A sense of fairness has long been considered purely human—but animals also react with frustration when they are treated unequally by a person. For instance, a well-known video shows monkeys throwing the offered cucumber at their trainer when a conspecific receives sweet grapes as a reward for the same task.
Click here for original story, Long-tailed macaques react with disappointment when their expectations are not met: Evolutionary insights into fairness
Source: Phys.org