Do Panther Chameleons Bask to Regulate Endogenous Vitamin D3 Production?

Basking by ectothermic vertebrates is thought to have evolved for thermoregulation. However, another beneficial effect of sunlight exposure, specifically the ultraviolet B (UV-B) component, includes endogenous production of vitamin D3. In the laboratory, panther chameleons exhibited a positive phototaxis to greater visible, ultraviolet A (UV-A) and UV-B light. However, with equivalent high irradiances of UV-A or UV-B, their response to UV-B was significantly greater than it was to UV-A. Exposure of in vitro skin patches of panther chameleons to high UV-B (90 μW/cm2) for 1 h significantly enhanced vitamin D3 concentration. Voluntary exposure to higher UV-B irradiance (70 vs. 1 μW/cm2) resulted in greater circulating 25-hydroxyvitamin D3 in female panther chameleons (604 vs. 92 ng/mL). Depending on dietary intake of vitamin D3, chameleons adjusted their exposure time to UV-B irradiation as if regulating their endogenous production of this vital hormone. When dietary intake was low (1-3 IU/g), they exposed themselves to significantly more UV-producing light; when intake was high (9-129 IU/g), they exposed themselves to less. Vitamin D3 photoregulation seems to be an important additional component of the function of basking.