This paper makes the case that reciprocity may be a strong force in chimpanzee society: "the majority of highly affiliative and cooperative dyads are actually unrelated or distantly related."
I think they make a pretty convincing case except that I wish they would have resolved kinship more than either r=. 5 or r=0. It would have been nice to see .25 and .125. I wonder if that would have at least nuanced the results a bit more? They were using a decent number of markers, but have no idea if they could have gotten .25 and .125.
Update: I think that the main significance of this paper is to show that chimps, like humans, have an appreciable degree of machiavelian intelligence.
The limited impact of kinship on cooperation in wild chimpanzees
Kevin E. Langergraber, John C. Mitani, and Linda Vigilant
PNAS Online before print
Abstract: The complex cooperative behavior exhibited by wild chimpanzees generates considerable theoretical and empirical interest, yet we know very little about the mechanisms responsible for its evolution. Here, we investigate the influence of kinship on the cooperative behavior of male chimpanzees living in an unusually large community at Ngogo in Kibale National Park, Uganda. Using long-term field observations and molecular genetic techniques to identify kin relations between individuals, we show that male chimpanzees clearly prefer to affiliate and cooperate with their maternal brothers in several behavioral contexts. Despite these results, additional analyses reveal that the impact of kinship is limited; paternal brothers do not selectively affiliate and cooperate, probably because they cannot be reliably recognized, and the majority of highly affiliative and cooperative dyads are actually unrelated or distantly related. These findings add to a growing body of research that indicates that animals cooperate with each other to obtain both direct and indirect fitness benefits and that complex cooperation can occur between kin and nonkin alike.