Abstract
Aggregations of different-looking animals are frequently seen in nature, despite well-documented selection pressures on individuals to maintain phenotypically homogenous groups. Two well-known theories, the 'confusion effect' (reduced ability of a predator to accurately target an individual in a group) and the 'oddity effect' (preferential targeting of phenotypically distinct, 'odd', individuals) act together to predict the evolution of behaviours in prey that lead to groups of animals that are homogeneous in appearance. In contrast, a recently proposed mechanism suggests that mixed groups could be maintained if one species in a mixed group is more conspicuous against the habitat than the other, as confusion effects generated by the conspicuous species impede predator targeting of the cryptic species; thus, cryptic species benefit from association with conspicuous ones. We test these contrasting predictions from the perspective of both predators and prey, and show that cryptic individual Daphnia are at reduced risk of predation from three-spine sticklebacks Gasterosteus aculeatus when in mixed-phenotype groups, a risk that is reduced further as the number of conspicuous individuals increases, supporting the hypothesis for the evolution of mixed groups. In contrast, while the preference for associating with colour-matched conspecifics by mollies (Poecilia sphenops) was reduced when they were cryptic, we found no evidence for active association with conspicuous conspecifics. We conclude that prey animals must balance the relative risks of oddity and conspicuousness in their social decisions, and that this could potentially lead to the evolution of mixed-phenotype grouping as a response to predation risk alone.
Highlights
Predators play a key role in ecological communities with well-‐established direct and indirect effects on prey abundance and behaviour (Sih et al 1985; Lima and Dill 1990; Schmitz et al.2004)
We conclude that prey animals must balance the relative risks of oddity and conspicuousness in their social decisions, and that this could potentially lead to the evolution of mixed phenotype grouping as a response to predation risk alone
The two theoretical frameworks for the evolution of mixed phenotype groups that we have investigated predict different suites of predator targeting of prey, prey risk and prey group choice behaviour
Summary
Predators play a key role in ecological communities with well-‐established direct and indirect effects on prey abundance and behaviour (Sih et al 1985; Lima and Dill 1990; Schmitz et al.2004). Theoretical understanding of predator-‐prey interactions is generally based on the assumption of homogeneous, randomly distributed prey. The assumptions of prey homogeneity can be violated in two important ways: 1) a violation of the assumption of spatial homogeneity via prey aggregation and 2) a violation of the assumption of phenotypic homogeneity where prey differ in appearance or behaviour. Including these factors into models of predator-‐prey interactions can have significant effects on their predictions (Fryxell et al 2007; Pettorelli et al 2011)
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