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Chimps found using spears

Feb. 22, 2007
Courtesy National Geographic Society
and World Science staff

Chimps in Sen­e­gal are reg­u­lar­ly mak­ing and us­ing spears to hunt oth­er, small pri­ma­tes, with­out hu­man help, ac­cord­ing to re­search led by an an­thro­po­l­o­gist. 

It’s the first study to re­port reg­u­lar tool use by non-hu­mans while hunt­ing oth­er ver­te­brates, ac­cord­ing to the U.S. Na­tion­al Ge­o­graph­ic So­ci­e­ty, which helped fund the work.

A bush­ba­by (Otole­mur gar­netti) was a re­ported vic­tim of a spear-wielding chimp. (Im­age cour­te­sy U.S. Nat'l Hu­man Ge­nome Inst.)

Anthropologist Jill Pruetz of Io­wa State Uni­ver­si­ty in Ames, Io­wa, and Pa­co Ber­to­la­ni, a grad­u­ate stu­dent at the Uni­ver­si­ty of Cam­bridge, U.K., doc­u­mented 22 cases of chimps mak­ing spears to use in hunt­ing down smaller pri­ma­tes in cav­i­ties of hol­low branches or tree trunks.

Chimps made the spears of live bran­ches that they trimmed, then sharp­ened with their teeth, Pru­etz and Ber­to­la­ni said. They found the act­i­vi­ties at Fon­goli, Sen­e­gal, in 2005 and 2006. 

A pa­per on the find­ings is to ap­pear in the March 6 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy. The pa­per was on­line in the jour­nal start­ing to­day.

“We came up­on the dis­cov­ery quite un­ex­pect­ed­ly,” said Pruetz. 

“There were hints that this be­hav­ior might oc­cur, but it was one time at a dif­fer­ent site. Then I talked to [Bertolani] and he told me that he saw a fe­male hunt with tools. When he looked through orig­i­nal da­ta... we real­ized he had oth­er ev­i­dence and ob­ser­va­tions of them prob­a­bly do­ing the same thing. While in Sen­e­gal for the spring se­mes­ter, I saw about 13 dif­fer­ent hunt­ing bouts. So it real­ly is ha­bit­u­al.”

Chimps repeatedly jabbed tools in­to hol­low trunks or branches and smelled and/or licked them up­on ex­trac­tion, the re­search­ers said. Two of the 22 cases were judged as mere­ly play­ful—in the case of an in­fant male—or ex­plor­a­to­ry. In all oth­er cases, the sci­ent­ists said the chimps poked with such force that prey could have been in­jured. They de­s­cribed just one case in which a chimp ex­tracted a bush­ba­by, a smal­ler pri­mate, us­ing a spear. 

Senegal, marked in green.

Al­though hunt­ing is pre­dom­i­nantly an adult male ac­tiv­i­ty with chimps, on­ly one adult male of 11 ma­les in the chimp com­mu­ni­ty was seen in the tool-assisted hunt­ing, the in­ves­ti­ga­tors said. The rest were ad­o­les­cent or youn­ger chimps of both sexes.

“In the chimp lit­er­a­ture, there is a lot of dis­cus­sion about hunt­ing by adult ma­les, be­cause ba­si­cal­ly, they’re the on­ly ones that do it, and they don’t use tools,” said Pruetz.

“Fema­les are rare­ly in­volved. And so this was just kind of as­tound­ing on a num­ber of dif­fer­ent lev­els. It’s not on­ly chimps hunt­ing with tools, but fe­ma­les—and the ones who hunt­ed the most with them were ad­o­les­cent fe­ma­les.

“It’s clas­sic in pri­ma­tes that when there is a new in­no­va­tion, particularly in terms of tool use, the young­er gen­er­a­tions pick it up very quick­ly. The last ones to pick up are adults, main­ly the ma­les,” she said. “This is be­cause im­ma­tures learn from the ones they are most af­fil­i­at­ed with, their moth­ers.”

