Chimpanzees adjust their warning calls if they think a fellow primate hasn’t picked up on a nearby threat, a new study finds.
The results, published in the journal Science Advances, reveal that humans and one of their closest living relatives may share a very special ability, one that could potentially shed light on the origins of language.
Humans have succeeded as a species in large part because of our complex social communication skills — and underlying those skills is the ability to understand another individual’s perspective, and adjust what we say accordingly. “In humans there’s a strong convention, if I’m talking to you I should generally tell you things you don’t already know,” lead author Catherine Crockford, a primatologist at Max Planck Institute for Evolutionary Anthropology in Germany, said in an email. “Right now, if I tell you things you know already, you’d probably get impatient quite quickly.”
But when an animal makes a call, does it know what its peers know or don’t know? Or is it just expressing its own internal feelings?
“There’s an assumption though, that animals don’t do this,” Crockford said. “Take alarm calls. Lots of animals give alarm calls. If a monkey gives an alarm call to a snake, it’s thought to be purely motivated by the signalers’ own perspective, like their own state of arousal, their own emotional state.”
To find out, Crockford and her colleagues ran a series of experiments in which they placed a fake snake made of chicken wire and plaster of Paris in the expected path of wild chimpanzees in the Budongo Forest in Uganda. In the first set of tests, the researchers video-recorded how chimps who encountered the fake snakes reacted. The chimps would make warning calls, or “alert hoos,” to a nearby chimp, and even sit by the snake to make the nearby chimp aware of the danger, especially if the two chimps shared a bond.
In the second set of tests, the scientists played audio recordings to make a wild chimpanzee think a fellow chimp was nearby but out of sight. They played one of two calls: a rest hoo, or one to three alert hoos. Then they watched to see what happened once the chimpanzee came across the fake snake.
The researchers found that if a chimpanzee heard the rest hoo audio recording, it was more likely to make an effort signaling the danger than if it had heard the alert hoo.
Here’s what this means: The chimpanzees couldn’t see any other chimp, or its reaction to their call. So they could not have been reacting to a fellow chimp’s movement or behaviors (for example, approaching or avoiding the snake).
“(It) shows that there is more to animal communication than we thought, specifically that social cognition influences … vocal and nonvocal behavior,” Crockford said.