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 @krnel):
Yup, I've heard of that one too, and it seems so familiar because I think I had that included the cucumber-grape thing, if not then I read it before possibly in relation to the overjustification effect. Thanks for the valuable feedback ;)