Portal de Periódicos da CAPES

Chimpanzees: The Culture-Zone Concept Becomes Untidy

2006; Elsevier BV; Volume: 16; Issue: 16 Linguagem: Inglês

10.1016/j.cub.2006.07.031

1879-0445

Richard W. Wrangham,

Animal Behavior and Reproduction

The discovery that chimpanzees in Cameroon use stone tools reinvigorates a long-standing puzzle: what explains the geographical distribution of chimpanzee traditions? The discovery that chimpanzees in Cameroon use stone tools reinvigorates a long-standing puzzle: what explains the geographical distribution of chimpanzee traditions? In 1844 a missionary to Liberia called Thomas Savage claimed that wild chimpanzees crack nuts "with stones precisely in the manner of human beings" [1Savage T.S. Wyman J. Observations on the external characters and habits of the Troglodytes Niger, Geoff.; and on its organization.Boston J. Nat. Hist. 1844; 4: 362-386Google Scholar]. More than a century later he was proved right. In certain locations chimpanzees smash nuts both skilfully and often. In the two best-studied populations, in Guinea and the Ivory Coast, chimpanzees spend 12–15% of their annual feeding time cracking nuts with stone or wood hammers, and during the three or four high-production months individuals thereby obtain more than 3500 kilocalories of fat-rich seeds per day [2Yamakoshi G. Ecology of tool use in wild chimpanzees: toward reconstruction of early hominid evolution.in: Matsuzawa T. Primate Origins of Human Cognition and Behavior. Springer-Verlag, Tokyo2001: 537-556Google Scholar]. Nut-smashing makes at least six species of inaccessible foods available and contributes a major part of the diet, particularly when fruits are scarce. The resulting food supply has even been suggested to foster an accelerated reproductive rate and a relatively gregarious grouping pattern. Chimpanzees might therefore be expected to smash nuts wherever the right combination of foods and raw materials are present. Yet they do not. Throughout Africa there are many populations where rocks, logs and nuts appear sufficiently abundant to allow the nut-smashing adaptation to flourish. In most such places chimpanzees ignore the rich nut offerings [3McGrew W.C. Why don't chimpanzees in Gabon crack nuts?.Int. J. Primatol. 1997; 18: 353-374Crossref Scopus (89) Google Scholar]. The reason why some chimpanzees fail to exploit these valuable food supplies has appeared uncontroversial until now. The limiting variable seemed to be the rate of invention. This hypothesis derived from evidence that nut-smashing was pervasive throughout far western Africa, but was found nowhere east of the N'Zo-Sassandra River, a supposed geographical boundary for chimpanzees that runs from savanna in the north of the Ivory Coast south to the Atlantic coast. To the east of this river, surveys found no evidence of nut-smashing. To the west it was close to universal, though populations were found to differ in details such as tool choice and food species. The implication was that nut-smashing is a behavioural tradition constrained in its spread by an uncrossable barrier. It was absent elsewhere because it had not been invented a second time [4Boesch C. Marchesi P. Marchesi N. Fruth B. Joulian F. Is nut cracking in wild chimpanzees a cultural behaviour?.J. Hum. Evol. 1994; 26: 325-338Crossref Scopus (116) Google Scholar]. This cultural explanation was supported by surveys showing that there are no relevant ecological or genetic differences between populations that do and do not smash nuts. It also fit evidence that, like other chimpanzee tool-using such as termite-fishing, the behavior is spread by social learning [5Lonsdorf E.V. What is the role of mothers in the acquisition of termite-fishing behaviors in wild chimpanzees (Pan troglodytes schweinfurthii)?.Anim. Cogn. 2006; 9: 36-46Crossref PubMed Scopus (140) Google Scholar]. For example, nut-smashing proficiency improves with age, and is particularly dependent on a sensitive period for learning the skills between three and five years of age, when juveniles pay attention to adult performance [6Biro D. Inoue-Nakamura N. Tonooka R. Yamakoshi G. Sousa C. Matsuzawa T. Cultural innovation and transmission of tool use in wild chimpanzees: evidence from field experiments.Anim. Cogn. 2003; 6: 213-223Crossref PubMed Scopus (308) Google Scholar]. So chimpanzees outside the 'culture-zone' were apparently just unlucky: there were simply no models from which to learn the valuable skill. The premise of a discrete culture-zone is now challenged by Morgan and Abwe's [7Morgan B.J. Abwe E.E. Chimpanzees use stone hammers in Cameroon.Curr. Biol. 2006; 16: R632-R633Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar] report, in this issue of Current Biology, of chimpanzees smashing nuts in Cameroon's Ebo Forest. The Ebo Forest lies 1700 kilometers to the east of the previously known closest nut-smashers. Possibly the disjunct distribution means that the original culture-zone was larger: nut-smashing might have gone extinct between the N'Zo-Sassandra and Ebo. Alternatively, it may imply that nut-smashing has been invented and become established more than once. Either way, the question prompted by the Ebo culture is why is it not more widespread? Nut-smashing is one of the best-studied chimpanzee behaviors because it is easily observed, has high value and is obviously relevant to understanding human evolution, given that stone-tool use — first evidenced in the fossil record about 2.6 million years ago — accompanied the emergence of the genus Homo[8Mercader J. Panger M. Boesch C. Excavation of a chimpanzee stone tool site in the African rainforest.Science. 