Disentangling the effects of group size and density on shoaling decisions of three-spined sticklebacks (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>) |
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Authors: | Joachim G Frommen Meike Hiermes Theo C M Bakker |
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Institution: | (1) Institute for Evolutionary Biology and Ecology, University of Bonn, An der Immenburg 1, 53121 Bonn, Germany;(2) Konrad Lorenz Institute for Ethology, Austrian Academy of Sciences, Savoyenstr. 1a, 1160 Vienna, Austria |
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Abstract: | Many animals live in groups most of their life. One function of this behaviour is an increased predator protection whereas
larger groups provide better protection than smaller ones. A causal explanation is that due to a higher number of shoal members
the individual risk of being predated will decrease (“dilution effect”). Additionally, shoaling leads to increased predator
confusion. This “confusion effect” can be strengthened by an increased group density, which often correlates with group size.
Many studies found that individuals prefer the larger of two groups. However, whether this preference is due to a larger group
size or because of an increased density of the larger group remained unclear. To disentangle these factors we gave three-spined
sticklebacks (Gasterosteus aculeatus) the choice between shoals of (1) different group size and density, (2) different group size, but equal density and (3) equal
group sizes, but different densities. As expected, test fish preferred the larger and denser shoal over the smaller, less
dense one. This preference was lost when shoal size differed but density was kept constant. When shoal size was equal but
density differed, test fish preferred the less dense shoal. However, this was only the case when test fish chose between two
relatively dense shoals. On the other hand, when overall density was low, test fish did not discriminate between shoals of
different densities. This result may be explained in terms of predator avoidance. The results show that shoaling preferences
might not always be influenced by a higher number of group members but also by the density and cohesiveness of the respective
groups.
An erratum to this article can be found at |
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Keywords: | Fish Safety in numbers Schooling Selfish herd Grouping Predation Confusion effect |
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