I once used a video about capuchins getting unequal payment in one of my classes. The monkeys would either get a sweet grape or a bland piece of cucumber if they give an experimenter a rock. If one capuchin would receive a cucumber while the other would receive a grape. The monkey that would receive the cucumber sees the other monkey receiving a grape and would get mad at the experimenter.
Although it's sad for the monkey, this video of the experiment is quite funny to watch. (not trying to hijack your post 
RE: Ravens Remember Getting an Unfair Deal and Who Did it to Them