1809. Chimps That Hunt Offer a New View on Evolution

By James Gorman, The New York Times, April 14, 2015

An adult male chimpanzee in Senegal, left, uses a tree branch with a modified end to stab into a tree cavity in a video image from the TV show "Life Story." A 10-year study found that females in Senegal play an unexpectedly big role in hunting. 

Studies of hunters and gatherers — and of chimpanzees, which are often used as stand-ins for human ancestors — have cast bigger, faster and more powerful males in the hunter role.

Now, a 10-year study of chimpanzees in Senegal shows females playing an unexpectedly big role in hunting and males, surprisingly, letting smaller and weaker hunters keep their prey.

The results do not overturn the idea of dominant male hunters, said Jill D. Pruetz of Iowa State University, who led the study. But they may offer a new frame of reference on hunting, tools and human evolution. “We need to broaden our perspective,” she said.

Among the 30 or so chimps Dr. Pruetz and her colleagues observed, called the Fongoli band, males caught 70 percent of the prey, mostly by chasing and running it down. But these chimps are very unusual in one respect. They are the only apes that regularly hunt other animals with tools — broken tree branches. And females do the majority of that hunting for small primates called bush babies.

Craig Stanford, an anthropologist at the University of Southern California who has written extensively on chimp hunting and human evolution, said the research was “really important” because it solidified the evidence for chimps hunting with tools, which Dr. Pruetz had reported in earlier papers.

It also clearly shows “the females are more involved than in other places,” he said, adding that it provides new evidence to already documented observations that female chimps are “much more avid tool users than males are.”

All chimpanzees eat a variety of plant and animal foods, including insects like termites. And all chimpanzees eat some other animals. The most familiar examples of chimpanzee hunting are bands of the apes chasing red colobus monkeys through the trees in the rain forests of East Africa.

In this kind of pursuit, the largest, strongest, fastest chimps dominate — and those are adult males. When females and smaller chimps do catch an animal, an adult male may simply take it away, although the meat is eventually shared. The theft rate in other groups of chimps is around 25 percent, Dr. Pruetz said. Those other chimps do not hunt with tools.

The Fongoli chimpanzees live in a mix of savanna and woodlands where prey is not as abundant as in rain forests. There are no red colobus monkeys, and although the chimps do hunt young vervet monkeys and baboons, the much smaller bush babies are their main prey.

Dr. Pruetz argues that less food may have prompted both technological and social innovation, resulting in new ways to hunt and new social interactions as well. Humans evolved in a similar environment, and, as she and her colleagues write in Royal Society Open Science, “tool-assisted hunting could have similarly been important for early hominins.”

The tools in question are broken branches that Dr. Pruetz calls jabbing tools. The season for bush baby hunting is June, when the temperature may be well over 100 and the humidity is suffocating. The Fongoli chimps find the bush babies in their dens in trees. Chimps will stab and poke one of the small animals, sometimes wounding but not impaling it, until it comes out of its hiding place. The chimps will grab it, Dr. Pruetz said, and immediately “bite the head off.”

Females, even those with infants, and juvenile chimps can do this kind of hunting. The process does not put a premium on speed and strength as the chase does, so big males do not have an advantage. But there is more than technique and technology involved. There is social change.

By and large, said Dr. Pruetz, the adult males, which could take away a kill, show a “respect of ownership.” Theft rates are only about 5 percent. The chimps she studies also have more mixed-sex social groups than chimp bands in East Africa.

Travis Pickering, an anthropologist at the University of Wisconsin, said that with less food available it seems that the Fongoli chimps, “have to be more inventive” and that “these hunting weapons even the playing field for non-adults and females.”

Early hominins may have been in a similar situation, he said. Hunting among human ancestors “very quickly became a male-dominated activity,” he said, but “female hominins could very well have been the inventors of weapons.”

When it comes to getting food, deciding who does what depends on definitions. Collecting insects, for example, is defined as gathering, not hunting. In the case of the bush babies, however, though they are small, they struggle and flee, and will bite. Any bite, no matter how small, can pose the danger of infection, so the pursuit of bush babies qualifies as hunting, Dr. Pruetz says, and Dr. Stanford and Dr. Pickering agrees.