For camouflaged prey animals which benefit from remaining undetected by predators, increased conspicuousness due to bilaterally symmetrical colouration is costly. The ubiquity of symmetrical body patterns in nature is assumed to be due to tight genetic and developmental constraints. Given these evolutionary restrictions, we investigated how animals might have evolved to optimise their surface colouration in order to reduce the predation cost of bilaterally symmetrical colouration. One way of doing this is by placing discrete, high contrast markings away from the axis of symmetry (or midline). Artificial camouflaged prey with symmetry placed at different distances from the axis were used in both laboratory visual search tasks with humans and field survival experiments with wild avian predators. By measuring response time (Experiment 1) and predation rate (Experiment 2), I showed that for targets that were symmetrical only near their midline, detectability was the same as for targets that were symmetrical all over their surface. Prey were significantly less conspicuous when symmetry was placed outside of this ‘critical zone’. To see whether living animals have evolved as predicted from the trends suggested by these experiments, the saliency of features at different distances from the midline of the cryptically coloured forewings from 36 Lepidopteran species was measured. Measures of both absolute salience and relative to wing area demonstrated that salience is indeed greatest away from the ‘critical zone’ near the axis of symmetry. My results suggest similarities in the mechanisms of bilateral symmetry detection by mammalian and avian predators. They also suggest that evolution has resulted in pattern elements being positioned in such a way that the detectability cost imposed by symmetry is reduced.
- bilateral symmetry
- visual search