I was recently thinking about how I read a study about how animals won't harm other animals in exchange for resources; https://en.wikipedia.org/wiki/Inequity_aversion_in_animals
I came across this section.
The monkeys tended to refuse to participate in a food-for-token exchange task once they saw another monkey get rewarded more desirable food for equal effort.
It turns out even animals know that unequal pay is unfair.