Monkeys strike for justice

[Ayinde]

Capuchin umbrage suggests sense of fairness extends beyond humans.

18 September 2003


Monkeys strike for equal pay. They down tools if they see another monkey get a bigger reward for doing the same job, US researchers have found1.

The experiments show that notions of justice extend beyond humans, says Sarah Brosnan of Emory University in Atlanta, Georgia. This is probably an innate ability that evolved in our primate ancestor, she believes: "You need a sense of fairness to live in large, complex groups."

Brosnan and her colleague Frans de Waal taught brown capuchin monkeys (Cebus apella) to swap plastic tokens for food. Normally, monkeys were happy to exchange a token for some cucumber.

But the monkeys took offence if they saw a neighbour getting a grape for a token. In about half of such trials, the short-changed capuchin either refused to hand over its token, or rejected the reward. Some threw the token or cucumber clean out of their cage.

The animals' umbrage was even greater if another monkey got a grape for nothing. About 80% rebelled in some way in this situation.

"It's a really neat discovery," says primatologist Charles Janson of the State University of New York at Stony Brook. "The monkey is clearly paying attention to what its neighbour is doing, and realizing that it's getting a better reward."

But it's not clear how important this ability is in the forests of South America, where the brown capuchin lives, warns Janson. "Capuchin monkeys can learn to do all sorts of things in captivity that they never do in the wild," he says.

Capuchins don't hold a grudge, says Brosnan. They worked with her on future trials, and the inequality did not create trouble between animals. "The monkeys were clearly not thrilled, but they weren't visibly anxious," she says.

Only female monkeys show this pique, the researchers found. Males were much less sensitive to inequality. Their minds may have been on other things, says Janson: "Males care about sex, and females care about food. The males might not consider the food differences worth worrying about."

Previous experiments with humans have shown that they become less cooperative if treated unfairly, and punish uncooperative people even if their own reward declines as a result. This is akin to a monkey throwing away the cucumber that it has already worked for. Brosnan is now studying chimps to see if they share this trait with us and capuchins.

http://www.nature.com/nsu/030915/030915-8.html

[kristine]

Same story different reporting...

Researchers find monkeys know when they're getting ripped off

By Tim Friend, USA TODAY

The sense of fairness that many people regard as a uniquely human quality may actually be found in non-human primates and perhaps throughout the animal kingdom, new research suggests.

The notions that people are created equal, that we should play by the rules and that we should be given equal pay for equal work is traditionally thought to come from our culture and socialization. The sense of fairness also has been tied to human emotions such as greed, envy, moral indignation and anger, says Sarah Brosnan of Emory University in Atlanta and the Yerkes National Primate Research Center.


But in a study conducted with captive capuchin monkeys reared at Yerkes, Brosnan and primatologist Frans de Waal found that their subjects also display these sensibilities and speculate that they may be tied to similar emotions in the monkeys.


"The social emotions may be underpinning the primate sense of fairness just as they do in humans," Brosnan says. Their research, published in today's Nature, is the first to demonstrate the sense of fairness in any animal.


The researchers set up a system in which female capuchins were given tokens, which the monkeys could exchange for so-so cucumbers or highly preferred grapes. The monkeys did not know in advance which of the treats they would receive.


When pairs of females were both given cucumbers in exchange for their tokens everything was fine. But when one capuchin saw the other receive a grape while it received only a cucumber, the seeds of unrest were sown.


In some cases, the monkey that was being short-changed would pay its token but refuse to accept the cucumber. Sometimes it would refuse to pay and would stop participating in the experiment. Refusing a food item of any type is very rare behavior in a capuchin, Brosnan says.


In more extreme cases, when one capuchin saw the other receive a grape for free without having to pay the token the unfairly treated capuchin would throw away its cucumber although it had paid for it.


Brosnan and de Waal say the behavior strongly suggests that the sense of fairness is an evolved trait that predates humans and may be widespread in social primates. Such a trait would be needed to foster the cooperation essential for survival, including gathering food and defending against predators. The scientists are conducting similar experiments in chimpanzees.


Brosnan says they have not looked at other species, but she suspects the trait could be present in any social species that displays cooperative behavior, from guppies to whales to lions to wild dogs.