For the first time, great apes have been observed making and using tools to hunt mammals, according to a new study. The discovery offers insight into the evolution of hunting behavior in early humans.
No fewer than 22 times, researchers documented wild chimpanzees on an African savanna fashioning sticks into "spears" to hunt small primates called lesser bush babies.
In each case a chimpanzee modified a branch by breaking off one or two ends and, frequently, using its teeth to sharpen the stick. The ape then jabbed the spear into hollows in tree trunks where bush babies sleep.
When hunting in the hollows, "almost without fail, every time they would withdraw the tool, they would sniff it or lick it, and then proceed to stab it in there again," said Jill Pruetz, an anthropologist with Iowa State University who led the research in Senegal.
"And they did it so forcibly that our assumption is the bush babies would have been injured if there were always bush babies in the hollow," she continued.
Anthropologist and study co-author Paco Bertolani witnessed the single case in which a chimpanzee successfully extracted a bush baby with a spear. The ape subsequently tore apart and ate the smaller primate.
Bertolani "couldn't tell for sure if the bush baby was dead or not" when it was first taken from the hollow, Pruetz said of the graduate student from England's University of Cambridge.
"But it didn't make any vocalizations, didn't attempt to escape—that sort of thing. So we are hypothesizing they are using the tools to incapacitate the bush babies."
Primatologist Craig Stanford, who was not involved in the research, called the 22 observed instances of spearmaking "good evidence."
But the observation of only "one actual kill—and no visual evidence of the spear being used as a spear—weakens it," the University of Southern California (USC) professor said in an email.
The new report was published online today in the journal Current Biology. The National Geographic Society's Committee for Research and Exploration partially funded the project. (National Geographic News is part of the National Geographic Society.)
In the 1990s Stanford observed male chimpanzees hunting colobus monkeys with their bare hands.
The new discovery of chimps hunting with tools is "stunning," Stanford said in a telephone interview.
"Except for one anecdote many years ago, there's never really been any evidence or suggestion that chimps would use weapons when they were hunting," he said.
The earlier anecdote—reportedly based on a single observation—described a female chimpanzee's use of a tool to rouse a squirrel from a tree hollow in Tanzania.
Chimpanzees are well-known toolmakers. In the 1960s primatologist Jane Goodall famously observed chimps using sticks to fish termites out of mounds.
And Stanford's research has shown that chimpanzees are highly efficient hunters of colobus monkeys.
"But we've never discovered chimp populations that made the cognitive leap to put those two [skills] together and use weapons to assist in their hunting," Stanford said.
"And clearly this is what these guys are doing."
Mothers and Children
What makes the discovery all the more remarkable, project leader Pruetz said, is who the hunters are: predominantly mature females and immatures—youngsters between about two and ten years old.
"We don't think of chimpanzee hunting in terms of the females and immatures," she said.
The new finding shows that females and immatures do hunt. It also suggests that females played a role in the evolution of tool use and hunting among early human ancestral species, she added.
Chimpanzees are modern humans' closest living relatives. And Pruetz's research site is a savannah similar to the open environment that early human ancestors are believed to have moved into millions of years ago.
"Looking at our closest living relatives in a habitat that is fairly similar to what we see characterizing early hominids six million years ago" can help researchers understand early human ancestors' behavior and ecology, she said.
USC's Stanford likens chimpanzees to a window to a past poorly preserved in the archaeological record.
Hunting "is something that the chimps do that almost certainly early, early hominids did too. They were just using a material—wood—that does not leave any archaeological trace," he said.
Putting Too Fine a Point on It?
In their paper, Pruetz and Bertolani describe a deliberate toolmaking process.
The tools, on average, are about 24 inches (60 centimeters) long and 0.4 inch (11 millimeters) around.
The researchers refer to the tools as spears. Pruetz said they differ from throwing spears, in the sense that they are jabbed into tree trunks and branches, not tossed.
USC's Stanford said the word "spear" is an overstatement that makes the chimpanzees sound too much like early humans.
He prefers "bludgeon."
"They seem to be using it to hit the animal hard, and having a point on the end certainly helps," he said.
"But I think it's not clear whether the point that they made is in fact even sharp enough to penetrate the animal."