2002; 296: 1452-1455Crossref PubMed Scopus (165) Google Scholar]. But even with less intense scrutiny, the same kind of geographical puzzle applies to many other chimpanzee behaviors. If invention is the chief constraint on distribution, a series of discrete culture-zones should be evident within which neighboring populations show similar traditions. But of the dozens of chimpanzee behaviors that appear to be social traditions, from ant-dipping and palm-pounding to leaf-clipping and hand-clasp-grooming, many have distributions at least as quirky as nut-smashing [9Whiten A. Goodall J. McGrew W.C. Nishida T. Reynolds V. Sugiyama Y. Tutin C.E.G. Wrangham R.W. Boesch C. Charting cultural variation in chimpanzees.Behaviour. 2001; 138: 1481-1516Crossref Scopus (295) Google Scholar]. A tradition shared by two or more widely separated populations is often mysteriously absent in one or more intervening populations. Disjunct distributions are the norm, not the exception. The Ebo report [7Morgan B.J. Abwe E.E. Chimpanzees use stone hammers in Cameroon.Curr. Biol. 2006; 16: R632-R633Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar] thus adds to evidence that the distribution of chimpanzee traditions is unexpectedly dynamic. In some ways, this is not surprising. In captivity, the hand-clasp-grooming tradition has been seen to be invented and passed on [10de Waal F.B.M. Seres M. Propagation of handclasp grooming among captive chimpanzees.Am. J. Primatol. 1997; 43: 339-346Crossref PubMed Scopus (42) Google Scholar]. Nut-smashing also appears to be easily invented and transmitted [11Hayashi M. Mizuno Y. Matsuzawa T. How does stone-tool use emerge? Introduction of stones and nuts to naive chimpanzees in captivity.Primates. 2005; 46: 91-102Crossref PubMed Scopus (49) Google Scholar]. In sanctuaries, even gorillas and bonobos have learned by themselves to smash nuts, though these species are not known to do so in the wild. The cognitive demands for inventing such traditions thus appear to be easily met, not only by chimpanzees but also by other great apes. If inventions occur easily, a high rate of invention could in principle contribute to making the distribution of traditions disjunct. In practice, however, there is a problem with this explanation. Chimpanzees are an old species: they closely resemble bonobos, a morphologically derived sister species that split off at least 1 million years ago [12Wrangham R.W. Pilbeam D. African apes as time machines.in: Galdikas B.M.F. Briggs N. Sheeran L.K. Shapiro G.L. Goodall J. All apes great and small: chimpanzees, bonobos, and gorillas. Vol. 1. Kluwer Academic/Plenum, New York2001: 5-18Google Scholar]. If chimpanzees have been inventing and passing on traditions even for as short a period as 1 million years, the distribution of traditions would be limited by the rate of invention only if the rate of invention were vanishingly low — much less than 1 in every 10,000 years for example. The fact that chimpanzees have invented traditions while being observed by humans suggests that every population should have had ample opportunity to acquire it. So the rate of repeat invention appears too high to account for the distribution of a series of idiosyncratic sets of chimpanzee traditions. If invention alone cannot explain why the unpredictable location of traditions, we are forced to think about a little-studied topic: extinction. The obvious explanation for why Kibale chimpanzees do not dip for ants, Gombe chimpanzees do not hand-clasp-groom, or Bossou chimpanzees do not use leaf-napkins is that, although their ancestors did, the tradition died out. Why extinctions should happen regularly is unclear. Long-term studies will be needed to test how population bottlenecks, alternative fashions, individual personalities or other factors might promote rates of tradition extinction. Understanding the extinction of chimpanzee traditions holds promise for explaining why ape culture has never blossomed as it did, critically, for humans. Unfortunately the opportunities for studying apes are disappearing rapidly due to extinction not just of traditions, but of whole populations. But on the positive side, Ebo nut-smashing is only one of many recent tool-using discoveries that in the 21st century include chimpanzee tool-kits in the Congo and the first gorilla tools in the wild, as well as capuchin monkey stone-tool-use in Brazil [13Breuer T. Ndoundou-Hockemba M. Fishlock V. First observation of tool use in wild gorillas.PLoS Biol. 2005; 3: 2041-2043Crossref Scopus (135) Google Scholar, 14Sanz C. Morgan D. Gulick S. New insights into chimpanzees, tools, and termites from the Congo Basin.Am. Nat. 2004; 164: 567-581Crossref PubMed Scopus (165) Google Scholar, 15Visalberghi E. Fragaszy D.M. Izar P. Ottoni E. Terrestriality and tool use.Science. 2005; 308: 951Crossref PubMed Google Scholar]. There is still an opportunity to learn much about the distribution of cultural variants, let alone why they are vulnerable to extinction. Happily, as Morgan and Abwe [7Morgan B.J. Abwe E.E. Chimpanzees use stone hammers in Cameroon.Curr. Biol. 2006; 16: R632-R633Abstract Full Text Full Text PDF PubMed Scopus (58) Google Scholar] hint, the process of studying populations like Ebo often leads to the establishment of a long-term research program, one of the most effective ways to promote conservation. Their discovery thus promises to benefit both science and conservation. If the new tradition proves idiosyncratic Ebo will become a site of particular interest but whatever is found there, the big picture is clear: the cultural primatology of central Africa is still in its infancy.