A new study has shown that chimpanzees may be able to determine whether their partners know they are in
danger. This suggests that these primates are able to decide how ignorant or informed their peers are about
an unexpected situation.
The finding, made by a team of researchers at Ohio State University’s Comparative Cognition Project,
suggests that chimps share with humans the ability to perceive the knowledge state of a peer, and perhaps
the intention to protect that peer.
Earlier experiments with both rhesus and Japanese macaque monkeys failed to show the same abilities in
those animals. These new results strengthen the argument that in some ways, chimpanzees are closer to
humans than they are to other primates.
The studies were presented Aug. 16 in Chicago at the annual meeting of the American Psychological
Association. Sally Boysen, associate professor of psychology at Ohio State and director of the project, said
the fundamental question for the test was whether one chimpanzee could tell if another was ignorant of a
specific situation, in this case, of a threat or a reward.
Boysen and her colleagues tested three pairs of chimpanzees at the Ohio State colony. Two adult males,
Kermit and Darrell, who had been together for 18 years, were tested, along with a pair of females, Sarah
and Abagail, and a male and female — Bobby and Sheba.
For the tests, Boysen modeled both a treat and a threat to the chimps. She used grapes, a food the chimps
highly desired, as the hidden treat. A member of the research group hiding with a tranquilizer dart was the
threat. All of the animals in the study had previously been sedated by a dart or had seen a tranquilizer dart
used, and saw it as a threat.
In half of the test conditions, both animals in the pair were able to watch as either the grapes were hidden in
the cage, or a researcher with the tranquilizer dart hid as a predator.
For the rest of the experiments, one animal was placed in an adjoining cage with a clear view of the food or
threat while the other animal was kept off in a nearby room.
Boysen wanted to know if one animal would “tell” the other about the reward or threat. If they did, it would
mean one animal would have to decide how well informed the other was about a given situation.
When she tested the animals with the hidden grapes, absolutely nothing happened. No information was
exchanged between the two chimps.
“You wouldn’t expect it to work with the food since no chimpanzee is going to willingly inform another
about the presence of food that they themselves don’t have access to,” she said.
But when the grapes were replaced by the predator, the results changed dramatically. When Kermit was
released into the cage area where the researcher was hidden out of sight behind a wall, Darrell became
He turned to Kermit displaying fear grimaces and alarm vocalizations – two common warning gestures for
chimpanzees. Darrell’s hair also stood on end all over his body, which reflected his state of arousal. Kermit
mirrored the same fear responses, turned and left the cage area before getting close to the hidden predator.
“Based on what we believe about the emergence of these skills in humans,” Boysen said, “this suggests that
Darrell, in a sense, put himself in Kermit’s place. I think Darrell was aware that Kermit couldn’t have
known that the predator was hidden in there.”
During the research all six animals ran through the same experiments and the results were always the same
— the chimps “told” their peers of the hidden predator.
“This suggests that one chimp does recognize the different knowledge state in the other chimp,” Boysen
said. “Darrell didn’t know what the predator was going to do so he had to make a prediction, a rapid
assessment of the situation. He made the decision that there would be trouble if he didn’t let Kermit know
about the predator.”
Primatologists have long known that chimpanzees in the wild will give off warning calls when they see a
potential threat. Boysen said that these were generalized warnings — not specific ones. These experiments
removed most of the variables that exist in the wild and tested if one chimp would warn another specific
chimp of a threat.
“They responded dramatically when a threat was present but only when the other animal in the pair was
uninformed,” Boysen said.
Since the strongest reactions during the tests occurred with Kermit and Darrell, Boysen thinks that the
social bond between two animals may be the key to their ability to warn each other. The stronger that bond,
the greater the warnings the animals gave.
Boysen’s work is supported by grants from the National Institute of Mental Health.