Here’s an updated version of a classic experiment originated by Sarah Brosnan showing that capuchin monkeys resent what they perceive as unfair unequal pay. The monkeys in this test have been taught that they can buy a treat with a pebble. A bit of cucumber is enough to serve as a treat, but a grape is much preferred. When a monkey paid in cucumber pieces sees another monkey paid in grapes, the monkey paid in cucumber bits takes very obvious offense. Watch:
If you click through to YouTube you can find interesting related videos.
Incidentally, I’ve personally observed similar behavior in dogs.
(Updated 2015 Dec 17 to fix broken YouTube link.)