As an increasing number of field studies of chimpanzees (Pan troglodytes) have achieved long-term status across Africa, differences in the behavioural repertoires described have become apparent that suggest there is significant cultural variation. Here we present a systematic synthesis of this information from the seven most long-term studies, which together have accumulated 151 years of chimpanzee observation. This comprehensive analysis reveals patterns of variation that are far more extensive than have previously been documented for any animal species except humans. We find that 39 different behaviour patterns, including tool usage, grooming and courtship behaviours, are customary or habitual in some communities but are absent in others where ecological explanations have been discounted. Among mammalian and avian species, cultural variation has previously been identified only for single behaviour patterns, such as the local dialects of song-birds. The extensive, multiple variations now documented for chimpanzees are thus without parallel. Moreover, the combined repertoire of these behaviour patterns in each chimpanzee community is itself highly distinctive, a phenomenon characteristic of human cultures but previously unrecognised in non-human species.

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The last decade has seen a resurgence of interest in laterality of function in primates, especially in hand use as it links to handedness and language in Homo sapiens. Manual lateralization of behavior in humans reflects asymmetry in cerebral structure, which must have evolved from nonhuman progenitors. To what extent is hand function lateralized in our nearest living relations? First, we address current issues of theory and methodology: statistics, measurement, variables, setting, sensory modality, and sample size. Specific topics include preference vs. performance, posture, bimanuality, inheritance, and arm asymmetry. We categorize the published literature in a descriptive, classificatory framework of five levels that range from Level 1, ambilaterality, to Level 5, human-like handedness. In a meta-analysis we put 241 published data-sets to a methodological test of seven criteria and code the 48 survivors onto the levels framework, by taxonomic grouping (prosimian, New World monkey, Old World monkey, ape, chimpanzee). Primates at Level 1 are mostly wild or naturalistic populations performing spontaneous species-typical behavior patterns. Most primates are at Levels 2 and 3, that is, individually lateralized to either side, especially on complex, demanding or practiced tasks, usually as devised in captive settings. Only chimpanzees show signs of population-level bias (Levels 4 and 5) to the right, but only in captivity and only incompletely. We conclude that nonhuman primate hand function has not been shown to be lateralized at the species level-it is not the norm for any species, task, or setting, and so offers no easy model for the evolution of human handedness.

This paper aims to compile an exhaustive list of the behavioral patterns exhibited by the chimpanzees of the Mahale Mountains National Park, Tanzania. The compilation is based on the glossary compiled by Goodall (1989), but a substantial numbers of new terms have been added. Thus, we list 316 simple anatomical terms, 81 complex anatomical terms, 37 simple functional terms, and 81 complex functional terms, in addition to 116 synonyms. The behavioral patterns are divided into eight categories on the basis of degree of universality: (1) commonly seen in both Homo and two species of Pan, (1?) commonly seen in Homo and only one species of Pan, (2) patterns common to the genus Pan but not to Homo, (3) patterns common to the chimpanzee Pan troglodytes but not the bonobo Pan paniscus, (4) patterns common to eastern (P.t. schweinfurthii) and central (P.t. troglodytes) but not western (P.t. verus) chimpanzees, (5) patterns unique to the eastern chimpanzees, P.t. schweinfurthii, (6) patterns unique to the population of Mahale, (7) patterns unique to many individuals (at least most members of an age/sex class) of M group chimpanzees, (8) patterns limited to a single (idiosyncrasy) or a few individuals of M group. It is most likely that the behavior patterns of the last common ancestor of Homo and Pan are found in Categories 1 and 1? and less likely in Categories 2 and 3. It is possible that behavior patterns belonging to Categories 5, 6 or 7 are cultures.

Cultural variation among chimpanzee communities or unit-groups at nine long-term study sites was charted through a systematic, collaborative procedure in which the directors of the sites first agreed a candidate list of 65 behaviour patterns (here fully defined), then classified each pattern in relation to its local frequency of occurrence. Thirty-nine of the candidate behaviour patterns were discriminated as cultural variants, sufficiently frequent at one or more sites to be consistent with social transmission, yet absent at one or more others where environmental explanations were rejected. Each community exhibited a unique and substantial profile of such variants, far exceeding cultural variation reported before for any other non-human species. Evaluation of these pan-African distributions against three models for the diffusion of traditions identified multiple cases consistent with cultural evolution involving differentiation in form, function and targets of behaviour patterns.