Ecology of culture: do environmental factors influence foraging tool use in wild chimpanzees, Pan troglodytes verus?

Kathelijne Koops, William C. McGrew, Tetsuro Matsuza

抄録の要約：

 地域ごとに行動が違う場合、文化の違いによるといわれてきた。しかし生態学的な要因や生得的な制約も考慮する必要がある。 ギニアのニンバ山セリンバラ地区の野生チンパンジーを対象に、石をつかったアブラヤシの種割り、棒をつかったシロアリ釣り、棒をつかった サスライアリ釣りの、3種類の道具使用について、その生起頻度と、道具使用の対象や道具の素材がどれくらいその生息環境に分布しているかを調べた。 主食となる果実の入手可能性の季節変化も考慮した。その結果、必要があって道具を使うというよりは、たまたまでくわす頻度が高いと、道具を使って採食することがわかった。つまり、文化といっても、環境とのあいだの相互作用を考慮する必要がある。

Geographical variation in behaviour may be best explained in terms of culture if ecological and genetic explanations can be excluded. However, ecological conditions and genetic predispositions may in turn also affect cultural processes. We examined the in fl uence of environmental factors on foraging tool use among chimpanzees at the Seringbara study site in the Nimba Mountains, Guinea, where nut cracking and termite fi shing are absent, but ant dipping is present. We tested two ecological hypotheses to explain foraging tool use prevalence. The opportunity hypothesis states that encounter rates with nuts, insects or tools explain tool use patterns. We measured the density and distribution of nut trees, nuts, army ants, termites and potential tools in relation to the chimpanzees' ranging patterns. The necessity hypothesis states that tool use is a response to scarcity of preferred foods (i.e. ripe fruit). We measured the temporal availability of nuts, army ants and termites in relation to preferred food sources. Our fi ndings support the opportunity hypothesis: nut trees and Macrotermes mounds were rare and peripheral to the chimpanzees' range, whereas army ants were abundant and widespread. The necessity hypothesis did not explain tool-assisted insectivory, as both army ants and termites were available during ripe fruit scarcity, yet neither ant dipping nor termite fishing  functioned as fallback strategies. Nuts were absent at times of fruit scarcity and were not available as fallback foods. Our findings highlight the importance of considering environmental conditions in explaining foraging tool use by wild chimpanzees and emphasize the interplay between environment and culture.

Animal Behaviour online 20 November 2012

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