William C. McGrew

Cultures in chimpanzees

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.

Intergroup aggression in chimpanzees and humans.

The occurrence of fatal attacks during intergroup encounters among chimpanzees suggests that certain aspects of chimpanzee and human intergroup aggression may be explicable in similar ways. This paper addresses three questions: What conditions favor the evolution of lethal raiding in intergroup aggression? Why is intergroup aggression in these two species predominantly the domain of males? Under what circumstances do groups compete over access to females as opposed to material resources? Examination of comparative data on nonhuman primates and crosscultural study of foraging societies suggests that attacks are lethal because where there is sufficient imbalance of power their cost is trivial, that these attacks are a male and not a female activity because males are the philopatric sex, and that it is resources of reproductive interest to males that determine the causes of intergroup aggression.

On the other hand: Current issues in and meta-analysis of the behavioral laterality of hand function in nonhuman primates

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 nonhu- man 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. Yrbk Phys Anthropol 40:201- 232, 1997. r 1997 Wiley-Liss, Inc.

CHARTING CULTURAL VARIATION IN CHIMPANZEES

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.

Evidence for a Social Custom in Wild Chimpanzees

Can the concept of culture be applied validly to any of the natural behaviours exhibited by non-human primates? We compare aspects of social grooming shown by two separate populations of wild chimpanzees (Pan troglodytes) in western Tanzania. We present as an example a behaviour pattern, the grooming-hand-clasp, which occurs commonly in the chimpanzees of Kasoge but is absent in the chimpanzees of Gombe. After discussing the problems of operationally defining culture, we present eight necessary criteria of culture which are capable of empirical verification: innovation, dissemination, standardisation, durability, diffusion, tradition, non-subsistence, and natural adaptiveness. These are applied to the behaviour of wild chimpanzees andJapanese monkeys (Macacafuscata). We conclude that no single behaviour pattern yet reported satisfies all eight criteria but that the grooming-handclasp should qualify as a social custom.

AN APE'S VIEW OF THE OLDOWAN

When in human evolution did our ancestors cease behaving like apes? In this article we address this question by interpreting the earliest known archaeological evidence, the Oldowan, in light of what primatologists know about modem apes, especially the chimpanzee (Pan troglodytes). Our analyses consider aspects of Oldowan tools and tool-making and those aspects of Oldowan subsistence that can be reconstructed from artefacts. We conclude that all the behaviour that can be inferred from Oldowan tools and sites falls within the range of the ape adaptive grade. There is nothing exclusively human-like about this oldest known archaeological evidence. However, the Oldowan did include two specific behavioural patterns that, while still within the ape adaptive grade, are almost unknown for modern apes and which point in the direction of adaptations found later in hominid evolution. These are carrying tools or food for thousands of metres and competing with large carmvores for animal prey.

Chimpanzees in a hot, dry and open habitat: Mt. Assirik, Senegal, West Africa

The habitat of the chimpanzees of Mt. Assirik, in the Parc National du Niokolo-Koba, Senegal, is described in terms of rainfall, temperature and vegetation. The results are compared with those collected at five other sites of study elsewhere in Africa. Mt. Assirik is the driest site at which chimpanzees have been studied, in terms of annual rainfall, proportion of dry months, and number of rainy days. Mt. Assirik is also the hottest such site: the coolest mean maximum temperature at Mt. Assirik exceeds the hottest such temperature at any other site. Mt. Assirik is the only site where chimpanzees have been studied in which the majority of vegetation is grassland. Forest consitutes less than 3% of the surface area. In summary, Mt. Assirik presents a truly open savanna habitat and is thus unique amongst sites where chimpanzees have been studied. These results are compared with data from a tropical foraging human society, the !Kung San of southern Africa. The !Kung Sans habitat is drier on most (but not all) criteria, but Mt. Assirik is hotter. The climate and vegetation of Mt. Assirik strikingly resemble those reconstructed for the Plio-Pleistocene in eastern Africa. This suggests that the chimpanzees of Mt. Assirik provide a useful model for inferring the processes of adaptation in early hominids.

Laterality of Hand Use Pays Off in Foraging Success for Wild Chimpanzees

The aim of this study was to see if behavioral lateralization in hand use benefits a lateralized organism in nature. We recorded wild chimpanzees (Pan troglodytes schweinfurthii) at Gombe, Tanzania, fishing for termites (Macrotermes spp.), an extractive foraging task using elementary technology. We compared individual apes who were completely lateralized, using only one hand or the other for the task, versus those who were incompletely lateralized, using either hand. Exclusively lateralized individuals were more efficient, that is, gathered more prey per unit effort, but were no different in success or error rate from incompletely lateralized apes. This is the first demonstration of a payoff to laterality of behavioral function in primates in conditions of ecological validity.

Laterality of function in apes: a meta-analysis of methods

Abstract Asymmetry of cerebral structure in human primates is correlated with laterality of function, which by homology, predicts that the same will hold for other hominoids (Hylobatidae, Pongidae). To what extent is the cerebral asymmetry of living apes reflected in functional laterality? To test this hypothesis, we analysed all 58 published sources of relevance for Pan (bonobo, chimpanzee) Gorilla (gorilla), Pongo (orang-utan), and Hylobates (gibbons). From coding the methods in terms of eight variables (function, context, sample, age, task, number, trials, complexity), we found that most studies focused on: hand use rather than other organs; captive rather than wild subjects; small rather than large samples of subjects; immature rather than adult subjects; spontaneous rather than induced measures; few rather than many tasks; few rather than many trials; simple rather than complex tasks. Other independent or dependent variables have been treated inconsistently or rarely, e.g., sex, performance asymmetries. Thus, the hypothesis cannot yet be tested, and such issues as population-level handedness versus individual hand preferences remain unresolved.

Wide-Ranging Chimpanzees at Mt. Assirik, Senegal

A 4-year study of the ecology and ethology of wild chimpanzees (Pan troglodytes verus)was carried out in far western Africa. Contacts with chimpanzees and the locations of their nests were noted to determine which types of habitat were most used and to estimate the density of the population and the size of its home range. The results show that this community has one of the lowest densities and largest home ranges of all populations of chimpanzees studied so far. As such, it may provide a useful model for the reconstruction of hominid evolution in the Plio-Pleistocene.