Research published in the October issue of The American Naturalist shows that Australian green tree frogs survive the dry season with the help of the same phenomenon that fogs up eyeglasses in the winter.
Christopher R. Tracy and colleagues found the Australian Green Treefrog, Litoria caerulea, remain active during the dry season with apparently no available water and at temperatures that approach their lower critical temperature. The authors hypothesized that when the frogs retreated to their refugia condensation forms on their cold skin — just like it does on a eye glasses or the widows of your car. They used frogs retrieved from natural dens and artificial dens and found the frogs absorb the condensing moisture through their skin, to maintain hydration during periods of little or no rain.
Before this study, the frogs' dry-season excursions were a bit mysterious.
"Every once in a while, we would find frogs sitting on a stick under the open sky, on nights when it was so cold they could barely move," said Dr. Chris Tracy, who led the research. "It was a real puzzle."
Tracy and his colleagues thought this behavior might enable the frogs collect condensation, but the hypothesis had never been tested.
The researchers designed a series of experiments using real frog dens in eucalyptus trees and artificial ones made from PVC pipe. They wanted to see if the frogs could collect enough moisture through condensation to compensate for what they lost being in the cold. They found that a cold night out cost a frog as much as .07 grams of water. However, a frog could gain nearly .4 grams, or nearly 1 percent of its total body weight, in water upon returning to the warm den.
The researchers also tested how well a frog's skin could absorb water, and found that as much as 60 percent of each water drop could be absorbed.
The results show that frogs can use condensation to hydrate themselves. And in a place as arid as the Australian savannahs during the dry season, where there is essentially no rain from June through August, every little bit counts.
"When there's no water available, even a small amount can mean the difference between surviving the dry season or not," Tracy said.
Christopher R. Tracy, Nathalie Laurence, Keith A. Christian, 2011, Condensation onto the Skin as a Means for Water Gain by Tree Frogs in Tropical Australia." The American Naturalist 178:553-558.(October 2011)