Primates show signs of empathy, but can other mammals sense and respond to emotional distress in another individual? Yes, say Peggy Mason and her co-workers at the University of Chicago in Illinois, who report that rats will liberate a trapped individual even when they do not receive a reward for doing so.

In the experiments, one rat was trapped inside a container within a larger arena in which another rat roamed free. By day six or seven, on average, the roaming rat learned to free the trapped one. When a container holding chocolate was added to the arena, the liberators took roughly the same amount of time to free a trapped rat as to access the treat.

Distressed rats typically freeze in response to another distressed rat. The fact that the creatures can control such urges to help another shows that empathy can motivate behaviour in animals other than primates, the authors suggest.

Science 334, 1427–1430 (2011)