Keeping warm isn't the only reason lizards bask in the sun. A new study has found that chameleons set their sunbathing schedule depending on how much vitamin D they need.
"It's a longstanding assumption that thermoregulation is the only reason that lizards bask. Our results suggest that in addition to thermoregulation, vitamin D regulation appears to have a significant impact on basking behaviour as well," said Kristopher Karsten, a biologist at Texas Christian University who led the study.
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. However, in order to produce vitamin D, the skin must be exposed to UV radiation.
To test whether chameleons alter their sunning behaviour based on dietary vitamin D intake, Karsten observed the behaviour 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 shade.
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.
On the other hand, chameleons with high vitamin D diets limited their UV exposure.
"It appears that panther chameleons have the ability to gauge their internal vitamin D levels and alter their basking behaviour accordingly," Karsten said.
It''s not clear, however, by what mechanism they are able to sense their internal vitamin D levels, but Karsten believes that 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 chameleons," Karsten said.
The study is published in the May/June issue of Physiological and Biochemical Zoology.