a selfish outcome in which only the chimpanzee who gave the token received a reward. The other tokens were of a different color that resulted in a prosocial outcome in which both chimpanzees received a food reward. The individual making the selection was rewarded no matter what. Their only choice was whether or not their friend would benefit as well.
Because the individual making the choice always received a reward there was no incentive in the test design to encourage the prosocial option. The choice was each individual's alone. Twenty-one pairs were tested in all, with each individual repeating the test on three different occasions and never sitting across from the same partner twice. If chimpanzees were indeed motivated only by selfish interests it would be expected that they would be more likely to choose a reward only for themselves (or it should be 50-50 if they were choosing randomly). But individuals were significantly more likely to choose the prosocial outcome, indicating that Pan troglodytes clearly considered others when making their choice.
"Offered a free choice between a prosocial and selfish option," the scientists conclude, "chimpanzees overwhelmingly favored the former to the advantage of their partner." Like Darwin, the chimpanzees in the study made a prosocial choice and decided to share the reward rather than enjoy it all for themselves.
This result should not be surprising to those who have followed De Waal's research (see my in-depth interview about his work here) as well as similar studies concerning primate cooperative behavior. For example, bonobos (Pan paniscus) are as closely related to humans as chimpanzees are (sharing 99.3% and 99.2% of our DNA respectively) and have previously been shown to cooperatively share their food with others. Researchers Brian Hare and Suzy Kwetuenda working at the Lola ya Bonobo Sanctuary in the Democratic Republic of Congo reported last year that bonobos, when given the choice, "preferred to release a recipient from an adjacent room and feed together instead of eating all the food alone."
Earlier studies have shown that chimpanzees engage in spontaneous altruism when witnessing an unfamiliar chimp trying to reach food on the other side of a door. As reported last year by Alicia Melis, Michael Tomasello, and colleagues, chimpanzees were much more likely than not to pull a chain that offered access to this food even though the altruist received nothing themselves. Chimpanzees have also been shown to provide assistance when presented with an unfamiliar human struggling to reach an object just out of reach. In 2007 Felix Warneken and colleagues found that chimps behaved identically to human toddlers under this scenario and were only too willing to help a stranger in need, even if that meant climbing over a series of obstacles in order to do so. Offering a reward for their assistance had no effect on the display of generosity in either study. Service, it seems, was its own reward.
While Caesar and his rising ape army chose to rebel against humanity, it's comforting to know that real chimpanzees retain the charitable natures that our two species share thanks to millions of years of evolution promoting these cooperative bonds.
Discover more about this groundbreaking study at Scientific American.