Aposematic signals and the relationship between conspicuousness and distinctiveness

It has long been recognized that prey that invest in toxic or other defenses often advertise these defenses by means of conspicuously coloured signals. One question that remains unanswered is why conspicuousness is such a universal trait of aposematic signals. Conspicuousness may allow more rapid avoidance learning by predators or improved retention of such learning. An alternative or complementary explanation is that defended species should adopt a conspicuous signal of their defence to make them visually distinct from inconspicuous undefended prey. Here, we use a neural network model of prey detection and attack decision making by a predator in combination with evolving, virtual prey to shed light on the relative importance of conspicuousness against the background and distinctiveness from other species as mechanisms underlying aposematic signalling. Our model suggests that prey conspicuousness may result from selection for distinctiveness, but that selection for distinctiveness does not result in maximization of conspicuousness. On the other hand, our model does not justify the exclusion of the possibility that conspicuousness as such may be a beneficial attribute of warning coloration. It is likely that the relative importance of the two selective forces (for conspicuousness and for distinctiveness) will differ on a case-by-case basis, however there is no empirical or logical reason for the current neglect of evolutionary pressure for distinctiveness. Thus, we suggest that description of aposematism as the teaming of a secondary defence with a conspicuous signal may be overly simplistic; we would rather that the signal were described as conspicuous and/or distinctive.