Abstract
A key feature of human behavioral diversity is that it can be constrained by cultural preference (“cultural override”); that is, population-specific preferences can override resource availability. Here we investigate whether a similar phenomenon can be found in one of our closest relatives, as well as the potential impacts of ecological differences on feeding behavior. Our study subjects were different subpopulations of Eastern chimpanzees (Pan troglodytes schweinfurthii) occupying two very different habitats, moist tropical lowland forests vs. moist tropical forest–savanna mosaic on opposite sides of a major river. Given differences in encounter rates of different kinds of tool sites on both sides of the Uele River, we predicted that these subpopulations would differ in their likelihood of using tools to prey on two insect species despite similar availability. In surveys conducted over a 9-year period at 19 different survey regions in northern Democratic Republic of Congo (10 in lowland forest and 9 in mosaic), we collected and analyzed data on chimpanzee tool-assisted exploitation of insects. To determine the availability of insect species eaten by the chimpanzees, we counted insects and their mounds on transects and recces at 12 of these sites. For stick tools used to harvest epigaeic Dorylus and ponerine ants, we evaluated seasonal, geographical, and prey-availability factors that might influence their occurrence, using nest encounter rate as a proxy to control for chimpanzee abundance. Across the 19 survey regions spanning both sides of the Uele, we found little difference in the availability of epigaeic Dorylus and ponerine ants. Despite this, tool encounter rates for epigaeic Dorylus, but not ponerine, ants were significantly higher in the mosaic to the north of the Uele. Furthermore, we found no evidence for termite fishing anywhere, despite the availability of Macrotermes mounds throughout the region and the fact that chimpanzees at a number of other study sites use tools to harvest these termites. Instead, the chimpanzees of this region used a novel percussive technique to harvest two other types of termites, Cubitermes sp. and Thoracotermes macrothorax. This mismatch between prey availability and predation is consistent with cultural override, but given the different habitats on the two sides of the Uele River, we cannot fully rule out the influence of ecological factors. Comparing our findings with those of similar studies of other chimpanzee populations promises to contribute to our understanding of the evolution of behavioral diversity in humans and our closest cousins.
Highlights
Human behavior is characterized by its remarkable diversity in most observable domains
We found epigaeic Dorylus and ponerine ants on both sites of the Uele River
For epigaeic Dorylus ants, tool site encounter rates were significantly higher north of the Uele River, and the chimpanzees used a long tool type not found to the south (Table II; Fig. 4a)
Summary
Human behavior is characterized by its remarkable diversity in most observable domains. Clothing, architecture, housing style, language, pet choice and food preference can all differ profoundly between cultures (Murdock 1981; see Electronic Supplementary Material [ESM] 1) Much of this variation is assumed to be based on culture, defined by Whiten et al (1999) as “behavior that is transmitted repeatedly through social or observational learning to become a population-level characteristic.”. It is clear that socially learned preferences impact which foods human cultures see as edible and/or desirable, as well as the details of their tools, housing, and clothing This bias toward culturally approved solutions and preferences, or “cultural override,” has a strong influence on human behavior (Haun et al 2006) and could represent one of the factors that differentiate human culture from other animal traditions (i.e., learned behaviors that show continuity across generations; McGrew 2004). This raises the question of whether such a divorce between resource availability and the development of material traditions to harvest those resources can be recognized in other species, in particular in our close cousins the chimpanzees (Pan troglodytes)
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