Colour change is widespread among ectotherms and can occur as a singular event or be rhythmic, induced by cyclic environmental factors or regulated by internal clocks. Despite the importance of colour change in reptile ecology, few studies have investigated the occurrence of a daily rhythm in lizard pigmentation and its potential entrainment cues, as well as changes in near-infrared reflectance, which is likely to be critical for thermoregulation.

We tested whether the skin colour of the bearded dragon lizard, Pogona vitticeps, displays an endogenous circadian rhythm and how the photoperiod may affect this rhythm. We subjected 11 lizards to four photoperiodic regimens at a constant temperature: constant darkness and 6, 12 and 18 hours of light per day; and measured their dorsal skin reflectance every 3 hours for 72 hours after a habituation period. We observed that a proportion of lizards exhibited a significant rhythm under constant darkness. Under the artificial light/dark cycles, this endogenous rhythm synchronised to the different photoperiods, demonstrating the role of light as an entrainment cue. In addition, the difference between ultraviolet-visible and near-infrared reflectance variations could imply specific uses of each spectrum range for different behavioural functions [1].

These findings clearly show that the pigmentation of P. vitticeps follows a circadian rhythm regulated by internal clocks and that can be entrained by cyclic environmental factors. Further studies of such colour rhythms might considerably improve our understanding of the adaptive functional significance of colour change and its underlying physiological mechanisms in ectotherms.