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 Harvard's 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 prey's 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 the addition of these two impact hunters that increased hunting likelihood. It is not entirely clear what special qualities these chimpanzees might posses that would contribute to their role in hunting groups. Of the two impact hunters one was the alpha male, but one was not particularly high-ranking or outstanding in terms of physical appearance. What appears to be significant is that these particular chimpanzees display a unique willingness to hunt, and have a high success rate when hunting. The findings of this study may also be applicable to the hunting behavior of other animals that hunt in this way, such as dolphins and lions.
This study also addresses the motivation for hunting in groups. It has previously been suggested that hunting with others increases the amount of meat that each individual obtains. However, chimpanzees obtain most of their calories from ripe fruit, and hunting does not increase in response to a scarcity of fruit. Instead, the researchers theorize that meat provides important micronutrients that are difficult to obtain solely from fruit, and because of this, even a small scrap of meat is beneficial.
Gilby and his colleagues have shown that as the number of hunters increases, so does the likelihood that an individual will receive a piece of meat. Therefore, there is an incentive for a given male to join a hunt. However, in large groups, hunters and non-hunters were equally likely to obtain meat, suggesting that an individual should refrain from hunting thus avoiding the inherent risks and dangers of chasing prey. This is exactly what the researchers found - the likelihood that a given male hunted increased rapidly with party size, but only to a point. In large groups, there was no effect of party size on an individual's hunting probability.
While predation among chimpanzees is cooperative, the researchers found that hunting does not appear to be a collaborative process. Males were not more likely to hunt for reasons of social bonding, nor was there an association between the presence of a sexually receptive female and an increased likelihood of hunting.
The research was funded by the National Science Foundation.