Chimpanzees have a lot to gain from climbing the social ladder. It now appears that lower-ranking male chimps strengthen bonds with their friends in high places by alerting them to some good eats. Researchers experimentally captured this communication—amounting to “hey buddy, there’s some food over here”—for the first time among a society of wild chimpanzees in Uganda.
When one party intentionally alerts another to a third object, scientists call it “triadic communication.” Such intentions have been difficult for scientists to ascertain among groups of chimps, since it is impossible to determine an animal’s motivations simply by observing it. Chimps often emit food grunts, a seemingly gleeful noise that sounds like a happy pig munching on slop. Some researchers think these grunts—and all chimp vocalizations—are reflexive emotional reactions to food and predators, while others have pointed to social maneuvering as a possible cause for some chimp chatter.
To explore the motivations behind food grunts, a team led by psychologists Anne Schel and Katie Slocombe of the University of York, United Kingdom, recorded chimp greeting calls in the Budongo Forest, Uganda. The scientists waited for a chimp to chow down alone, then played recordings of a familiar male’s greeting call. If the call came from a more dominant male in the society—especially one that was already friends with the feeding male—the chimp grunted to alert the newcomer to his bounty. In the video below, the researchers have just played the greeting call of a higher-ranking male to Frank, a male chimp who had been eating alone in silence for several minutes. Frank immediately calls back with his own greeting and several low “food grunts.”
Out of 42 trials conducted by the team, the feeding chimp made the specific food-sharing grunt 10 times. In those cases, Slocombe says, the chimps only alerted their friends who ranked higher on the social pecking order within their society. “It shows they are really quite selective,” she says.
Letting their friends in on a food source could be a way that chimps form alliances with one another to ensure their own protection if a bigger male comes along, the researchers believe. If the chimps had made the food grunt no matter who was coming, Slocombe says, she would have deduced that the calls were an automatic response to either the food or the newcomer. They reported their results in the November issue of Animal Behaviour.
However, one primatologist disputes the team’s interpretation. A simpler emotional response could explain the food grunts, says primate behavioral ecologist Brandon Wheeler at the German Primate Center in Göttingen. “I feel like they set up a straw-man argument,” Wheeler says. Slocomb and Schel cited the complex circumstances as the reason why the vocalizations were voluntary, but Wheeler believes the presence of a chimp’s friend merely triggers an innate emotional response. Called the “audience effect,” this same phenomenon happens when you watch a funny movie with a friend instead of viewing it alone. The friend’s presence makes you more likely to laugh. So instead of “Hey buddy, there’s some food over here,” says Wheeler, the chimps might simply be saying, “Mmmm!”
Slocombe disagrees, stating that Wheeler’s explanation—while plausible—is not a better or simpler way to explain the observations. Whatever cues are driving the grunts are too complex for innate behavior to explain them, she maintains. “Friendship with another is a result of repeated affiliative interactions and is dynamic, changing sometimes very quickly over time,” Slocombe says. “It is not parsimonious to assume that friendship with another triggers an innate response.”
“There’s certainly a lot of advancement here,” Michael Corballis, cognitive neuroscientist at the University of Auckland in New Zealand, says of the study. Corballis is of the belief that hand and body gestures are tools of communication within chimps. But he also says that this study shows that there is likely more intention behind chimp jabber than researchers previously thought. He suggests that if chimps are grunting to their friends intentionally, that may mean they have theory of mind, or the mental flexibility to put themselves in the shoes of other chimps. Theory of mind is a cognitive milestone in animals, but Slocombe and Schel cautioned that their study could not determine if chimps have the ability.
The food grunts could be “mmm!” or, “hey buddy, there’s some food here,” or a third option, “hey buddy, come join me.” Since chimps can’t tell researchers what their intentions were, it’s impossible to really know for sure, Schel says.