Sunbathing: good for your bones (if you’re a lizard)

Cold-blooded animals don’t produce their own heat, and thus they must sunbathe, basking in the sun’s rays to raise their body temperatures so they have eonugh energy to  go about their business. But for some ectotherms, the sun isn’t just a source of heat.  A new study shows that in the panther chameleon, native to Madagascar, basking behavior also controls production of essential vitamin D.

In reptiles, vitamin D3 chaperones calcium in and out of the blood, and so is important for the production of strong bones. Insects, the lizards’ preferred meal, don’t provide much vitamin D; however, exposure to sufficient UV radiation triggers production of the essential vitamin in lizards’ skin. So when D levels drop, sunning squamates adjust their behavior to soak up more daylight.

Kristopher Karsten of Texas Christian University and his colleagues fed one group of chameleons crickets dusted with vitamin D powder, and fed another group plain old crickets. In outdoor trials, the plain-cricket lizards basked for longer periods of time than the D-supplemented lizards.

The most interesting thing about this study is perhaps not the fact that lizards control their internal vitamin D, since there’s a growing understanding that lizards bask for many other reasons than just thermoregulation, such as visibility and mating. The more compelling bit is the accuracy and precision with which the lizards control their vitamin D. Within groups, the lizards showed very little variation in realized D levels. Like body temperature, which for most lizards has a narrow optimal range, D levels are probably under strong selection for an optimum, above or below which the lizards don’t achieve the best possible bone strength.

Karsten, K., Ferguson, G., Chen, T., & Holick, M. (2009). Panther Chameleons, Furcifer pardalis, Behaviorally Regulate Optimal Exposure to UV Depending on Dietary Vitamin D3 StatusPhysiological and Biochemical Zoology, 82 (3), 218-225 DOI: 10.1086/597525