Article 11

Dim Forest, Bright ChimpsIn the rain forest of Ivory Coast, chimpanzees meet

the challenge of life by hunting cooperativelyand using crude tools

Christophe Boesch andHedwige Boesch-Achermann

Tal National Park, Ivory Coast, De-cember 3, 1985. Drumming, barking,and screaming, chimps rush through theundergrowth, little more than blackshadows. Their goal is to join a groupof other chimps noisily clusteringaround Brutus, the dominant male ofthis seventy-member chimpanzee com-munity. For a few moments, Brutus,proud and self-confident, stands fairlystill, holding a shocked, barely movingred colobus monkey in his hand. Thenhe begins to move through the group,followed closely by his favorite femalesand most of the adult males. He seemsto savor this moment of uncontested su-

periority, the culmination of a hunt highup in the canopy. But the victory is nothis alone. Cooperation is essential tocaptUring one of these monkeys, andBrutus will break apart and share thishighly prized delicacy with most of themain participants of the hunt and withthe females. Recipients of large portionswill, in turn, share more or less gener-ously with their offspring, relatives, andfriends.

In 1979, we began a long-term studyof the previously unknown chimpanzeesof Tal National Park, 1,600 square milesof tropical rain forest in the Republic ofthe Ivory Coast (Cote d'Ivoire). Early

on, we were most interested in thechimps' use of natural hammers-branches and stones-to crack open thefive species of hard-shelled nuts that areabundant here. A sea otter lying on itsback, cracking an abalone shell with arock, is a familiar picture, but no pri-mate had ever before been observed inthe. wild using stones as hammers. EastAfrica's savanna chimps, studied for de-cades by Jane Goodall in Gombe, Tanza-nia, use twigs to extract ants and termitesfrom their nests or honey from a bees'nest, but they have never been seen usinghammerstones.

As our work progressed, we weresurprised by the many ways in whichthe life of the Tal forest chimpanzeesdiffers from that of their savanna coun-terparts, and as evidence accumulated,differences. in how the two populationshunt proved the most intriguing. JaneGoodall had found that chimpanzeeshunt monkeys, antelope, and:wild pigs,findings confirmed by Japanese biolo-gist Toshida Nishida, who conducted along-term study 120miles south of Gom-be, in the MahaleMountains.So we werenot surprised to discover that the Talchimps eat meat. What intrigued us wasthe degree to which they hunt coopera-tively. In 1953 Raymond Dart proposed

that group hunting and cooperation werekey ingredients in the evolution ofHomo sapiens. The argument has beenmodified considerably since Dart firstput it forward, and group hunting hasalso been observed in some social car-nivores (lions and African wild dogs, forinstance), and even some birds of prey.Nevertheless, many anthropologists stillhold that hunting cooperatively andsharing food played a central role in thedrama that enabled early hominids,some 1.8 million years ago, to developthe social systems that are so typicallyhuman.

We hoped that what we learnedabout the behavior of forest chimpan-zees would shed new light on prevailingtheories of human evolution. Before wecould even begin, however; we had tohabituate a community of chimps to our.presence. Five long years passed beforewe were able to move with th'em ontheir daily trips through the forest, ofwhich "our" group appeared to claimsometwelve square miles. Chimpanzeesare alert and shy animals, and the lim-ited field of view in the rain forest-

. about sixty-five feet at best-madefinding them more difficult. We had torely on sound, mostly their vocalizationsand drumming on trees. Males often

Reprintedwith permission from Natural History, September1991, pp. 50, 52-56. «J 1991 by the American Museumof Natural History. 63

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drum regularly while moving throughthe forest: pant-hooting, they draw neara big buttress tree; then, at full speedthey fly over the buttress, hitting it re-peatedly with their hands and feet.Such drumming may resound more thanhalf a mile in the forest. In the begin-ning, our ignorance about how theymoved and who was drumming led tofailure more often than not, but eventu-ally we learned that the dominant malesdrummed during the day to let othergroup members know the direction oftravel. On some days, however: intermit-tent drumming about dawn was the onlysignal for the whole day. If we were outof earshot at the time, we were oftenreduced to guessing.

