Animals know when they are being treated unfairly (and they don`t like it)
(PHYSorg.com Biology)

Humans beings appear to be hardwired to have a sense of fairness. This is puzzling from an evolutionary perspective, which you would have thought would mean we were predisposed to seek advantage for ourselves and our families wherever possible. But in fact a sense of fairness is important for humans to be able to help each other. Human cooperation is based on reciprocal altruism - we help people because they`ve either helped us in the past or they may help us in the future.