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 altruistic in response to a general level of cooperation in the population.
Theoretically, such “generalised reciprocity” shouldn’t exist. In large groups, dirty rats will take advantage of helpful strangers and offer nothing in return. It persists, says Taborsky, because exploited animals move away. “An animal is more likely to leave the group if it didn’t receive cooperation in the past,” he says. “This leads to cooperative and uncooperative groups in a population.” If cooperative groups are better at exploiting the environment, generalised reciprocity remains in the population. (PLoS Biology, DOI: 10.1371/journal.pbio.0050196).