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.)
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Here's video of a chimp using such a spear to retrieve his busy baby prize.