Envious capuchin monkeys react badly to raw deals

This article is reposted from the old Wordpress incarnation of Not Exactly Rocket Science.

Two years ago, Sarah Brosnan and Frans de Waal at the Yerkes National Primate Research Center found that brown capuchin monkeys also react badly to receiving raw deals. Forget bananas - capuchins love the taste of grapes and far prefer them over cucumber. If monkeys were rewarded for completing a task with cucumber while their peers were given succulent grapes, they were more likely to shun both task and reward.

That suggested that the human ability to compare own efforts and rewards with those of our peers evolved much earlier in our history than we previously thought. Of course, animal behaviour researchers always need to be careful that they're not reading too much into the actions of the animals they study.

It's easy to suggest that the monkeys were motivated by envy, fuelled by directly weighing up their rewards with those of others. But they could equally be driven by greed of frustration. They could simply have coveted the better reward regardless of the fact that it was given to their partner. Alternatively, they could have been frustrated at being given grapes in previous trials and having to contend with cucumbers.

To rule out these alternative explanations, de Waal and Brosnan tasked graduate student, Megan van Wolkenten with repeating their earlier study with subtle tweaks. Their new results firmly show that monkeys can indeed spot unjust deals and respond with envy and apathy.

The trio worked with 13 capuchins who were asked to hand over a small granite rock in exchange for a cucumber or grape reward. They tested the monkeys in pairs, sat in adjacent wire cages so that each individual could see what its partner was getting.

If both partners were rewarded equally, they completed the task about 90% of the time, regardless of whether they were given grapes or cucumbers. Even if they were shown their future rewards before the experimenters reached for the rock tokens, they didn't make any special efforts to earn the grapes.

That suggests that they're not being greedy after all and are more than happy to work for a cucumber reward if their peers are rewarded equally. However, if monkeys were given cucumbers while their partners received grapes, they only cooperated 80% of the time and as the trials continued, they were more and more likely to refuse.

The researchers also found that monkeys were just as likely to hand over the tokens, regardless of whether they received a grape or a cucumber in the previous round. That effectively discounts the frustration angle, which suggests that cucumbers fail to meet the lofty expectations set by grapes.

The trio of researchers also found that the monkeys weren't just fussed about rewards. They also compared their efforts to those of their partners and were less likely to cooperate if they had gone to more trouble to get their rewards.

If one monkey exchanged tokens for cucumbers while their partner got one for free, it was still happy to complete the task 90% of the time. But if it had to hand over three rocks for the same reward, it only complied 75% of the time. The monkeys became even more indignant if their slacker partners were given grapes for slacking. Now, they were making more effort and getting poorer rewards and their tendency to hand over rocks fell to new lows.

However, if both partners were given grapes, they were willing to do whatever it took to get them, inequity be damned. It seems that capuchins aren't willing to act disdainfully in the face of really good rewards.

Together, these new results show that capuchins react negatively to unequal rewards and are motivated neither by greed nor frustration. Capuchins hunt squirrels as a team and once food is found, they willingly share it out among the group. Their intolerance for unequal handouts would foster greater cooperation among monkey troupes by preventing any individuals from monopolising the spoils.

In other studies, pairs of capuchins who cooperate for unequal rewards do better in the long run if they swap who gets the lion's share. De Waal speculates that this need to share the spoils of a hunt could be the origin of our own disdain for inequality.

Even so, de Waal notes that the monkeys' aversion to injustice isn't on a par with humans. They don't like getting less than their peers, but they don't react to getting more. If anything, this worsens any inequality since monkeys that do badly end up shunning the task and its reward altogether, while the one that's better off continues to be rewarded.

It may be that in a more realistic situation, monkeys that were ripped off could just leave and find other social partners, but only further research would tell.

Reference: van Wolkenten, M., Brosnan, S., de Waal, F. (2007). Inequity responses of monkeys modified by effort.Proceedings of the National Academy of Sciences, 104(47), 18854-18859.