April 21, 2009 — Keeping warm isn’t the only
reason lizards and other cold-blooded critters bask in the sun.
According to a study published in the May/June issue of Physiological
and Biochemical Zoology, chameleons alter their sunbathing behavior
based on their need for vitamin D.
"It’s a longstanding assumption that thermoregulation is the only
reason that lizards bask," says Kristopher Karsten, a biologist at
Texas Christian University who led the study. "Our results suggest that
in addition to thermoregulation, vitamin D regulation appears to have a
significant impact on basking behavior as well."
Chameleons, like humans and most other vertebrates, get vitamin D in
two ways: They can absorb it from food, and they can produce it in
their skin. In order to produce vitamin D, however, the skin must be
exposed to UV radiation.
To test whether chameleons alter their sunning behavior based on
dietary vitamin D intake, Karsten observed the behavior of two
different groups of chameleons. One group had high internal vitamin D
levels, thanks to a diet of crickets dusted with a vitamin D powder.
The other group ate regular crickets and had low vitamin D. The
chameleons were then placed in individual outdoor enclosures that
offered open area for direct sun, and a tree to offer filtered sun and
Chameleons generally move from sun to shade throughout the day. But
Karsten found that chameleons fed the low vitamin D diet compensated by
increasing their exposure to the sun’s UV rays. Chameleons with high
vitamin D diets, on the other hand, limited their UV exposure.
"It appears that panther chameleons have the ability to gauge their
internal vitamin D levels and alter their basking behavior
accordingly," Karsten says.
And they do it with remarkable accuracy.
"The chameleons were as effective as mathematically possible by our
methods at regulating toward optimal UV exposure for their vitamin D
profile," he says. "We thought they were probably pretty good at
regulating their UV exposure; we just didn’t think they’d be this good."
It’s not clear, however, by what mechanism they are able to sense
their internal vitamin D levels, but Karsten thinks there may be a
brain receptor sensitive to the vitamin.
"Given the ability for panther chameleons to precisely, accurately
and effectively adjust basking behavior as a direct result of vitamin
D3, [a brain vitamin D receptor] seems likely to occur in panther
Source : University of Chicago Press Journals