Blushing and suntanning (or sunburning) are probably the best we can hope for on a spectrum of natural skin tone change; but imagine having access to a whole palette of colours that you could change at will. While chameleons are best known for their pyrotechnic colour displays, dowdier bearded dragons (Pogona vitticeps) are also capable of switching between muted shades in a matter of seconds. ‘There are three main potential benefits to colour change’, says Viviana Cadena, from the University of Melbourne, Australia, explaining that bearded dragons probably change colour to regulate their body temperature; to communicate with the opposition while defending territory and during courtship; and to blend in with the surroundings for camouflage. But Cadena and her colleagues Kathleen Smith, Devi Stuart-Fox and John Endler wanted to understand how bearded dragons alter their colour when the shade of their surroundings changes and when the light intensity alters as a cloud passes over.
‘This is important to dragons in the wild because it would affect how well they can use colour change for camouflage to avoid being spotted by a predator’, says Cadena, who headed out to northwestern Victoria (near Mildura) and the red desert around Alice Springs with Smith to collect wild bearded dragons. ‘We chose these two populations because their colours and the appearance of the habitats in which they live differed the most amongst all bearded dragon populations’, says Cadena, recalling that the lizards were fun to work with, but difficult to catch. ‘Spotting them in the wild takes some practice, and their skin is rough and you have to hold them steady because they try to wiggle to get out of your hands’, she laughs a she remembers the scratches incurred. But once the lighter (southern) and darker (northern) dragons were back in the Melbourne lab, Cadena and Smith photographed their responses to play sand (similar to the yellow sand tone near Mildura), red desert sand from Alice Springs and black sand from a local pet store. Then the scientists transferred the dragons to the yellow play sand before photographing how the reptiles responded as they varied the light from intense daylight to overcast conditions and early sunrise.
‘We found that both populations of bearded dragons were able to change colour to the same extent, but this varied depending on the colour of the background we exposed them to’, says Cadena, who saw that once the Mildura lizards had adjusted their colour to match their new surroundings, their hue was always yellower than the more orange-toned Alice Springs lizards. And Cadena was surprised to find that both populations were able to achieve better colour matches with their surroundings when the light was lowest. ‘We think this could be because there might be a higher risk of predation at low light levels at dawn and dusk, when many predators are most active’, says Cadena.
Having confirmed that that each population is capable of matching its surroundings, Cadena says, ‘We believe that the differences in colouration we found between the two populations help the lizards better adapt to the looks of their own habitats’. And she and her colleagues are now eager to find out whether the reptile spectacular extends further into regions of the optical spectrum that our limited vision cannot see.
