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. Recent findings suggest that prey which 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 examine the patterns of anti-predator colour evolution and test whether species with hidden colours serve as an intermediate stage in the evolution of aposematism, focusing on the colour-diverse snake family Elapidae. Phylogenetic comparative analysis revealed key patterns in anti-predator colour evolution: (i) a few major transitions influenced the overall distribution of anti-predator coloration, and (ii) aposematism evolved multiple times, with hidden coloration as a major precursory state. We also quantified associations between colour patterns and defensive behaviours that reveal ventral surfaces (i.e. hidden signals). We found that venter-revealing defensive behaviours are frequently associated with hidden colour signals, and that the evolution of these behaviours and hidden colours are associated. Our results highlight the importance of hidden colours in evolutionary transitions to aposematism and the co-evolution of defensive behaviour with anti-predator coloration in snakes.