Chimpanzee Hunting Behavior and Human Evolution
Chimpanzees are efficient predators that use meat as a political and reproductive tool. Are there implications for the evolution of human behavior?
As far back as the 1960s, the American primatologist Geza Teleki proposed that the predatory behavior of the Gombe chimpanzees had a strong social basis. The Dutch primatologist Adrian Kortlandt suggested that hunting was
a form of social display, in which male chimpanzees revealed their prowess to other members of the community. Although Richard Wrangham, of Harvard University, suggested that meat consumption was nutritionally based, he also noticed that certain aspects of their hunting behavior could not be accounted for by nutritional needs alone.
Even if hunting does have its social consequences, the nutritional value of meat cannot be denied. After all, even an infant monkey is a high-quality package of protein and fat that is difficult to find in any plant food. Among the more compelling arguments for the nutritional importance of meat in the chimpanzee's diet is Wrangham's observation that Gombe chimpanzees lose weight during the peak dry-season months. Wrangham suggested that these months correspond to a period of food shortage in the Gombe forest. Perhaps not coincidentally, nearly 40 percent of the colobus kills at Gombe take place during the dry-season months of August and September. The chimpanzees of another community in the Mahale Mountains of Tanzania also have a peak hunting period, when about 60 percent of the kills take place. In this instance, however, the hunting peak occurs in a two-month period during the early wet season. Whether this period corresponds to a food shortage or whether something else drives the seasonal pattern of hunting is not known.
One observation that must be explained is chimpanzees' tendency to hunt in groups. Since these animals live in a fission-fusion society, in which there is very little group cohesion (beyond mothers and their young),
the size and membership of a hunting party may vary from a single chimpanzee to 35 individuals. However, the most intense periods of hunting tend to occur when the foraging parties are large, and the size of a hunting party is related to the success of a hunt. A lone hunter captures a colobus about 30 percent of the time, whereas a hunting party of 10 or more individuals is successful in nearly every hunt. My fieldwork has shown, however, that
there is no relation between the number of hunters in a party and the amount of meat available for each individual. Chimpanzees do not join hunting parties expecting to increase their own intake of meat.
We might look toward the social aspects of chimpanzee societies to understand their hunting patterns. One clue to the significance of meat in a chimpanzee society comes from the observation that males do most of the hunting. During
the past decade, adult and adolescent males made over 90 percent of the kills at Gombe. Although females occasionally hunt, they more often receive a share of meat from the male who captured the prey.
This state of affairs sets up an interesting dynamic between males and females. Sometimes a begging female does not receive any meat until after the male copulates with her (even while clutching the freshly killed carcass). Some other observations are also telling. Not only does the size of a hunting party increase in proportion to the number of estrous females present, but
the presence of an estrous female independently increases the likelihood that there will be a hunt. Such observations suggest that male chimpanzees use meat as a tool to gain access to sexually receptive females. But females appear to be getting reproductive benefits as well: William McGrew of Miami University in Ohio showed that female chimpanzees at Gombe that receive generous shares of meat produce more offspring that survive.
The distribution of the kill to other male chimpanzees also hints at another social role for meat. The Japanese primatologist Toshisada Nishida and his colleagues in the Mahale Mountains showed that the alpha male Ntilogi
distributes meat to his allies but consistently withholds it from his rivals. Such behavior, they suggest, reveals that meat can be used as a political tool in chimpanzee society. Further studies should tell us whether such actions have consequences for alliances between males.
Although there appear to be a number of social and nutritional advantages to hunting, the motivation to begin any particular hunt is not always evident. How does a chimpanzee decide that the potential benefits of a successful hunt outweigh the potential risk of injury from a prey's bite? How do the costs and benefits of foraging for plant foods compare to those of forgoing a hunt? Part of the solution may lie in the nutritional components of the
plants in the chimpanzees' diverse diet. My colleagues and I are currently attempting to assess these components and explore other factors that may be involved in the chimpanzees' decision-making process.