Within-group spatial position and vigilance: a role also for competition? The case of impalas (Aepyceros melampus) with a controlled food supply
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Theory predicts that individuals at the periphery of a group should be at higher risk than their more central conspecifics since they would be the first to be encountered by an approaching terrestrial predator. As a result, it is expected that peripheral individuals display higher vigilance levels. However, the role of conspecifics in this “edge effect” may have been previously overlooked, and taking into account the possible role of within-group competition is needed. Vigilance behavior in relation to within-group spatial position was studied in impalas (Aepyceros melampus) feeding on standardized patches. We also controlled for food distribution in order to accurately define a “central” as opposed to a “peripheral” position. Our data clearly supported an edge effect, with peripheral individuals spending more time vigilant than their central conspecifics. Data on social interactions suggest that it was easier for a foraging individual to defend its feeding patch with its head lowered, and that more interactions occurred at the center of the group. Together, these results indicate that central foragers may reduce their vigilance rates in response to increased competition. Disentangling how the effects of competition and predation risk contribute to the edge effect requires further investigations.
KeywordsVigilance Food control Edge effect Predation Competition
We are grateful to C. Bonenfant, S. Devillard, J.-M. Gaillard, M. Garel, M. Guillemain, M. Hewison, J. O’Brien and three anonymous referees for comments on an earlier draft. This project was developed within the HERD Project (CIRAD/CNRS). We are grateful to Zimbabwe Parks and Wildlife Management Authority for their support, and to the CNRS-NRF PICS program “Plant-herbivore dynamics in changing environments - developing appropriate models for adaptive management” for funding. Many thanks also to S. Le Bel (CIRAD-Zimbabwe) for facilitating the operations. We are also grateful to all the kids from Main Camp for their joyful support and pods collection, and we thank all the inhabitants of Main Camp for their understanding and tolerance as they adapted their routes during the observation sessions.
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