claudiusx":e6bcg883 said:
They don't have some magic feature to make them less vulnerable to UV radiation on comand. They can darken and lighten their skin to absorb more or less heat. This has nothing to do with being able to be protected from the harmful UV rays.
-Brandon
Actually,they do vary the skin UV reflectivity markedly .
Nothing magical about this , it's simply an evolutionary adaptation they and many reptiles native to Australia have which is a consequence of the extreme levels of UV they are exposed in Australia especially in the more tropical areas ( most of the natural range of P.Vitticeps ) and in the less sparcely wooded areas (much of Australia is more akin the tropical savannah).
from the literature :
the bearded dragon P. vitticeps, like many other species of lizard, possesses a black peritoneum which ensures protection of deeper tissues against damage from near-UV light .
and
results establish the existence of an endogenous circadian rhythm in the dorsal skin reflectance of the bearded dragon P. vitticeps. All the rhythmic lizards displayed relatively well-synchronised sinusoidal rhythms of ∼10% reflectance amplitude, with a maximum reflectance (i.e. maximum skin lightening) reached in late evening and a minimum reflectance (i.e. maximum skin darkening) reached in late morning. Skin darkening is caused by the dispersion of melanosomes (melanin-bearing organelles) within melanophores, whereas skin lightening results from their aggregation around the perinuclear region
and
While P. vitticeps may modify both visible (UV-Vis range) and NIR reflectance to accommodate requirements of signalling or camouflage and thermoregulation during light phases, they are in a rest state during dark phases and have little need to display colour change for other purposes, which may explain their higher proportion of UV-Vis reflectance. This variation in the proportion of UV-Vis reflectance may be a function of a change in the spacing of reflecting platelets in iridophores, in combination with melanosome dispersion. Light could act as a stimulus for the pituitary to release MSH, resulting in the dispersion of melanosomes and the aggregation of reflecting platelets. The aggregated reflecting platelets could thus be associated with a decrease in the proportion of UV-Vis reflectance in relation to NIR reflectance, although the mechanisms moderating NIR reflectance are currently unknown.
ref
Cyclic Colour Change in the Bearded Dragon Pogona vitticeps under Different Photoperiods
PLoS One. 2014; 9(10): e111504.
Marie Fan, Devi Stuart-Fox, and Viviana Cadena
ref
Bagnara JT, Hadley ME (1973) Chromatophores and Color Change: the Comparative Physiology of Animal Pigmentation. Prentice Hall Inc, Englewood Cliffs.
ref
Porter WP, Norris KS (1969) Lizard Reflectivity Change and Its Effect on Light Transmission through Body Wall. Science 163: 482–484
ref
Christian KA, Bedford GS, Shannahan ST (1996) Solar Absorptance of Some Australian Lizards and Its Relationship to Temperature. Aust J Zool 44: 59–67