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Glistening. When green tree frogs hop from the cool outdoors to their snug tree hollow dens, they collect beads of moisture (shown)—and then drink up.

Christopher R. Tracy

Australian Frogs Do the Dew

Green tree frogs survive by using lemonade physics. According to a new study, the amphibians spend some time in the chilly outdoors and then hop into a warm tree den, a move that produces condensation on their skin just like on a cool glass of lemonade left out of the fridge. Then the frogs soak up the dew, keeping them well hydrated during long, rainless months.

Green tree frogs (Litoria caerulea), which can grow to about 10 centimeters long, aren't hard to spot in Australia's tropical savannas. On even the driest nights, these intrepid hoppers leave their hiding places in old tree hollows to perch for hours on twigs or termite mounds. Like most amphibians, the frogs don't drink with their mouths but with their whole bodies, soaking in water through their skin. Their porous skin, however, puts them at risk for drying out in the open air. And northern Australia can get cold for a frog, dropping to about 13°C. So the question was, "Why were these frogs coming out on these cold nights when it was so cold they could hardly move?" says study co-author Christopher Tracy, a physiological ecologist at the University of Melbourne in Australia.

Perhaps for a quick drink. Like that glass of lemonade, a cold frog can, theoretically, collect condensation if it hops back into a hot and humid environment, such as a tree hollow. On cool nights, these snug dens are about 10°C warmer than the outdoors and can reach relative humidity of 90% to 100%.

To test their lemonade theory, Tracy and colleagues chilled frogs, either in the night air or in ice baths, then popped them into a well-used hollow. Soon, many started to glisten with dew. And they seemed to be absorbing those drips, too, gaining about 1% of their body mass after just 15 minutes in the tree, the team will report next month in The American Naturalist.

These crafty frogs certainly aren't without precedent. In 1969, U.S. scientists Robert Lasiewski and George Bartholomew first showed that tarantulas, geckos, and toads could, at least in the lab, collect and possibly drink their own dew. But whether any of these cool critters actually did that in the wild remained unclear. "It is a convincing study after 40-plus years of assumption," says physiologist Roger Seymour of the University of Adelaide in Australia, who studied under Lasiewski and Bartholomew.

Dew harvesting could come in handy between June and August, when rainfall trickles to zero in parts of northern Australia, Tracy adds. He calculates that the frogs actually come out ahead from their nightly forays, gaining more water from dew than they lose by evaporation through their skin—but only just. "In a situation where there's no other water available," he says, "any little bit helps."

The green tree frogs' technique may be more common than people suspect, says Harvey Lillywhite, a physiological ecologist at the University of Florida in Gainesville. Once, he lifted a rock on a warming day in arid Australia and found several geckos covered in tiny drops of condensed water. Other animals tap the air for a cool drink in similar ways. Beetles living in the Namib Desert of Africa, for instance, scuttle to the tops of dunes and then stick their butts in the air to trap passing fog. "But this paper actually quantifies the water gain and shows that the animal makes a water profit," Lillywhite says.

But environmental physiologist Stanley Hillman of Portland State University in Oregon says that if the frogs do mine condensation, it might be useful only in certain circumstances. The green tree frogs, which may or may not be good judges of weather, gain more water than they lose only under a narrow range of conditions, he says. If their tree hollows are too cool or the outside air too hot, they could wind up going thirsty. In the dry season, losing water can be death: "It would be a mistake you wouldn't make often," Hillman says.