National Geographic, DC
02-22-07
Chimps Use "Spears" to Hunt Mammals, Study Says
John Roach
for National Geographic News
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 (bush baby photo).
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.
Picture of chimpanzee eating bush baby, likely hunted with a spear
Enlarge Photo
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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.
(Photo gallery: Jane Goodall encounters chimps that are unafraid of humans.)
And Stanford's research has shown that chimpanzees are highly efficient hunters
of colobus monkeys (watch video of chimps hunting 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."
(Related news: "Chimp 'Stone Age' Finds Are Earliest Nonhuman Ape Tools,
Study Says" [February 13, 2007].)
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."
Jill Pruetz's work with chimpanzees will be featured in an upcoming
NOVA/National Geographic special on PBS (airdate to be determined).