During these difficult early days, onefeature of the chimps' routine proved tobe our salvation: nut cracking is a noisybusiness. So noisy, in fact, that in theearly days of French colonial rule, oneofficer apparently even proposed thetheory that some unknown tribe wasforging iron in the impenetrable anddangerous jungle.

Guided by the sounds made by thechimps as they cracked open nuts,which they often did for hours at a time,we were gradually able to get withinsixty feet of the animals. We still seldomsaw the chimps themselves (they fled ifwe came too close), but even so, the evi-dence left after a session of nut crackingtaught us a great deal about what typesof nuts they were eating, what sorts ofhammer and anvil tools they were using,and-thanks to the very distinctive noisea nut makes when it finally splitsopen-how many hits were needed tocrack a nut and how many nuts couldbe opened per minute.

After some months, we began catch-ing glimpses of the chimpanzees beforethey fled, and after a little more time,we were able to draw close enough towatch them at work. The chimps gathernuts from the ground. Some nuts aretougher to crack than others. Nuts of thePanda oleosa tree are the most demand-ing, harder than any of the foods pro-cessed by present-day hunter-gatherersand breaking open only when a force of3,500 pounds is applied. The stone ham-mers U!?edby the Tal chimps range fromstones of ten ounces to granite blocks

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of four to forty-five pounds. Stones ofany size, however, are a rarity in the for-est and are seldom.conveniently placednear a nut-bearing tree. By observingclosely, and in some cases imitating theway the chimps handle hammerstones,we learned that they have an impressiveability to find just the right tool for thejob at hand. Tal chimps could rememberthe positions of many of the stones scat-tered, often out of sight, around a pandatree. Without having to run around re-checking the stones, they would selectone of appropriate size that was closestto the tree. These mental abilities in spa-tial representation compare with someof those of nine-year-old humans.

To extract the four kernels from in-side a panda nut, a chimp must use ahammer with extreme precision. Timeand time again, we have been impressedto see a chimpanzee raise a twenty-pound stone above its head, strike a nutwith ten or more powerful blows, andthen, using the same hammer, switch todelicate little taps from a height of onlyfour inches. To finish the job, thechimps often break off a small piece oftwig and use it to extract the last tinyfragments of kernel from the shell. In-triguingly, females crack panda nutsmore often than males, a gender differ-ence in tool use that seems to be morepronounced in the forest chimps than intheir savanna counterparts.

After five years of fieldwork, wewere finally able to follow the chimpan-zees at close range, and gradually, wegained insights into their way of hunt-ing. One morning, for example, we fol-lowed a group of six male chimps on athree-hour patrol that had taken theminto foreign territory to the north. (Ourstudy group is one of five chimpanzeegroups more or less evenly distributed inthe Tal forest.) As always during theseapproximately monthly incursions,which seem to be for the purpose of ter-ritorial defense, the chimps were totallysilent, clearly on edge and on the lookoutfor trouble. Once the patrol was over,however,and they were back within theirown borders, the chimps Shiftedtheir at-tention to hunting. They were after mon-keys, the most abundant mammals in theforest. Traveling in large, multi-speciesgroups, some of the forest's ten species

of monkeys are more apt than others towind up as a meal for the chimps. Therelatively sluggish and 1arge (almostthirty pounds) red colobus monkeys arethe chimps' usual fare. (Antelope alsolive in the forest, but in our ten years atTal, we have never seen a chimp'catch,or even pursue, one. In contrast, Gombechimps at times do come across fawns,and when they do, they seize the oppor-tunity-and the fawn.)

The six males moved on silently,peering up into the vegetation and stop-ping from time to time to listen for thesound of monkeys. None fed orgroomed; all focused on the hunt. Wefollowed one old male, Falstaff, closely,for he tolerates us completely and is oneof the keenest and most experiencedhunters. Even from the rear, Falstaff setthe pace; whenever he stopped, theothers paused to wait for him. Afterthirty minutes, we heard the unmistak-able noises of monkeys jumping frombranch to branch. Silently,. the chimpsturned in the direction of the sounds,scanning the canopy. Just then, a dianamonkey spotted them and gave an alarmcall. Dianas are very alert and fast; theyare also about half the weight of colobusmonkeys. The chimps quickly gave upand continued their search for easier,meatier prey.

