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FEATURE ARTICLE

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?

Craig Stanford

This article originally appeared in the May-June 1995 issue of American Scientist.

In the early 1960s, when the british primatologist Jane Goodall first observed wild chimpanzees hunting and eating meat in Gombe National Park, Tanzania, it was widely believed that these animals were strict vegetarians. Skeptics suggested that the diet of the Gombe chimpanzees was aberrant. Others suggested that the quantity of meat the chimpanzees ate was trivial. After more than 30 years of research, however, it is now clear that meat is a natural part of the chimpanzees' diet. Indeed, hunting has been observed at most of the other sites where chimpanzees are studied across central Africa. And, it turns out, a chimpanzee community may eat several hundred kilograms of meat in a single year.

To many anthropologists this is a surprising development. Of all the higher primates, only human beings and chimpanzees hunt and eat meat on a regular basis. The similarities pose an intriguing prospect: Might the close evolutionary relationship between chimpanzees and human beings provide some clues to the evolution of our own behavior? We do know that the earliest bipedal hominids, the australopithecines, evolved in Africa about 5 million years ago and that they shared a common ancestor with modern chimpanzees shortly before that time. Unfortunately, the evidence for the occurrence of meat-eating among the early australopithecines is spotty at best. Primitive stone tools that were made 2.5 million years ago suggest that early hominids had the means to carve the flesh from large carcasses, but we know very little about their diets before that time. Were they hunters or perhaps, as many anthropologists now argue, scavengers? The behavior of chimpanzees may provide a window through which we can see much that has been lost in the fossil record.

There are also some interesting subtleties to the chimpanzees' hunting behavior that need to be addressed. Although chimpanzees can and do hunt alone, they often form large hunting parties consisting of more than 10 adult males, plus females and juveniles. Chimpanzees also go on "hunting binges" in which they kill a large number of monkeys and other animals over a period of several days or weeks. Such binges have always been a little mysterious. What could incite a chimpanzee to suddenly forgo plant foraging and turn to hunting? Are there social or ecological factors associated with the impetus to hunt? What ecological effects does the chimpanzees' predatory behavior have on their prey?

In the past five years I have been mindful of such questions as I observed the chimpanzees and their primary prey at Gombe, the red colobus monkey. Although we are only beginning to understand some of the causes and consequences of the chimpanzees' actions, what we have discovered is more complicated and more interesting than anyone suspected. For chimpanzees, meat is not only another way to get nutrients like fat and protein, but a means to make political bonds and gain access to sexually receptive females.

Why Hunt?

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.

Predator and Prey

The chimpanzees' desire for meat has also led me to investigate the ecological consequences of their hunting. Although only three percent of their feeding time is dedicated to eating meat, a chimpanzee community may kill more than 150 animals in a single year. Wrangham and Emily van Zinnicq Bergmann-Riss have noted that chimpanzees prey on more than 25 species of vertebrates at Gombe, including monkeys, wild pigs and small antelopes. Despite the chimpanzees' diverse appetite, their most frequent victim is the red colobus monkey. Over the past decade the colobus account for more than 80 percent of the prey eaten by the chimpanzees at Gombe.

A look at the intensity of predation and the distribution of colobus in the chimpanzees' home range sheds light on the predators' effect, and may have implications for the meat-foraging patterns of early hominids. There are about 500 colobus monkeys within the 18-square-kilometer hunting range of the Gombe chimpanzees. On the basis of observed kills, plus the estimated number of kills per day on which no human observer was following the animals, it appears that about 75 to 175 colobus are killed by chimpanzees every year. This translates into a mortality rate of between 15 and 35 percent, depending on the frequency of hunting in a given year. Although a mortality rate of 15 percent from predation has been recorded for other species of mammals, these estimates only include predation by chimpanzees. Mortality from other predators (such as leopards and eagles) is not known. A census of the colobus population in the central valley of the chimpanzees' hunting range (where more than a third of the hunts take place) shows that the average group contained 19 colobus. In the remote regions of the chimpanzees' domain (where only about three percent of the hunts take place), the average colobus group consisted of 34 animals. It appears that the chimpanzees' hunting reduces the size of the colobus groups by almost 50 percent in the community's core area.

Gombe chimpanzees appear to have a predilection for immature colobus. Nearly 75 percent of the colobus they capture are infants or juveniles. The immature animals are caught in greater proportions than their numbers in the colobus population would dictate. A census shows that the chimpanzees' choice of young animals skews the composition of the colobus population. In the core hunting area, immature animals make up about 17 percent of the colobus population, whereas they comprise nearly 40 percent of the population in the remote hunting areas.

