National Geographic has this article about an experiment showing that monkeys know when they are being ripped-off and get upset about it.
In the experiment the monkeys were paired up and given simple tasks to do for a reward. When monkeys saw their peers getting better rewards for the same task, they “went on strike” and refused to participate. In some cases a monkey would get plain old cucumber while its partner would receive a delicious grape for the same task. In those cases the monkey who got ripped off “often refused to conduct future exchanges, would not eat the cucumbers they received, and in some cases, threw their rewards at the researchers.”
Very interesting – we humans love it when animals exhibit human-like behavior. It’s also very interesting to me because it shows that some monkeys have evolved a view of what is “fair”. This is evolutionarily useful for a species whose survival depends a lot on complex social interactions.