Shortly after, we heard the charac-teristic cough of a red colobus monkey.Suddenly Rousseau and Macho, twotwenty-year-olds, burst into action, run-ning toward the cough. Falstaff seemedsurprised by their precipitousness, butafter a moment's hesitation, he also ran.Now the hunting barks of the chimpsmixed with the sharp alarm calls of themonkeys. Hurrying behind Falstaff, wesaw him climb up a conveniently situ-ated tree. His position, combined withthose of Schubert and Ulysse, two ma-ture chimps in their prime, effectivelyblocked off three of the monkeys' pos-sible escape routes. But in,another tree,nowhere near any escape route and thususeless, waited the last of the hunters,Kendo, eighteen years old and the leastexperienced of the group. The monkeys,taking advantage of Falstaff's delay andKendo's error, escaped.

The six males moved on and withinfive minutes picked up the sounds of an-

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other group of red colobus. This time,the chimps approached cautiously, no-body hurrying. They screened the can-opy intently to locate the monkeys,which were still unaware of the ap-proaching danger. Macho and Schubertchose two adjacent trees, both full ofmonkeys, and started climbing very qui-etly, taking care not to move anybranches. Meanwhile, the other fourchimps blocked off anticipated escaperoutes. When Schubert was halfway up,the monkeys finally detected the twochimps. As we watched the: colobusmonkeys take off in literal panic, the ap-propriateness of the chimpanzees' sci-entific name-Pan came to mind: witha certain stretch of the imagination,the fleeing monkeys could be shep-herds and shepherdesses frightened atthe sudden appearance of Pan, the wildGreek god of the woods, shepherds,and their flocks.

Taking off in the expected direction,the monkeys were trailed by Macho andSchubert. The chimps let go with loudhunting barks. Trying to escape, twocolobus monkeys jumped into smallertrees lower in the canopy. With this,Rousseau and Kendo, who had beenwatching from the ground, sped up intothe trees and tried to grab them. Only athird of the weight of the chimps, how-ever, the monkeys managed to make itto the next tree along branches too smallfor their pursuers. But Falstaff had an-ticipated this move and was waiting forthem. In the following confusion, Fal-staff seized a juvenile and killed it witha bite to the neck. As the chimps met ina rush on the ground, Falstaff began toeat, sharing with Schubert and Rous-seau. A juvenile colobus does not pro-vide much meat, however, and this time,not all the chimps got a share. Frustratedindividuals soon started off on anotherhunt, and relative calm returned fairlyquickly: this sort of hunt, by a smallband of chimps acting on their own atthe edge of their territory, does not gen-erate the kind of high excitement thatprevails when more members of thecommunity are involved.

So far we have observed some 200monkey hunts and have concluded thatsuccess requires a minimum of threemotivated hunters acting cooperatively.

Alone or in pairs, chimps succeed lessthan 15 percent of the time, but when

. three or four act as a group, more thanhalf the hunts result in a kill. Thechimps.seem well aware of the odds; 92percent of all the hunts we observedwere group affairs.

Gombe chimps also hunt red colobusmonkeys, but the percentage of grouphunts is much lower: only 36 percent.In addition, we learned from Jane Goo-dall that even when Gombe chimps' dohunt in groups, their strategies are dif-ferent. When Tal chimps anive under-agroup of monkeys, the hunters scatter,often silently,usually out of sight of oneanother but each aware of the others' po-sitions. As the hunt progresses, theygradually close in, encircling the quarry.Such movements require that eachchimp coordinate his movements withthose of the other hunters, as well .aswith those of the prey, at all times.

Coordinated hunts account for 63percent of all those observed at Tal butonly 7 percent of those at Gombe. JaneGoodall says. that in a Gombe grouphunt, the chimpanzees typically traveltogether until they anive at a tree withmonkeys. Then, as the chimps beginclimbing nearby trees, they scatter aseach pursues a different target. Goodallgained the impression that Gombechimps boost their success by huntingindependently but simultaneously,therebydisorganizingtheir prey; our impressionisthat the Tal chimps owe their success tobeing organizedthemselves.

