Higher survival of aposematic prey in close encounters with predators: an experimental study of detection distance

Aposematic animals are often conspicuous. It has been hypothesized that one function of conspicuousness in such prey is to be detected from afar by potential predators: the ‘detection distance hypothesis’. The hypothesis states that predators are less prone to attack at long detection range because more time is allowed for making the ‘correct’ decision not to attack the unprofitable prey. The detection distance hypothesis has gained some experimental support in that time-limited predators make more mistakes. To investigate effects of prey presentation distance we performed two experiments. First, in experiment 1, we investigated at what distance chicks, Gallus gallus domesticus, could see the difference in colour between aposematic and plain mealworms. Birds chose the correct track in a two-way choice when prey were at 20, 40 and 60 cm distance but not at 80 cm. Second, in experiment 2, fifth-instar larvae of the aposematic bug Lygaeus equestris were presented to experienced chicks at 2, 20 or 60 cm distance. We found no difference in attack probability between distances. However, prey mortality was significantly lower for the shortest presentation distance. In conclusion, we found no support for the hypothesis that aposematic prey benefit from long-range detection; in fact they benefit from short-distance detection. This result, and others, suggests that the conspicuousness of aposematic prey at a distance may simply be a by-product of an efficient signalling function after detection.