Abstract
BackgroundGrouping 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 FindingsWe 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 SignificanceOur 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.
Highlights
Group living is widespread across the animal kingdom, in prey species, as it carries a number of proposed anti-predator benefits
Our results suggest that predation risk and individual body size act together to influence shoaling decisions
We found that larger-bodied test fish showed a significantly stronger preference for the size-matched shoal compared to smaller-bodied test fish (GLM, p,0.001, table 1, figure 1), but found only a marginal effect of predation risk (GLM, p = 0.059), no effect of the nonmatched shoal size (GLM, p = 0.487, fig 1, table 1), and no significant interactions
Summary
Group living is widespread across the animal kingdom, in prey species, as it carries a number of proposed anti-predator benefits. These include the dilution of individual risk [1,2], the many-eyes theory of increased vigilance [3,4], and the confusion effect, where a predator has difficulty in targeting a specific individual for attack [5], all of which are increased in larger groups. The confusion effect is enhanced when prey are morphologically and behaviourally similar [6], but when phenotypically distinct individuals occur within a group, predators preferentially target these individuals, enhancing their success [7,8]. Animals should choose group mates close in appearance to themselves (eg. similar size), whilst choosing a large group
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