Skip to main content

Research Repository

Advanced Search

Balancing the dilution and oddity effects: Decisions depend on body size

Morrell, Lesley J.; Askwith, Beth; Rodgers, Gwendolen M.; Ward, Jonathan R.


Beth Askwith

Gwendolen M. Rodgers

Jonathan R. Ward


Stuart A. Sandin


Background Grouping behaviour, common across the animal kingdom, is known to reduce an individual's risk of predation; particularly through dilution of individual risk and predator confusion (predator inability to single out an individual for attack). Theory predicts greater risk of predation to individuals more conspicuous to predators by difference in appearance from the group (the ‘oddity’ effect). Thus, animals should choose group mates close in appearance to themselves (eg. similar size), whilst also choosing a large group. Methodology and Principal Findings We used the Trinidadian guppy (Poecilia reticulata), a well known model species of group-living freshwater fish, in a series of binary choice trials investigating the outcome of conflict between preferences for large and phenotypically matched groups along a predation risk gradient. We found body-size dependent differences in the resultant social decisions. Large fish preferred shoaling with size-matched individuals, while small fish demonstrated no preference. There was a trend towards reduced preferences for the matched shoal under increased predation risk. Small fish were more active than large fish, moving between shoals more frequently. Activity levels increased as predation risk decreased. We found no effect of unmatched shoal size on preferences or activity. Conclusions and Significance Our results suggest that predation risk and individual body size act together to influence shoaling decisions. Oddity was more important for large than small fish, reducing in importance at higher predation risks. Dilution was potentially of limited importance at these shoal sizes. Activity levels may relate to how much sampling of each shoal was needed by the test fish during decision making. Predation pressure may select for better decision makers to survive to larger size, or that older, larger fish have learned to make shoaling decisions more efficiently, and this, combined with their size relative to shoal-mates, and attractiveness as prey items influences shoaling decisions.


Morrell, L. J., Askwith, B., Rodgers, G. M., & Ward, J. R. (2011). Balancing the dilution and oddity effects: Decisions depend on body size. PLoS ONE, 6(7), e14819.

Journal Article Type Article
Acceptance Date Dec 16, 2010
Online Publication Date Jul 5, 2011
Publication Date Jul 11, 2011
Deposit Date Apr 1, 2015
Publicly Available Date Apr 1, 2015
Journal PLoS ONE
Print ISSN 1932-6203
Electronic ISSN 1932-6203
Publisher Public Library of Science
Peer Reviewed Peer Reviewed
Volume 6
Issue 7
Article Number ARTN e14819
Pages e14819
Keywords Predation; Freshwater fish; Physiological parameters; Rivers; Fishes; Decision making; Xiphophorus; Animal behaviour
Public URL
Publisher URL
Additional Information Copy of article first published in PLoS ONE, 2011, v.6, issue 7


Article.pdf (332 Kb)

Copyright Statement
© 2011 Rodgers et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

You might also like

Downloadable Citations