Behavioral and energetic costs of group membership in a coral reef fish |
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Authors: | J Wilson White Robert R Warner |
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Institution: | (1) Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA 93106, USA;(2) Department of Wildlife, Fish and Conservation Biology, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA |
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Abstract: | Animals in social aggregations often spend more time foraging than solitary conspecifics. This may be a product of the relative
safety afforded by aggregations: group members can devote more time to foraging and less time to antipredator behaviors than
solitary animals (the “risk reduction” effect). All else being equal, risk reduction should result in higher food intake for
grouped animals. However, intragroup competition may force group members to spend more time foraging in order to obtain the
same food ration as solitary individuals (the “resource competition” effect). We compared these opposing explanations of foraging
time allocation in a coral reef fish, bluehead wrasse (Thalassoma bifasciatum). Aggregations of juvenile bluehead wrasse experience safety-in-numbers, and preliminary observations suggested that juveniles
in aggregations spent more time foraging for copepods in the water column than solitary juveniles. However, the risk reduction
and resource competition hypotheses are indistinguishable on the basis of behavioral observations alone. Therefore, we collected
behavioral, dietary, and growth data (using otolith growth rings) for bluehead wrasse at multiple reefs around a Caribbean
island. Despite spending more time foraging in the water column, grouped fish did not capture more prey items and had slower
growth rates than solitary fish. Thus, the increased foraging time of grouped fish appears to reflect resource competition,
not risk reduction. This competition may limit the size and frequency of aggregations among juvenile bluehead wrasse, which
have been shown to experience reduced mortality rates in larger groups. Bluehead wrasse recruits also spent less time foraging
but grew faster at sites where planktonic copepod prey were more abundant. This suggests the possibility that large-scale
spatiotemporal variability in the abundance of planktonic copepods over coral reefs may produce corresponding variability
in the dynamics of reef fish populations.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. |
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Keywords: | Foraging behavior Group-size effect Growth rate Resource competition Risk reduction |
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