The find­ings sup­port a the­o­ry that fe­ma­les might have played a role in the ev­o­lu­tion of tool tech­nol­o­gy among early hu­mans, Pruetz said. Those tech­nolo­gies would have in­clud­ed both hunt­ing- and gathering-related ac­tiv­i­ties. “The com­bi­na­tion of hunt­ing and tool use at Fon­goli, be­hav­iors long con­sid­ered hall­marks of our own spe­cies, makes the pop­u­la­tion es­pe­cial­ly in­trigu­ing,” wrote the sci­ent­ists in the Cur­rent Bio­lo­gy paper. 

“The ob­ser­va­tion that in­di­vid­u­als hunt­ing with tools in­clude fe­ma­les and im­ma­ture chim­panzees sug­gests that we should re­think tra­di­tion­al ex­pla­na­tions for the ev­o­lu­tion of such be­hav­ior in our own line­age. Learn­ing more about the unique be­hav­iors of chim­panzees in such an en­vi­ron­ment, be­fore they disap­pear, can pro­vide im­por­tant clues about the chal­lenges fac­ing our ear­li­est an­ces­tors.”

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Chimps in Senegal are regularly making and using spears to hunt other, small primates, without human help, according to research led by an Iowa State University anthro pologist. It’s the first study to report regular tool use by non-humans while hunting other vertebrates, according to the U.S. National Geographic Society, which helped fund the research. The university’s Jill Pruetz, along with Paco Bertolani, a graduate student at the University of Cambridge, U.K., documented 22 cases of the chimps making tools to use in hunting down smaller primates in cavities of hollow branches or tree trunks. They made the discovery at their research site in Fongoli, Senegal, in 2005 and 2006. A paper on the study is to appear in the March 6 issue of the research journal Current Biology. The article will be available online in the professional journal on Thursday, Feb. 22. “We came upon the discovery quite unexpectedly,” said Pruetz. “There were hints that this behavior might occur, but it was one time at a different site. Then I talked to [Bertolani] and he told me that he saw a female hunt with tools. When he looked through original data that was collected, we realized he had other evidence and observations of them probably doing the same thing. While in Senegal for the spring semester, I saw about 13 different hunting bouts. So it really is habitual.” Chimpanzees jabbed tools into hollow trunks or branches multiple times and smelled and/or licked them upon extraction. Two of the 22 cases were judged as merely playful—in the case of an infant male—or exploratory. In all other cases, researchers said the chimps poked with such force that prey could have been injured. They witnessed just one case in which a chimp extracted a bushbaby, a smaller primate, using of the spear. Although hunting is predominantly an adult male activity with chimps, only one adult male of 11 males in the community was seen in the tool-assisted hunting, the invest igators said. The reported incidents included one adult female, one adult male, three adolescent females, two adolescent males, one juvenile female, one juvenile male, and one infant male. “In the chimp literature, there is a lot of discussion about hunting by adult males, because basically, they’re the only ones that do it, and they don’t use tools,” said Pruetz. “Females are rarely involved. And so this was just kind of astounding on a number of different levels. It’s not only chimps hunting with tools, but females—and the ones who hunted the most with them were adolescent females. “It’s classic in primates that when there is a new innovation, particularly in terms of tool use, the younger generations pick it up very quickly. The last ones to pick up are adults, mainly the males,” she said. “This is because immatures learn from the ones they are most affiliated with, their mothers.” They authors concluded that the findings support a theory that females might have played a role in the evolution of tool technology among the earliest humans. Those technologies included hunting-related behavior, in addition to gathering-related activities. “The combination of hunting and tool use at Fongoli, behaviors long considered hallmarks of our own species, makes the population especially intriguing,” they wrote. “The observation that indi viduals hunting with tools include females and immature chimpanzees suggests that we should rethink traditional explanations for the evolution of such behavior in our own lineage. Learning more about the unique behaviors of chimpanzees in such an environment, before they disappear, can provide important clues about the challenges facing our earliest ancestors.”