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Conradt, L.; Krause, J.; Couzin, I. D.; Roper, T. J. |
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Title |
“Leading According to Need” in Self-Organizing Groups |
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Year |
2009 |
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The American Naturalist |
Abbreviated Journal |
Am Nat |
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Volume |
173 |
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3 |
Pages |
304-312 |
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behavioral synchrony, collective group decisions, democracy and egalitarianism in animals, public goods experiments, sexual segregation, social choice theory |
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Abstract |
Self‐organizing‐system approaches have shed significant light on the mechanisms underlying synchronized movements by large groups of animals, such as shoals of fish, flocks of birds, or herds of ungulates. However, these approaches rarely consider conflicts of interest between group members, although there is reason to suppose that such conflicts are commonplace. Here, we demonstrate that, where conflicts exist, individual members of self‐organizing groups can, in principle, increase their influence on group movement destination by strategically changing simple behavioral parameters (namely, movement speed, assertiveness, and social attraction range). However, they do so at the expense of an increased risk of group fragmentation and a decrease in movement efficiency. We argue that the resulting trade‐offs faced by each group member render it likely that group movements are led by those members for which reaching a particular destination is most crucial or group cohesion is least important. We term this phenomenon leading according to “need” or “social indifference,” respectively. Both kinds of leading can occur in the absence of knowledge of or communication about the needs of other group members and without the assumption of altruistic cooperation. We discuss our findings in the light of observations on fish and other vertebrates. |
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Equine Behaviour @ team @ |
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5121 |
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Author |
Cameron, E.Z.; du Toit, J.T. |
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Title |
Winning by a neck: tall giraffes avoid competing with shorter browsers |
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Journal Article |
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Year |
2007 |
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The American naturalist |
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Am Nat |
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169 |
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1 |
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130-135 |
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Acacia/growth & development; Animals; Feeding Behavior/*physiology; Neck/*anatomy & histology; Plant Leaves/growth & development; Ruminants/*anatomy & histology/*physiology; South Africa |
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Abstract |
With their vertically elongated body form, giraffes generally feed above the level of other browsers within the savanna browsing guild, despite having access to foliage at lower levels. They ingest more leaf mass per bite when foraging high in the tree, perhaps because smaller, more selective browsers deplete shoots at lower levels or because trees differentially allocate resources to promote shoot growth in the upper canopy. We erected exclosures around individual Acacia nigrescens trees in the greater Kruger ecosystem, South Africa. After a complete growing season, we found no differences in leaf biomass per shoot across height zones in excluded trees but significant differences in control trees. We conclude that giraffes preferentially browse at high levels in the canopy to avoid competition with smaller browsers. Our findings are analogous with those from studies of grazing guilds and demonstrate that resource partitioning can be driven by competition when smaller foragers displace larger foragers from shared resources. This provides the first experimental support for the classic evolutionary hypothesis that vertical elongation of the giraffe body is an outcome of competition within the browsing ungulate guild. |
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Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa. ezcameron@zoology.up.ac.za |
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1537-5323 |
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PMID:17206591 |
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refbase @ user @ |
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410 |
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