In vertebrates, vitamin D plays an important role in maintaining the body's calcium levels through the calcium-phosphorous hormonal regulation system. Animals with low vitamin D levels suffer from calcium deficiency, which leads to numerous maladies. Thus, animals need to maintain their vitamin D stores to stay healthy, and do so either by absorbing vitamin D from their food or by producing it themselves. However, producing your own supply of vitamin D requires ultraviolet-B (UV-B) radiation, and since animals clearly differ in the time they spend sunbathing, the relative importance of these two vitamin D sources varies considerably among animals. Generally, creatures that eat food with low vitamin D levels appear to rely mostly on UV-B-generated vitamin D,while nocturnal and other animals that rarely see sunlight appear to be unable to generate their own and presumably depend on rich dietary vitamin D sources instead.
Ferguson and colleagues at Texas Christian University wondered if this general pattern of attaining vitamin D applies to closely related species that live in the same place, but experience different UV-B and dietary vitamin D availabilities. To find out, they compared the natural UV-B exposure, the skin's ability to photobiosynthesize (generate its own) vitamin D, and dietary vitamin D levels of two Jamaican lizards: Anolis lineotopus merope, a shade-dweller, and Anolis sagrei, which prefers to bask in the sun.
To quantify natural UV-B exposure, Ferguson and colleagues watched adult lizards of the two species for nine continuous hours, recording each lizard's location and sun-exposure (whether they were sitting in full-sun, filtered-sun or shade) every five minutes. To assess the amount of UV-B irradiation the lizards experienced in these locations, on a subsequent day they re-enacted each lizard's movements on the observation day using artificial `lizard models'. The models were ampules that contained provitamin D, which is converted to vitamin D in proportion to UV-B exposure. To measure how effective the lizards were at generating their own vitamin D, the team assessed skin sensitivity for vitamin D biosynthesis by exposing skin samples from each species to 0, 20, 40 or 60 minutes of UV-B irradiation and measuring vitamin D production. Finally, they assessed the lizards' natural levels of dietary vitamin D by determining vitamin D levels in the stomach contents of wild-caught lizards.
The team found that the shade-dweller A. lineotopus meropeexperienced less UV-B irradiation than the sun-loving A. sagrei but,unexpectedly, they didn't compensate for this by eating vitamin D-rich food;in fact, the sun-lovers had more vitamin D in their food. So how do the shade-dwellers cope? Ferguson et al. provide an explanation: the shade-dweller's skin is much more sensitive to UV-B-induced vitamin D biosynthesis than that of A. sagrei; they are simply better at producing vitamin D, despite the lack of sun. Ferguson et al. argue that the reduced skin sensitivity of A. sagrei to vitamin D biosynthesis may reflect a lesser need for self-generated vitamin D, since there is enough of it in their diet, and a greater need for UV-B sunscreen to protect their skin from the damaging influence of UV-B. Thus, it appears that these lizards can adjust their skin's UV-B sensitivity, so they can strike a balance between avoiding UV-B damage and generating their own vitamin D.