Just why the Gombe and Tal chimpshave developed such different huntingstrategies is difficult to explain, and weplan to spend some time at Gombe inthe hope of finding out. In the mean-time, the mere existence of differencesis interesting enough and may perhapsforce changes in our understanding ofhuman evolution. Most currently ac-cepted theories propose that some threemillion years ago, a dramatic climatechange in Africa east of the Rift Valleyturned dense forest into open, drier habi-tat. Adapting to the difficulties of lifeunder these new conditions, our ances-tors supposedly evolved into cooperativehunters and began sharing food theycaught. Supporters of this idea point outthat plant and animal remains indicative

11. Dim Forest, Bright Chimps

of dry, open environments have beenfound at all early hominid excavationsites in Tanzania, Kenya, South Africa,and Ethiopia. That the large majority ofapes in Africa today live west of the Rift -Valley appears to many' anthropologiststo lend further support to the idea thata change in environment caused thecommon ancestor of apes and humansto evolve along a different line fromthose remaining in the forest.

Our observations, however, suggestquite another line of thought. Life indense, dim forest may require more so-phisticated behavior than is commonlyassumed: compared with their savannarelatives, Tal chimps show greater com-plexity in both hunting and tool use. Talchimps use tools in nineteen differentways and have six different ways ofmaking them, compared with sixteenuses and three methods of manufactureat Gombe.

Anthropologist colleagues of minehave told me that the discovery thatsome chimpanzees are accomplished us-ers of hammerstones forces them to lookwith a fresh eye at stone tools turned upat excavation sites. The important roleplayed by female Tal chimps in tool usealso raises the possibility that in thecourse of human evolution, -women mayhave been decisive in the developmentof many of the sophisticated manipulat-ive skills characteristic of our species.Tal mothers also appear to pass on theirskills by actively teaching their off-spring. We have observed mothers pro-viding their young with hammers andthen stepping in to help when the inex-perienced youngsters encounter diffi-culty. This help may include carefullyshowing how to position the nut or holdthe hammer properly. Such behavior hasnever been observed at Gombe.

Similarly, food sharing, for a longtime said to be unique to humans, seemsmore general in forest than in savannachimpanzees. Tal chimp mothers sharewith their young up to 60 percent of thenuts they open, at least until the latterbecome sufficiently adept, generally atabout six years old. They also shareother foods acquired with tools, includ-ing honey, ants, and bone marrow.Gombe mothers share such foods muchless often, even with their infants. Tal

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chimps also share meat more frequentlythan do their Gombe relatives, some-times dividing a chunk up and givingportions away, sometimes simply allow-ing. beggars to grab pieces.

Any comparison between chimpan-zees and our hominid ancestors can onlybe suggestive,not definitive.But our stud-ies lead us to believe that the process ofhominization may have begun inde-pendently of the drying of the environ-ment. Savanna life could even havedelayed theprocess; many anthropologistshave been struck by how slowly homi-nid-associated remains, such as the handax, changed after their first appearancein the Olduvai age.

Will we have the time to discovermore about the huntingstrategiesor other,perhaps as yet undiscovered abilities of

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these forest chimpanzees? Africa'stropical rain forests, and their inhabi-tants, are threatened with extinction byextensive logging, largely to provide theWestern world with tropical timber andsuch products as coffee, cocoa, and rnb-ber. Ivory Coast has lost 90 percent ofits original forest, and less than 5 per-cent of the remainder can be consideredpristine. The climate has changed dra-matically. The harmattan, a cold, drywind from the Sahara previously un-known in the forest, has now sweptthrough the Tal forest every year since1986. Rainfall has diminished; all therivulets in our study region are now dryfor several months of the year.

In addition, the chimpanzee, biologi-cally very close to humans, is in demandfor research on AIDS and hepatitis vac-

cines. Captive-bred chimps are avail-able, but they cost about twenty timesmore than wild-caught animals. Chimpstaken from the wild for these purposesare generally young, their mothers hav-ing been shot during capture. For everychimp arriving at its sad destination,nine others may well have died in theforest or on the way.>Such priorities-cheap coffee and cocoa and chimpan-zees~o not do the economies of ThirdWorld countries any good in the longrun, and they bring suffering and deathto innocent victims in the forest. Ourhope is that Brotus, Falstaff, and theirfamilies will survive, and that we andothers will have the opportunity to learnabout them well into the future. Butthere is no denying that modem timeswork against them and us.