Local behavioral variations and acquisition processes of cultural behavior patterns among wild chimpanzees

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We visited six field sites of chimpanzees: Mahale, Gombe (Tanzania), Kanyawara, Ngogo, Budongo (Uganda), and Bossou (Guinea) and compared their behavioral patterns among them. We also studied the development of the cultural behavior at Mahale. We discovered many local differences in behavior such as patterns of social scratch, grooming vocalizations, play, water contact behavior, intimidation displays, courtship displays, and self-medications. A play pattern named "Leaf-pile pulling" is shown by many juveniles and old infants of Mahale and has persisted for at least more than 10 years. However, this pattern has not been confirmed to exist as a tradition elsewhere. Juveniles at Mahale show various play patterns in response to running water of streams. Facing with the water surface, several male juveniles and adolescents may shake their head longitudinally violently as if enjoying the movements of their own mirror images. Several juveniles engage in so-called "leaf-sponging" and also use … More the leaf midrib for the similar purpose. Such diversified patterns of water play may be fashions or incipient traditions since they have a likelihood of having been transmitted from a juvenile to another, but there are some subtle behavioral differences between 2000 and 1994 as observed in video clips. It is very likely that intimidation displays such as "Throw splash" and courtship displays such as "Leaf-clipping" and "Shrub-bend" are among the traditions of Mahale chimpanzees. Patterns such as "Heel kick," "Palm squash of ecto-parasites," and "mutual touching of sexual skin" that have been newly recorded at Bossou have never been observed at Mahale. From a longitudinal study at Mahale, it has proved that individuals acquire cultural patterns sequentially: first feeding behavior then grooming behavior, and followed by tool-using and courtship displays, and finally intimidation displays. It is likely that they acquire almost all traditions by the weaning period. Social learning process has been suggested for the formation of "Grooming-hand-clasp," in which mothers may teach their infant offspring how to grasp hands by "molding." An adult female was shown to keep the mode of clasp pattern of her old group even 20 years after her immigration to a new group. Innovation by individuals likely occurs repeatedly among wild chimpanzees, but may rarely be fixed as a cultural pattern. The fact that we can find many local behavioral differences in chimpanzees may be partly explained by the slow process of dissemination from a group to another. In order to investigate the similarity between fathers and offspring, we have collected DNA sample such as feces from all the individuals of M group and are analyzing genes on the Y-chromosome. Less