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?
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.