The ability to change one's skin colour is an extremely useful trait. Some species use dynamic changes in pigmentation to signal to potential mates or rivals. Others use it to camouflage themselves to fit in with their surroundings and avoid predators. Colour change can also assist in body temperature regulation by allowing for greater (or lesser) amounts of heat absorption as required. This last function is especially important in ectotherms, organisms that rely almost exclusively on external sources of heat to maintain an optimal body temperature. Darker pigmentation has a lower reflectance, which allows the animal to absorb more heat from the sun; conversely, lighter skin has a higher reflectance, which can reduce the amount of heat gained. As such, some ectotherms have been shown to change colour throughout the day, visibly darkening to assist in morning basking and lightening as temperatures increase and the need to absorb heat declines. While the existence of such regular changes in pigmentation is well known, what triggers these changes remains under-studied.

To test the hypothesis that photoperiod (daily cycles of light and dark) is a cue for colour change in lizards, Marie Fan and her colleagues at the University of Melbourne, Australia, acclimated inland bearded dragons (Pogona vitticeps) to four different photoperiods (full dark and 6, 12 and 18 h of light per day). Bearded dragons are a rather large and charismatic Australian lizard and are known from previous work to use colour change to assist in thermoregulation, making them the ideal animals to test the scientists' theory. Fan and colleagues also wished to discern whether the changes in pigmentation occurred immediately in response to environmental cues, such as changes in light, or whether they occurred under regulation from a circadian (daily) clock. The team captured lizards from the wild, brought them into the lab and placed them at a constant temperature for 3 days under one of the four experimental photoperiods. Then, on the third day, the researchers measured the reflectance of the animal's skin once every 3 h for 72 h, to allow them to visualize the changes that occurred in the lizard's skin pigmentation throughout the day and night, and to determine whether there was a distinct rhythm to any pigmentation fluctuation.

The team found that the lizard's pigmentation consistently changed along with the light cycle: lightening in the dark and darkening during the day. Under the variable light conditions, the timing of the colour changes reflected the artificial day and night times with peak reflectance (lightest colouring) occurring after the start of the dark phase. The circadian rhythms persisted even for lizards kept under 24 h of darkness. These findings show that not only does the skin colour of bearded dragons respond to an internal clock but also the clock can be manipulated and entrained by changes in photoperiod. The presence of a circadian rhythm allows the lizards to change colour in anticipation of changes in environmental cues. That changes in pigmentation occur in a manner that would be conducive to increasing body temperature during the daytime active phase is indicative of a control for thermoregulatory purposes. But it remains to be seen exactly what triggers the changes and whether the circadian clock would react differently under variable environmental conditions.

Cyclic colour change in the bearded dragon Pogona vitticeps under different photoperiods