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First published online May 24, 2005
Journal of Experimental Biology 208, vi (2005)
Copyright © 2005 The Company of Biologists Limited
doi: 10.1242/jeb.01652
Outside JEB |
JAMAICAN LIZARDS' BALANCING ACT
University of British Columbia
jstecyk{at}interchange.ubc.ca
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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 merope experienced 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.
References
Ferguson, G. W., Gehrmann, W. H., Karsten, K. B., Landwer, A. J., Carman, E. N., Chen, T. C. and Holick, M. F. (2005). Ultraviolet exposure and vitamin D synthesis in a sun-dwelling and a shade-dwelling species of Anolis: are there adaptations of lower ultraviolet B and dietary vitamin D3 in availability in the shade? Physiol. Biochem. Zool. 78,193 -200.[CrossRef][Medline]
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