Monkeys and other primates go ballistic if they are not treated same or receive unequal pay, the same goes for humans under the similar circumstances, if one get’s more than the other for the same job, task.
Two monkeys performed a simple task after which they received a reward, but not the same.
One monkey got cucumber and the other grapes. However, for the first monkey it did not take long to realize that the cucumber is not exactly the same as grapes.
The task was, in fact, to give a stone to the scientist, who after that gave them a treat.
Watch the reaction of a monkey who got a cucumber instead of grapes.