Abstract
Most studies suggest that during times of nutritional stress, an animal faced with two foraging choices should follow a risk-prone strategy, choosing the option with highest payoff variance. This “scarcity/risk” hypothesis was developed to account for the foraging patterns of small animals with high metabolic rates susceptible to the threat of starvation. In this paper, we propose that animals should also be risk-prone when their diet quality is particularly high, far exceeding that which is needed to survive. Under these circumstances, the costs of experiencing a low or negative payoff can easily be recouped. We suggest that large-bodied omnivores are most likely to adopt this “abundance/risk” strategy. We investigate this question among wild chimpanzees (Pan troglodytes) that choose between a risk-averse strategy of feeding on plant material and a risk-prone strategy of hunting red colobus monkeys. Using 14 years of data on the Kanyawara chimpanzees of Kibale National Park, Uganda, we find strong evidence that chimpanzees follow the “abundance/risk” strategy. Both hunting rate (hunts/100 observation hours) and the probability of hunting upon encountering red colobus monkeys were positively correlated with seasonal consumption of ripe drupe fruits, a class of preferred food associated with elevated reproductive performance by females. Critically, these results remained statistically significant after controlling for the potentially confounding effects of male chimpanzee party size and the presence of sexually receptive females. These findings suggest that the relationship between risk-sensitive foraging and diet quality depends upon the daily probability of starvation, the number of alternative foraging strategies, and the degree to which diet quality satisfies an animal’s nutritional requirements.
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Acknowledgments
Long-term research at Kanyawara was supported by funding from NSF Grant 0416125 to R. Wrangham. I. Gilby was also partially supported by NSF Grant 115-0431141. We thank the Uganda National Council for Science and Technology, the Uganda Wildlife Authority, and the Makerere University Biological Field Station for permission to conduct research within Kibale National Park. The research complies with the current laws of Uganda. This project would not have been possible without the hard work and dedication of the field research team, especially Francis Mugurusi, Christopher Muruuli, Peter Tuhairwe, Christopher Katongole, and the late Donor Muhangyi and John Barwogeza, as well as field managers Michael Wilson, Martin Muller, Katherin Pieta, Carole Hooven, Kimberly Duffy, and Emily Otali. We are grateful to David Watts and two anonymous reviewers for helpful comments on an earlier version of the manuscript.
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Gilby, I.C., Wrangham, R.W. Risk-prone hunting by chimpanzees (Pan troglodytes schweinfurthii) increases during periods of high diet quality. Behav Ecol Sociobiol 61, 1771–1779 (2007). https://doi.org/10.1007/s00265-007-0410-6
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DOI: https://doi.org/10.1007/s00265-007-0410-6