I've heard that monkeys have a sense of fairness, and can demand to be paid the prevailing wage.
Specifically, suppose you, an animal behaviorist, have two monkeys in side-by-side cages, clearly visible to one another, performing the same daily task. One day you feed a slice of banana to one of the monkeys, and a slice of cucumber (apparently less delectable to simian lips) to the other. The cucumber-fed monkey can become quite agitated about this and can even throw the cucumber back at the experimenter in disgust.
My own unscientific hypothesis is that the banana-fed monkey draws the conclusion that this lab is a strict meritocracy. That fellow across the way should quit whining and work harder.