Lizards Sunbathe for Vitamin D
May 4, 2009 -- Lizards may look lazy when basking in the sun, but the behavior serves a surprising purpose: They need vitamin D.
Not only that, suggests a new study, the cold-blooded creatures manage their time in the sun with extreme precision for ideal vitamin D production. Like people, lizards have compounds in their skin that, with exposure to the sun's ultraviolet light, are converted to a useable form of the essential nutrient.
"They were really, really good at hitting an optimum UV level," said Kristopher Karsten, a behavioral ecologist at Texas Christian University in Fort Worth. "It broadens our perspective of what they're really doing when they're sitting out there in the sun."
Lizards get some vitamin D from their diet. The rest they get from the sun. Dosage is key: Not enough D and the animals may get sick, grow slowly, have trouble reproducing, or even die. Too much D is toxic.
"It's a double-edged sword, where if they have too little, they're not going to do well, and if they have too much, they're not going to do well," Karsten said. "It's because there's an optimum amount associated with this that it interested us."
For two months, Karsten and colleagues raised six panther chameleons inside in their lab. Three of the animals ate crickets that were dusted with vitamin D powder. The other three ate a cricket diet that was vitamin-D deficient. By the end of the period, the first group had far higher D levels than the second.
Next, the chameleons were moved into round outdoor enclosures that measured about 6 feet wide and 6 feet tall. Each lizard's cage contained a plum tree or two as well as a black shade. The animals were free to move between sunny, UV-rich areas and shaded, low-UV areas. For 12 hours, from 7 a.m. and 7 p.m., the researchers noted how each lizard spent its time.
As expected, the vitamin D-deficient chameleons exposed themselves to more UV-light than those who already had plenty of D. What was surprising was how precise the animals were about their behavior. The reptiles timed their basking sessions to achieve ideal levels of D.
"They were extremely good at it," said Mark Acierno, a small animal internist at Louisiana State University in Baton Rouge. "We know now that vitamin D is very important, and they'll go out of their way to change their behavior to make sure they get enough."
Karsten suspects that the chameleons have a sensor in their brains that tells them how high or low their vitamin D levels are. The results of his study were published in the journal Physiological and Biochemical Zoology.
Until recently, most reptile sunbathing research had focused on basking's role in regulating body temperature. Gaining a more detailed understanding of why lizards do what they do, Acierno said, should help people, zoos, and other groups who raise exotic animals in captivity.
"If we want to keep them in captivity and we want them to live a normal life span in a healthy fashion," Acierno said, "We need to understand these things. In that sense, it's really important."