If rats benefit from the kindness of strangers they are more likely to assist an unfamiliar rat in future. In doing so, they provide the first evidence of an unusual form of altruism that appears to violate evolutionary theory.

Claudia Rutte and Michael Taborsky of the University of Berne, Switzerland, trained rats to pull a lever that released food for their partner in the next cage. If the rats subsequently received snacks released by lever-pulling strangers in neighbouring cages, they were more likely to lever-pull and so feed another unfamiliar rat in the future. In other words, the rats became …