In a simple experiment, researchers at the University of Chicago sought to find out whether a rat would release a fellow rat from an unpleasantly restrictive cage if it could. The answer was yes.
The free rat, occasionally hearing distress calls from its compatriot, learned to open the cage and did so with greater efficiency over time. It would release the other animal even if there wasn’t the payoff of a reunion with it. Astonishingly, if given access to a small hoard of chocolate chips, the free rat would usually save at least one treat for the captive — which is a lot to expect of a rat.
The researchers came to the unavoidable conclusion that what they were seeing was empathy — and apparently selfless behaviour driven by that mental state.