Key 'impact hunters' catalyze hunting among male c...( CAMBRIDGE Mass. Feb. 1 2008 While h...)

CAMBRIDGE, Mass., Feb. 1, 2008 While hunting among chimpanzees is a group effort, key males, known as impact hunters are highly influential within the group. They are more likely to initiate a hunt, and hunts rarely occur in their absence, according to a new study. The findings, which appear in the current issue of Animal Behaviour, shed light on how and why some animals cooperate to hunt for food, and how individual variation among chimpanzees contributes to collective predation.

The study was led by Ian Gilby, a postdoctoral researcher in the Department of Anthropology in Harvards Faculty of Arts and Sciences, with Lynn Eberly of the University of Minnesota and Richard Wrangham, Ruth Moore Professor of Biological Anthropology at Harvard.

Chimpanzees live in communities of 40 to 150, within which fluid subgroups of changing size and composition form. While their diet is largely ripe fruit, chimpanzees also prey upon red colobus monkeys, which are agile and live in the trees. For this study, the researchers followed the hunting patterns of 11 adult males over more than a decade, among which two chimpanzees were identified as impact hunters. The chimpanzees that were studied live in Kanyawara, in Kibale National Park, Uganda.

Our findings show that while hunting among chimpanzees is a group process, these individual males have a strong effect on whether or not others decide to hunt, says Gilby. They have a strong catalytic effect on the decision to hunt. When a party of chimpanzees encounters a troop of red colobus, the impact hunters tend to be the first to hunt. By doing so, they dilute the preys defenses, thereby reducing the costs of hunting for other chimpanzees. Hunts rarely occurred in the absence of the impact males.

If all chimpanzees were equally skilled at hunting, the addition of any given chimpanzee to a party would increase the likelihood of hunting. However, this did not appear to be the case; it was th
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