A potential deimatic display revealed in a lizard

Conspicuously coloured signals may evolve via sexual selection to be ornaments or armaments, thereby conferring a fitness advantage to their bearer. Conversely, conspicuous colours may also evolve under natural selection as either aposematic signals or deimatic displays that deter attacks from predators. While conspicuous colour patches may evolve for one purpose (e.g. quality indicators), they may later be co-opted for another (e.g. anti-predator defence). Phrynocephalus mystaceus is a cryptic agamid lizard with flaps in both sexes that when folded against the head are inconspicuous, but when deployed are predicted to be highly conspicuous and to increase the appearance of body size. We tested whether head flaps play a role in social signalling via courtship or as status signals during contests in both sexes. We also tested whether the head flaps have an anti-predator function by simulating predatory encounters. Head flaps were never deployed in courtship or during contests and, therefore, are unlikely to be under sexual selection. However, head flaps and their deployment during simulated predatory encounters were consistent with the predictions associated with deimatic display theory. First, head flaps were similar in form and function between sexes. Second, they were highly conspicuous to both avian and snake predators. Third, there was a rapid transition from crypsis to conspicuousness when they deployed their head flaps during a late stage of predation, the subjugation phase, consistent with an ambush. Confirmation of the deimatic display hypothesis will require future testing of receiver responses.