Toshisada Nishida

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.

The social group of wild chimpanzees in the Mahali Mountains

There are more than six large groups of wild chimpanzees in the study area, which is in the north-eastern part of the Mahali Mountains of Western Tanzainia. One of these groups was provisionized, that is, customarily fed sugar cane and bananas. The characteristics of the social group of wild chimpanzees are clarified by long-term observation of the baited population. The chimpanzees live in a clear-cut social unit which consists of adult males, adult females, and immature animals. The permanency, stable membership, and integrative nature of the unit-group were confirmed during the course of this study. The size of unit-groups ranges from 30 to 80 head.The unit-group generally splits up into temporary subgroups that repeat joining and parting. The size of the subgroups of the baited population ranges from one to 28 head, the mean being 8.1 head. The centralization of a unitgroup is mainly sustained by the high sociability of adult males. The random nature of the membership of subgroups is emphasized in this paper, although subgroups are usually composed by social bonds on the basis of similar age, sex, blood relationship, and/or sexual attraction.The inter-unit-group interaction is peaceful; the subordinate unit-group avoids the dominant one. The home ranges of unit-groups overlap each other extensively, the overlapping areas being used flexibly by both unit-groups on the basis of dominance-subordination relationship. The member-exchange among unit-groups may sometimes occur, but the extent of openness or closedness of a unit-group has not been well elucidated.

Alpha status and agonistic alliance in wild chimpanzees (Pan troglodytes schweinfurthii).

Intermale competition was studied in a habituated group of wild chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains of Tanzania. The group of 27 individuals included 3 adult males and 12 adult females. The beta-ranked male was observed to attack the alpha male and usurp his status. The beta male was supported by an ally, the gamma male. Earlier in the competition the gamma male had supported the alpha male. When the alpha male was displaced, he completely lost what had been an exclusive copulatory right. It was clear that alliance with the most subordinate male was a critically effective strategy for the major competitors. The alliance strategy of the gamma male invested him with important power. The strategy of changing alliance is referred to as “allegiance fickleness.” It is suggested that allegiance fickleness could provide a basis for individual selection of social intelligence.

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.

Natural history of a tool-using behavior by wild chimpanzees in feeding upon wood-boring ants

Various aspects of the tool-using behavior of the chimpanzees of the Mahale Mountains, Tanzania are described: natural history of the prey, cognitive mapping, selection of tool materials, types of tools, transport of tools, feeding height, feeding technique, handedness, feeding competition, etc. Feeding on Camponotus ants is the only tool-using behavior displayed habitually throughout a year by this habituated group of chimpanzees, although they show other types of tool-use in diverse contexts. The ontogeny of ant-fishing is strikingly similar to that of termite-fishing when chimpanzees are less than four years old but in later years, it seems to lag slightly behind that of termite-fishing, but to mature earlier than ant-dipping. The delaying factor in antfishing may be the motor skill involved in responding to the anti-predatory behavior of the insects. The chimpanzees of Mahale consume more than 25 species of insects. They are estimated to obtain 20% of the total intake of insects with the aid of tools. An average bout of 33·2 min yields about 300 ants, i.e. 10 to 25 grams. Chimpanzees are estimated to invest daily on average 1–2% of the daylight hours in ant-fishing behavior. The adaptive significance of this tool-using behavior is still unclear. Procurement of protein is unlikely to be the major function of the behavior, because the intake of 0·6–1·5 grams per 33·2 min is too slight. The behavior might be nutritionally-oriented (e.g. Vitamin-B group or other known nutrients) or non-nutrition-oriented or both. The chimpanzees of Mahale prey upon Camponotus and Crematogaster ants, but reject Dorylus ants, while those of Gombe prey upon Dorylus, but reject Camponotus and Crematogaster. Since the two habitats seem very similar ecologically, the difference in entomophagy might be due to traditional drift.

Aging and fertility patterns in wild chimpanzees provide insights into the evolution of menopause

Human menopause is remarkable in that reproductive senescence is markedly accelerated relative to somatic aging, leaving an extended postreproductive period for a large proportion of women. Functional explanations for this are debated, in part because comparative data from closely related species are inadequate. Existing studies of chimpanzees are based on very small samples and have not provided clear conclusions about the reproductive function of aging females. These studies have not examined whether reproductive senescence in chimpanzees exceeds the pace of general aging, as in humans, or occurs in parallel with declines in overall health, as in many other animals. In order to remedy these problems, we examined fertility and mortality patterns in six free-living chimpanzee populations. Chimpanzee and human birth rates show similar patterns of decline beginning in the fourth decade, suggesting that the physiology of reproductive senescence was relatively conserved in human evolution. However, in contrast to humans, chimpanzee fertility declines are consistent with declines in survivorship, and healthy females maintain high birth rates late into life. Thus, in contrast to recent claims, we find no evidence that menopause is a typical characteristic of chimpanzee life histories.

Predatory behavior among wild chimpanzees of the mahale mountains

Eighteen predatory and two cannibalistic episodes have been observed among wild chimpanzees of the Mahale Mountains. The prey consists mainly of juveniles of medium-sized mammals, six species of which were recorded as prey fauna for the first time in this study. Predatory behavior of Mahale chimpanzees seems more opportunistic and primitive than has ever been observed in the Gombe National Park in terms of capture technique, size of prey, co-operation in hunting, sexual difference in capture frequency, degree of consumption and extent of meat sharing. Cannibalism is an inter-unit-group phenomenon and may be an extension of infanticide, which may function in changing “mother” into “female.”

The leaf-clipping display: A newly-discovered expressive gesture in wild chimpanzees

Abstract Chimpanzees of the Mahale Mountains, Tanzania were discovered to show an expressive gesture using leaves, termed as “leaf-clipping display”. This behavioural signal is directed by an adult male to an estrous female as a possessive behaviour, or by an adolescent male as a courtship display, or by an estrous female to an adolescent male also as a solicitation of copulation. The signal also is used toward human observers as a signal of food-demanding. This behaviour pattern might originate in a displacement tool-making behaviour in conflict situations. The leaf-clipping display has not been observed in any other chimpanzee populations studied, and may probably be one example of the tradition drift in wild chimpanzees.

Local Differences in plant-feeding Habits of chimpanzees between the Mahale Mountains and Gombe National Park, Tanzania

Plant-feeding habits of chimpanzee populations ( Pan troglodytes schweinfurthii ) were compared between the Gombe National Park and Mahale Mountains, Tanzania. Comparison of food lists revealed 15 clear differences in the frequency of eating species which occur commonly in both areas. Feeding techniques also differed in at least three commonly eaten food types. Factors responsible for the local variation of the food items and feeding techniques are discussed.