Monkeys invest less energy in a task if they see other monkeys receiving better rewards for the same effort, researchers report. They say that their experiment provides new evidence that non-human primates can feel envy. The findings could also help explain why humans have such a keen sense of fairness, according to experts.

Previous studies have found that monkeys put less effort into a task when they see cage-mates receiving tastier treats for completing the same task. But scientists have not felt confident in saying why the poorly rewarded animals slack off.

Some people have suggested the primates that refuse to repeat the task are simply greedy and therefore only willing to work for a bigger reward. Alternately, it has been proposed that the monkeys stop performing the task because they have received large rewards in the past and feel frustrated by the measly amounts offered in later trials.

To understand the monkeys’ reluctance to participate in the task, Frans de Waal at the Yerkes National Primate Research Center in Atlanta, Georgia, US, and colleagues decided to try several variations on this experiment.


Fruits of labour

They trained 13 capuchin monkeys (Cebus apella) to retrieve a small rock and place it in the experimenter’s hands. In exchange for completing this task, the animals received a reward.

Pairs of monkeys were seated beside one another in a test booth, separated by a mesh partition. In one trial, the monkeys received the same sized cucumber reward for their efforts and 90% completed the task within 5 seconds.

But then the researchers gave one of the monkeys a grape instead of a cucumber. To a human this may seem like a minor detail, but monkeys go bananas over grapes, which they far prefer to cucumbers.

When the monkeys given cucumber saw their partners receive this grape reward, they invested less effort in future repetitions of the task, and completed it within 5 seconds only 80% of the time.

In a third scenario, the monkeys both received the same cucumber reward, but could see a bowl of grapes just beyond their reach. Under these circumstances, the animals performed the task with the same willingness as when the grapes were hidden. The researchers say that this rules out the possibility that the primates alter their behaviour out of greed.

Species variation

De Waal’s team also found that the monkeys exhibited the same patterns of behaviour regardless of whether they had received a grape or cucumber in preceding experiments, discounting the possibility that the animals slacked off out of frustration from unmet expectations.

They say the study “confirms that capuchin monkeys react negatively to situations in which they receive a less favourable reward than their partner for the same task. Our control procedures suggest that this response was due solely to the discrepancy between the monkey’s own and the other’s rewards and not to individual factors such as greed or frustration.”

Other experts, however, note that earlier studies have shown that monkeys and chimps do not always care about fairness. Keith Jensen at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, recently demonstrated that chimpanzees will accept a rotten deal from a fellow chimp.

Jensen says that understanding how non-human primates view fairness can provide clues about how humans evolved a great capacity for cooperation. But he adds: “Human cooperation is special”.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0707182104)