Capuchin monkeys are known for their ability to recognize when they're being treated inequitably, but it now appears the primates can even spot unfairness in situations that don't involve themselves.
The fluffy-faced monkeys judge the social interactions of others and hold biases against individuals behaving poorly, new research shows.
In a pair of studies, researchers investigated how capuchin monkeys in captivity reacted to different third-party social interactions. In one study, capuchins watched two actors engage in reciprocity exchanges, in which one actor handed over several balls to another, who then either reciprocated or selfishly kept all the balls. The second study involved a similar setup, but this time one actor helped or refused to help another actor who was struggling to open a container.
After each scene, the monkeys chose a treat from one of the actors — they consistently avoided treats from actors who refused to reciprocate or help. Capuchins in the wild may keep tabs on group members to figure out whom to avoid interacting with on a specific day, researchers said.
"The research implies capuchin monkeys are judging other individuals even when they aren't involved in the action, something that humans do all the time," said Sarah Brosnan, an ethnologist at Georgia State University, who wasn't involved in the new research. "It suggests the behavior may be deeply rooted in the primate family tree."
In all fairness
In 2003, Brosnan and her colleagues discovered capuchin monkeys have a sense of fairness. They trained captive monkeys to hand them an object in exchange for a cucumber slice or the preferable grape. If a capuchin saw another monkey receive a grape while it was given a cucumber, it would refuse the reward or even throw the cucumber at the researcher.
Subsequent research showed other cooperative primates, including chimpanzees, also know when they're being treated unfairly, but nobody has looked at whether nonhuman primates can spot inequity in situations that don't involve themselves.
"So we wondered if they're sensitive to third-party interactions," said James Anderson, a primatologist at the University of Stirling in Scotland and lead author of the new studies. "Can they form impressions of individuals based on how those individuals behave towards one another?"
To find out, Anderson and his colleagues tested capuchins' reactions to scenes of reciprocity. Two actors began with two containers each, one of which contained three balls. One actor held out an empty container to the second actor, who then placed her balls into the container. Next, the second actor similarly requested balls from the first actor. In half of the trials, the first actor refused to reciprocate and kept all six balls to herself (the actors switched places throughout the experiment and equally played the non-reciprocator role).
After each scene, both actors offered an identical treat to the monkey — the capuchin chose a treat by reaching toward one of the outstretched hands. The primates showed no preference when both actors reciprocated, but they consistently avoided taking treats from non-reciprocators, the researchers found in the study, detailed online recently in the journal Cognition.
The team then conducted "incomplete" and "impoverished" reciprocity sessions, in which the reciprocator gave over only one of her three balls or the single ball she started with, respectively. The monkeys showed no significant preferences in either cases, but were overall more likely to accept treats from impoverished than incomplete reciprocators, even if the receiving actors pretended to be satisfied with an incomplete exchange. "[The impoverished actor] gave everything she had started off with, so it's as if the monkeys accepted her intention to fully reciprocate," Anderson said.
In a companion study, published today (March 5) in the journal Nature Communications, the researchers tested how capuchins regard unhelpful people. Here, one actor struggled to open a container and requested help from the second actor, who either helped or turned away. Similar to before, the capuchins avoided accepting treats from unhelpful actors. [No Fair? 5 Animals With a Moral Compass]
The researchers then investigated what happens when both actors have a container. Again, if the second actor refused to help, the monkeys showed a sharp bias against her and accepted treats only from the other actor. However, if the actor didn't help because she was too occupied with her own container, the capuchins showed no biases, further suggesting the monkeys considered the actors' intentions. (The team also tested if the act of turning away, rather than being unhelpful, was specifically to blame for the monkeys' biases; it wasn't.)
Importantly, the objects handled in both studies had no relevance to the monkeys, Anderson said. If actors handled food, the monkeys would likely choose whoever they thought would give them the most treats.
Brosnan agreed: "Using food could have changed the capuchins' behaviors."
A widespread behavior?
"I think it's a really interesting study with implications for helping us understand how cooperation comes about," said Malini Suchak, a primatologist at Emory University, who wasn't involved in the research. Capuchins and some other primate species are very cooperative, so knowing who in their community will be the most reciprocal or helpful is important. "If you choose the wrong partner — a cheater — you've lost at that point."
Darby Proctor, also an Emory primatologist, says the research may help "inform us about our own evolution." If social evaluation isn't widespread among primates, it may mean the behavior evolved from some kind of selective pressure, she said.
Brosnan, on the other hand, wonders if the behavior exists in other animals, such as birds and fish that have been shown to gather information by "eavesdropping" on others.
But the experts agree that before looking at other species, researchers should see if capuchins really do judge the actions of their own kind. "I would want to see what they could get the capuchins to understand about two other capuchins," Proctor said, adding that the studies' results suggest the monkeys evaluate each other in the wild.
For now, Anderson is investigating what capuchins think of people who over-reciprocate. "Can monkeys develop a positive bias for a person who behaves generously?" he said.