Hidden colour signals as key drivers in the evolution of anti-predator coloration and defensive behaviours in snakes

The initial evolution of warning coloration ("aposematism") within a cryptic population of defended prey presents an evolutionary paradox. A recent phylogenetic analysis of amphibia suggests a new solution: prey that combine cryptic colours with conspicuous patches on concealed body parts ("hidden signallers"), may have mediated the transition of species from camouflage to aposematism. Here, we focus on the colour-diverse snake family Elapidae and test whether species with hidden colours could also serve as an intermediate stage in the evolution of aposematism in this group. Phylogenetic comparative analysis reveals several key patterns in their anti-predator colour evolution: (i) a few major transitions influenced the overall distribution of hidden-colours, camouflage, and aposematism in the group, and (ii) aposematism evolved multiple times, with hidden coloration a common precursory state, while direct transitions from camouflage to aposematism are also observed. We also quantify associations between colour patterns and defensive behaviours that reveal ventral surfaces (i.e. hidden signals). We find that venter-revealing defensive behaviours frequently co-occur with hidden colour signals. Our results suggest that aposematism can evolve through multiple routes and highlight the prevalence of co-evolution between venter-revealing defensive behaviour and anti-predator coloration in snakes.