Tolerance allows bonobos to outperform chimpanzees on a cooperative task.

To understand constraints on the evolution of cooperation, we compared the ability of bonobos and chimpanzees to cooperatively solve a food-retrieval problem. We addressed two hypotheses. The "emotional-reactivity hypothesis" predicts that bonobos will cooperate more successfully because tolerance levels are higher in bonobos. This prediction is inspired by studies of domesticated animals; such studies suggest that selection on emotional reactivity can influence the ability to solve social problems [1, 2]. In contrast, the "hunting hypothesis" predicts that chimpanzees will cooperate more successfully because only chimpanzees have been reported to cooperatively hunt in the wild [3-5]. We indexed emotional reactivity by measuring social tolerance while the animals were cofeeding and found that bonobos were more tolerant of cofeeding than chimpanzees. In addition, during cofeeding tests only bonobos exhibited socio-sexual behavior, and they played more. When presented with a task of retrieving food that was difficult to monopolize, bonobos and chimpanzees were equally cooperative. However, when the food reward was highly monopolizable, bonobos were more successful than chimpanzees at cooperating to retrieve it. These results support the emotional-reactivity hypothesis. Selection on temperament may in part explain the variance in cooperative ability across species, including hominoids.