Phenotypic assortment by body shape in wild-caught fish shoals
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Phenotypic variation plays a critical role in determining the structural organisation and ecological function of wild populations. Animal groups are often structured according to factors such as species, sex, body size and parasite load, but it is unclear whether body shape also influences patterns of social organisation, and thus contributes to population phenotypic structure. Here, we use geometric morphometric analyses to determine whether wild-caught shoals of a freshwater fish, the western rainbowfish (Melanotaenia australis), are structured according to body size and shape. Using randomisation analyses, we show that the level of variation in size and shape observed in natural group assemblages is lower than that expected under a null model of random shoal composition. In addition, we found evidence of further phenotypic structuring along an upstream-downstream environmental gradient. The putative benefits of morphological assortment include a reduction in predation risk (due to prey oddity and predator confusion effects) and increased hydrodynamic or foraging efficiency. We suggest that morphological variation is a neglected component of population social organisation that can affect population processes, such as patterns of gene flow, and ecological interactions, such as predator-prey dynamics.
KeywordsGroup structure Group living Oddity effect Confusion effect Social organisation
We would like to thank Damien Farine for excellent feedback on a previous version of this manuscript and Christos Ioannou, Joel Trexler, and two anonymous reviewers for comments that improved our work. We would also like to thank Monica Gagliano for field assistance, Steven Correia for help with the image analyses and Andrew Storey (Wetland Research and Management) and Nicole Gregory (Rio Tinto) for coordinating field site access. We are grateful to Rio Tinto for facilitating the fieldwork and for providing accommodation in the field.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
All procedures were approved by the University of Western Australia Animal Ethics Committee (approval number RA/3/100/691).
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