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

Abstract

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.