Overcoming the detectability costs of symmetrical colouration

For camouflaged prey, enhanced conspicuousness due to bilaterally symmetrical colouration increases predation risk. The ubiquity of symmetrical body patterns in nature is therefore paradoxical, perhaps explicable through tight developmental constraints. Placing patterns that would be salient when symmetrical (e.g. high contrast markings) away from the axis of symmetry is one possible strategy to reduce the predation cost of symmetrical colouration. Artificial camouflaged prey with symmetrical patterns placed at different distances from the axis were used in both visual search tasks with humans and survival experiments with wild avian predators. Targets were less conspicuous when symmetrical patterning was placed outside a ‘critical zone’ near the midline. To assess whether real animals have evolved as predicted from these experiments, the saliency of features at different distances from the midline was measured in the cryptically coloured forewings of 36 Lepidopteran species. Salience, both in absolute terms and relative to wing area, was greatest away from the axis of symmetry. Our work therefore demonstrates that prey morphologies may have evolved to exploit a loophole in the ability of mammalian and avian visual systems to spot symmetrical patterns.