Broadening the angle of view on aposematism: a comment on Skelhorn et al

Much of previous research on aposematism has dealt with questions related to the “initial evolution” of aposematism (i.e., arise of this often striking defense strategy under conditions where predators have no previous experience of it) and to natural selection on warning signals. With their review article, Skelhorn et al. (2016) redirect our attention to what one could call the everyday ecology of predation on defended prey: When in an established community there are prey species or individuals that differ in quantity or quality of protective chemical compounds, what are the factors that influence predators’ decision to consume the prey? Behavioral ecological theory states that forager behavior approximates optimality with respect to costs and benefits and that foraging decisions are also influenced by physiological state. A series of experiments conducted by the authors confirm that this also applies to predation on chemically defended prey. The authors raise a valid point: Although aposematism is usually not considered to provide perfect protection for prey, the adaptive aspects of predation on aposematic prey have seldom been considered. Skelhorn et al. (2016) make a point of how little we know about how different factors influence predator decision making (or about cognitive processes, as they call it). There is a value to identifying areas where more knowledge or new theoretical frameworks would be needed. However, there are several research topics and areas (e.g., herbivory on defended plants or pollination of flowers that vary in quality or quantity of reward they provide, optimal foraging theory, optimal defense theory) addressing questions that are similar or related to questions about information processing and decision making by predators consuming chemically defended prey and how the optimal decisions may vary with the physiological state of the consumer or the quality of the resource. Therefore, it would be even more fruitful to review and integrate such relevant knowledge from other areas to provide a foundation for the framework that the authors are asking for, to initiate the work needed to fill the indicated gaps of knowledge. Studies dealing with the evolution of aposematism have often treated this strategy as a special case, an evolutionary paradox or peculiarity, and yet we know that there is a large range variation both in the strength of the defense and in the visual appearance (distinctiveness and conspicuousness) of defended prey. Moreover, these 2 aspects do not necessarily covary, but there are a number of prey species that possess chemical defenses and yet do not appear particularly conspicuous or distinctive at least to humans. Therefore, one of the benefits of the approach proposed by Skelhorn et al. (2016) is that it makes us consider aposematic prey as something that is incorporated in the set of everyday foraging decisions that predators face. The approach would also allow us to take into account the large variability in defenses of prey. This broader angle of view may also help us to find answers to some profound questions: What is aposematism and how can we identify aposematic prey (without subjectivity), considering the enormous variability in visual appearance and in quantity and quality of defenses among prey? In which ways does the ecology of aposematic prey differ from other, defended or undefended prey? Instead of the dichotomic categorization of prey to aposematic and nonaposematic, we maybe should think of various prey benefitting from aposematism to different degrees and even different ways.