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Achilles' Heel of Sociality Revealed by Energetic Poverty Trap in Cursorial Hunters

Gregory S. A. Rasmussen, Markus Gusset, Franck Courchamp and David W. Macdonald
The American Naturalist
Vol. 172, No. 4 (October 2008), pp. 508-518
DOI: 10.1086/590965
Stable URL: http://www.jstor.org/stable/10.1086/590965
Page Count: 11
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Achilles' Heel of Sociality Revealed by Energetic Poverty Trap in Cursorial Hunters
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Abstract

Abstract: This study empirically tests two foundation ecological theories: (1) pack hunting is a driver for the evolution of sociality; and (2) species have a finite energy potential, whereby increased maintenance costs result in decreased reproductive effort. Using activity and prey data from 22 packs of African wild dogs (Lycaon pictus), we parameterized a model detailing the energetic cost/benefit of cooperative hunting. Larger pack size increased foraging time, prey size, and capture probability while reducing chase distance, resulting in a rapidly increasing net rate of energy intake up to a pack size of five, which peaked at 10 individuals and then declined. With a streamlined body plan necessary for hypercursoriality limiting stomach capacity in smaller packs, it was demonstrated that the group hunting benefit will rather accrue to widely foraging predators than to “sit‐and‐wait” ones. Reproductive effort, measured by the number of pups born, revealed smaller litters with decreasing pack size, validated finite energy theory, and highlighted a “poverty trap” where smaller groups have lower foraging gains, smaller litters, and increased vulnerability to extirpation. Consequently, these results demonstrated a mechanistic example of pervasive selection for maximal body size (Cope’s rule), leading to a macroevolutionary ratchet, where sociality linked to hypercursoriality is betrayed by an Achilles' heel.

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