Selectively targeting a segment of the colobus population does have consequences for the prey species. Consider the following scenario. Since colobus do not breed seasonally, if a female colobus loses an infant to predation she begins to cycle again and may produce another within seven months. The death of an adult, however, results in the loss of an individual who cannot be replaced for at least two years. In the latter instance, a reproductively capable colobus is absent from the population for a greater period of time. In other words, the effect of the chimpanzees' predation on the colobus would be more pronounced if breeding adults were the primary targets of the hunters.

The mortality rate of colobus monkeys is also tied to the composition of the chimpanzee population. Since male chimpanzees tend to be the hunters, they are a much greater threat to a colobus population than female chimpanzees. Indeed the mortality rate of the colobus monkeys is directly tied to the number of adult and adolescent males in a chimpanzee community. It is also turns out that some male chimpanzees are better hunters than others. During one four-year period, the chimpanzee Frodo single-handedly eliminated about 10 percent of the colobus in the Gombe home range. The presence of a single prolific hunter can thus have significant consequences for a colobus population.

Early Hominid Behavior

The early hominids were probably at least as socially complex as modern chimpanzees. The hunting ecology of the chimpanzee suggests the following: Most meat-eating took place within the home range of the social group and most frequently within a core area smaller than the total range. Most of the prey were small animals, weighing less than 25 kilograms. Most of the hunters would have been males, and the rate of success was linked to the number of hunters in a party. The meat was probably shared by members of the hunting party as well as by any females who might have been present. Meat may have been used by males for selfish political reasons and for gaining sexual access to females. If so, we would expect a degree of sexual selection for the best hunters.

Some recent fossil discoveries in Ethiopia support such an idea. A multinational expedition led by the American anthropologist Tim White, the Japanese anthropologist Gen Suwa and the Ethiopian anthropologist Berhane Asfaw recently unearthed the fossilized remains of the oldest known hominid species, Australopithecus ramidus. These primitive hominids lived about 4.4 million years ago, perhaps one million years after the evolutionary branching of the ancestral lines leading to modern chimpanzees and human beings. The fossil deposits indicate that these early australopithecines inhabited a forested environment, which they shared with colobus monkeys, small antelopes and other ground-dwelling vertebrates. As yet there is no direct evidence that the early hominids were preying on these animals; all we can say is that they had the opportunity to do so.

The fossils do suggest that some behavioral differences between chimpanzees and hominids had already emerged. For one thing, Australopithecus ramidus was a biped; its lower body was clearly adapted for walking on the ground. The first hominids may have continued to use trees for gathering fruit and for shelter at night, but their ground-dwelling habits would certainly have made it a little more difficult for them to catch the arboreal ancestors of modern colobus monkeys. Of course, this does not deny the possibility of hunting monkeys, since a hunting party could have flushed out an animal by driving it from one hunter to another once it was cornered in an isolated tree. Whether the hominids concentrated on colobus monkeys to the same extent that the Gombe chimpanzees do is another matter. Some other prey species, perhaps antelope, were also available.

The australopithecines also differed from modern chimpanzees in having relatively small canine teeth. But again, this does not mean that they did not eat meat. Although large canines are often taken to be an indicator of a meaty diet, they are more likely to be used as weapons by males in the competition for mates. Chimpanzees do not use their canines to capture adult colobus monkeys; rather they tend to grab the animals and flail them to death.

Finally, a number of anthropologists have suggested that the carcasses of large mammals were an important source of meat for early hominids once they had developed the use of stone tools. Did the early hominids eat meat before the development of stone tools 2.5 million years ago? Given the behavior of the chimpanzee, it seems likely that they did, but the relative significance of meat in their diet remains open to conjecture. Although a scavenging life-style is frequently suggested for the early hominids, modern chimpanzees in the wild have little interest in dead animals as food. When scavenging does take place, the female chimpanzees at Gombe do show more interest than do the males; the females are also more adept at using tools. The same may have been true of the earliest hominids.

The role of hunting in the lives of the earliest hominids was probably as complex and politically charged as it is in modern chimpanzees. The early hominids may even have been important predators in their ancient forest ecosystems. When we ask the question, "when did meat become an important part of the human diet?" we should look well before the evolutionary split between apes and human beings in our own family tree.

Bibliography

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