Reciprocity

“The evolution of cooperation among nonrelatives has been explained by direct, indirect, and strong reciprocity … indirect and strong reciprocity have been found only in humans so far. Here we show experimentally that cooperative behavior of female rats is influenced by prior receipt of help, irrespective of the identity of the partner.” Rutte C, Taborsky M (2007) Generalized Reciprocity in Rats. PLoS Biol 5(7).