Reciprocal altruism among rats

from arstechnica.com:

That species is Rattus norvegicus, commonly known as the Norway brown rat, well known to behavioral researchers around the world. In this study, the rats were tested for reciprocal altruism; altruistic behavior that is influenced by the rats’ prior experience. The rats were tasked with pulling a lever so that another rat would receive a food treat. Rats that had previously been helped were more likely pull the lever, and did so faster than those who had not been helped. The more recently that they had been on the receiving end of kindness, the more likely they were to help out others.