Can certain animals understand the concept of fairness? Do they get jealous? See this hilarious and eye opening experiment that answers these questions.
Two Capuchin monkeys were put side by side, and each one was given the task of handing over a rock in exchange for a treat. When both monkeys were given a slice of cucumber, neither monkey was upset. However, when inequity was created between them, trouble happened. If one monkey got a grape and the other one got a cucumber, the monkey who got the cucumber became very upset. This fairness study clearly demonstrates that monkeys can understand unequal treatment and will reject it.