Monkeys Can Tell What's Fair
Both capuchin monkeys and chimpanzees seem to have a concept of fairness, and will become emotionally distressed in unfair situations. If two monkeys receive unequal pay for the same task—one gets a tasty grape while another gets a cucumber slice, for example—the "cheated" party tends to get upset. This behavior was famously documented by primatologist Frans de Waal.
Key Facts In This Video
Initially, capuchin monkeys are happy to be rewarded with a cucumber for performing the same task. (0:20)
A capuchin monkey will become visibly upset if it sees that its comrade is getting a better reward for a task that both have performed. (0:42)
Chimpanzees have been known to reject a reward if they see that another chimp has received a worse one. (0:58)