Uncovering the effects of Müllerian mimicry on the evolution of conspicuousness in colour patterns

Variation in the conspicuousness of colour patterns is observed within and
among defended prey species. The evolution of conspicuous colour pattern
in defended species can be strongly impaired because of increased
detectability by predators. Nevertheless, such evolution of the colour
pattern can be favoured if changes in conspicuousness result in Müllerian
mimicry with other defended prey. Here, we develop a model describing the
population dynamics of a conspicuous defended prey species, and we assess
the invasion conditions of derived phenotypes that differ from the
ancestral phenotype by their conspicuousness. Such change in
conspicuousness may then modify their level of mimicry with the local
community of defended species. Derived colour pattern displayed in this
focal population can therefore be either exactly similar, partially
resembling or completely dissimilar to the local mimicry ring displaying
the ancestral colour pattern. We assume that predation risk depends (1) on
the number of individuals sharing a given colour pattern within the
population, (2) on the occurrence of co-mimetic defended species, and (3)
on the availability of alternative edible prey. Using a combination of
analytical derivations and numerical simulations, we show that colour
patterns that are less conspicuous than the ancestral one are generally
favoured within mimicry rings, unless reduced conspicuousness impairs
mimicry. By contrast, when a mutation affecting the colour pattern leads
to a shift toward a better protected mimicry ring, a more conspicuous
colour pattern can be favoured. The selected aposematic pattern then
depends on the local communities of defended and edible prey, as well as
on the detectability, memorability and level of mimicry of the colour
patterns.