Density-dependent warning coloration

Predators learn to avoid unpalatable prey more quickly when it is conspicuous than when it is cryptic1. When they are rare, however, conspicuous individuals suffer a greater risk of discovery by predators than the cryptic majority, and their frequency is still too low for predators to learn to avoid them2. We might therefore expect selection to favour unpalatable prey that is cryptic at low local densities and conspicuous at high local densities. I have found that such density-dependent warning coloration occurs in Schistocerca emarginata (=lineata) (Orthoptera: Acrididae) grasshoppers, providing insight into the biology of the desert locust, Schistocerca gregaria, and a new perspective on the evolution of warning coloration (aposematism).