Chimpanzees do not take advantage of very low cost opportunities to deliver food to unrelated group members.

We conducted experiments on two populations of chimpanzees, , to determine whether they would take advantage of opportunities to provide food rewards to familiar group members at little cost to themselves. In both of the experiments described here, chimpanzees were able to deliver identical rewards to themselves and to other members of their social groups. We compared the chimpanzees' behaviour when they were paired with another chimpanzee and when they were alone.

Endowment effects in chimpanzees.

Human behavior is not always consistent with standard rational choice predictions. Apparent deviations from rational choice predictions provide a promising arena for the merger of economics and biology [1-6]. Although little is known about the extent to which other species exhibit these seemingly irrational patterns [7-9], similarities across species would suggest a common evolutionary root to the phenomena.

Chimpanzees are indifferent to the welfare of unrelated group members.

Humans are an unusually prosocial species-we vote, give blood, recycle, give tithes and punish violators of social norms. Experimental evidence indicates that people willingly incur costs to help strangers in anonymous one-shot interactions, and that altruistic behaviour is motivated, at least in part, by empathy and concern for the welfare of others (hereafter referred to as other-regarding preferences). In contrast, cooperative behaviour in non-human primates is mainly limited to kin and reciprocating partners, and is virtually never extended to unfamiliar individuals.