Evolution of conspicuousness in defended species involved in Mullerian mimicry

Variation in warning pattern resulting in different levels of conspicuousness is observed within and among chemically-defended prey species. Toxin acquisition and predator avoidance behaviour have been shown to affect the evolutionary processes underlying such warning signal diversity. Predators learn to associate warning colour pattern with unpalatability and this results in a decrease in predation risk for individuals displaying a locally abundant warning pattern. This positive number-dependent selection generated by predators causes evolutionary convergence across defended prey species toward a common warning colour pattern (Mullerian mimicry). Mullerian mimicry has a strong effect on the local diversity of warning patterns in tight connexion with the local abundance of mimetic species. Moreover, because chemical defences often stem from diet, the levels of defences might depend on resource specialisation, therefore impacting species abundance and the evolution of aposematic colour patterns. To investigate the effect of Mullerian mimicry on the evolution of the conspicuousness of warning signal and unpalatability, we built a single-species model where warning signal and unpalatability can evolve, assuming that (1) predation risk depends on both the number of individuals sharing a warning signal within the studied species and the local abundance of mimetic species and (2) the level of unpalatability depends on resource specialisation, implying that specialists suffer from reduced carrying capacity. We found that Mullerian mimicry can promote the evolution of more conspicuous colouration in cryptic species. Nevertheless, by favouring locally abundant signals, Mullerian mimicry stabilizes the persistence of mildly conspicuous patterns, explaining for instance the evolution of partially transparent colour patterns in some chemically-defended butterflies. Our model also show that Mullerian mimicry limits the emergence of new aposematic signals associated with reduced unpalatability. Altogether, our theoretical approach highlights complex interplays between resource specialisation, mimicry and predator behaviour on the evolution of aposematic signals and associated defences.