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Referential gestural communication in wild chimpanzees (Pan troglodytes)

2006; Elsevier BV; Volume: 16; Issue: 6 Linguagem: Inglês

10.1016/j.cub.2006.02.037

1879-0445

Simone Pika, John C. Mitani,

Primate Behavior and Ecology

Humans commonly use referential gestures, for example pointing, which direct the attention of recipients to particular aspects of the environment [1McNeill D. Hand and Mind. The University of Chicago Press, Chicago1992Google Scholar]. The use of these gestures has been linked with cognitive capacities such as mental state attribution [2Camaioni L. The development of intentional communication: A re-analysis.in: Nadel J. Camaioni L. New perspectives in early communicative development. Routledge, London1993: 82-96Google Scholar, 3Tomasello M. Joint attention as social cognition.in: Moore C. Dunham P.J. Joint attention: Its origin and role in development. Erlbaum, New York1995: 103-130Google Scholar] because the recipient must infer the signaler's meaning. In our closest living relatives, the non-human primates, referential gestures have been reported only in captive chimpanzees interacting with their human experimenters [4Leavens D.A. Hopkins W.D. Thomas R.K. Referential communication by chimpanzees (Pan troglodytes).J. Comp. Psych. 2004; 118: 48-57Crossref PubMed Scopus (122) Google Scholar] and human-raised or language-trained apes ([5Gardner R.A. Gardner B. Teaching sign language to a chimpanzee.Science. 1969; 165: 664-672Crossref PubMed Scopus (633) Google Scholar, 6Miles H.L. The cognitive foundations for reference in a signing orang-utan.in: Parker S.T. Gibson K.R. Language and intelligence in monkeys and apes. Cambridge University Press, Cambridge1990: 511-539Crossref Google Scholar, 7Patterson F. Conversations with a gorilla.Nat. Geo. 1978; 134: 438-465Google Scholar]; but see also [8Plooij F.X. Some basic traits of language in wild chimpanzees?.in: Lock A. Action, gesture and symbol. Academic Press, London1978: 111-131Google Scholar]). Here we provide the first evidence for the widespread use of a referential gesture by wild chimpanzees (Pan troglodytes). Observations of the Ngogo community in Kibale National Park, Uganda, indicate that wild chimpanzees use 'directed scratches' to request grooming of specific body areas. The gesture involved one chimpanzee making a relatively loud and exaggerated scratching movement on a part of his body, which could be seen by his grooming partner (a movie clip of the 'directed scratch' can be found in the Supplemental Data available on-line with this issue). It occurred between pairs of adult males and was recorded 186 times in 101 (41%) of 249 grooming bouts. One hundred nineteen times (64%), the groomer stopped grooming and groomed the scratched spot. Eight times (4%) individuals simultaneously scratched and presented a body part and were groomed there immediately. In 59 cases (32%), the groomer continued to groom without touching the area scratched by the signaler. The gesture received significantly more positive than negative responses (p < 0.001; exact binominal test) and occurred in 61% (N=51) of all observed grooming dyads (N=84). It was performed on average 3.65 times per dyad and was used significantly more often in dyads consisting of high ranking males than other possible pairings (p < 0.001; df=6, linear-linear association; Figure 1). Three hypotheses may account for these observations. First, the 'directed scratch' may not represent a communicative signal, but instead reflect behavioural conformity due to stimulus enhancement [9Whiten A. Ham R. On the nature and evolution of imitation in the animal kingdom: Reappraisal of a century of research.Adv. Stud. Behav. 1992; 21: 239-283Crossref Scopus (522) Google Scholar]. For example, the scratching movement may simply facilitate grooming with the recipient, who simply learns the contingency rule "if he scratches my grooming is tolerated." If this hypothesis is true, then we would expect to find a positive response mainly from low-ranking males towards high-ranking males, but this is not the case (Figure 2). Second, 'directed scratching' might simply represent a physical response by an individual to parasites or dirt, thereby drawing the attention of the groomer to a potential area to groom. This hypothesis suggests that 'directed scratches' would be displayed uniformly across all grooming dyads, a prediction that does not accord with our observations (Figure 2). Third, the gesture may be used communicatively to indicate a precise spot on the body and to represent a desired future action, namely grooming. Consistent with this hypothesis is the finding that in the majority of cases, individuals immediately groomed the indicated spot. Our observations suggest that the recipient of the signal has an understanding of the intended meaning of the gesture and that wild chimpanzees use gestures to specify an area of the body to be groomed and to depict a desired future action. They therefore qualify as referential and iconic [10Bates E. Language and context: the acquisition of pragmatics. Academic Press, New York1976Google Scholar] and reflect greater signal specificity than related gestures such as 'raise arm' and 'present back' [11Goodall J. The chimpanzees of Gombe, Patterns of Behaviour. The Belknap Press of Harvard University Press, Cambridge1986Google Scholar], which request grooming of larger body areas that are difficult to access. In sum, the frequent use of and responsiveness to 'directed scratches' by male chimpanzees at Ngogo imply that some form of mental state attribution may be present in our closest living relatives. The editors of Current Biology welcome correspondence on any article in the journal, but reserve the right to reduce the length of any letter to be published. All Correspondence containing data or scientific argument will be refereed. Queries about articles for consideration in this format should be sent by e-mail to [email protected] We thank the Uganda Wildlife Authority, the Uganda National Council for Science and Technology and Makerere University for permission to work at the Makerere University Biological Field Station. We are grateful to A. Tumusiime, L. Ndagizi, G. Mbabazi, and A. Magoba for invaluable assistance in the field and S. Amsler, J. Lwanga and M. Wakefield for helpful discussion. For comments on earlier drafts and discussion we thank A.C. Arcadi, M. Tomasello, V.M. Janik and R.W. Byrne. Research was supported by NSF grants BCS-0215622 and IOB-0516644. Download .mov (2.53 MB) Help with mov files Movie S1