Surprising that rats reward kindness.

Rats can remember acts of kindness by other rats—and treat them accordingly, a new study says.

In experiments, Norwegian rats were most helpful to individuals that had previously helped them—perhaps to try and secure their assistance again, scientists suggest.

While rats are known to cooperate and assist one another, rewarding another rat for no immediate gain wasn't thought to be common behavior. (Also see"Rats Show Regret After Wrong Choices, Scientists Say.")

The study,  published February 24 in the journal Biology Letters, suggests otherwise.

In fact, a rat rewarding a fellow rat for help—an act called direct reciprocation—is a first among nonhumans, said study co-authorMichael Taborsky, a behavioral ecologist at the University of Bern in Switzerland.