[...] we show experimentally that cooperative behavior of female rats is influenced by prior receipt of help, irrespective of the identity of the partner. Rats that were trained in an instrumental cooperative task (pulling a stick in order to produce food for a partner) pulled more often for an unknown partner after they were helped than if they had not received help before.
The paper, published in PLoS Biology, is free to access. With the exception of direct reciprocity (the 'you've been nice to me, I'll be nice to you' type), no other mechanism for reciprocity had ever been demonstrated in nonhuman animals before.
'Altruistic behavior by previous social experience irrespective of partner identity' is fundamental to human society, and perhaps the single behaviour economic theory most struggles with.