Many arid dwellers have evolved ingenious methods of harvesting - from ingesting water vapour directly from the air, through to trapping moisture between their scales.
But for amphibians, drinking involves soaking water through their skin.
"While this may be good for absorbing moisture from the atmosphere in humid environments, it works in reverse too: frogs lose a lot of moisture through their skin in dry conditions," says physiological ecologist Dr Christopher Tracy from the University of Melbourne.
"Frogs that live in arid, or seasonally dry areas, usually avoid drying out by becoming dormant in burrows or tree hollows, or digging into muddy cracks."
Now, according to a study published in The American Naturalist , a chance observation has shown that green tree frogs have another trick up their sleeve.
"Green tree frogs remain active without access to free water for months on end," says Tracy.
An unrelated radio tracking project showed frogs moving from their protected tree burrows to perch on twigs or termite mounds, even on cool nights.
"We wondered why they would come out on nights that were so cold they could hardly move," he says. "At 13°C their metabolic rate, physiology, reaction times, even their ability to hop is barely 20 per cent of what they can do when it's warm.
"Even though they're what is known as sit and wait predators, it's unlikely they would have had the energy to forage, so we wondered if they were going out to get a drink."
Condensation effect
The researchers decided to test an idea from the 1960s that some species can gain water from condensation when they return to their warm hollow.They chilled frogs either in an ice water bath or by allowing them to cool to the ambient air temperature, then weighed them in a basket and lowered them into a large tree hollow that was normally shared by up to eight frogs each day.
The temperature at various depths inside the hollow, and of the frogs themselves was recorded. After 15 minutes the now-glistening frogs were retrieved and weighed again. The process was repeated in the lab using an artificial hollow.
Tracy and colleagues found that in both hollows water condensed on the cool frogs, especially on their heads and backs. They then absorbed the water through their porous skin, resulting in a weight gain of up to 0.4 gram, or close to one per cent of their body mass. Further calculations showed that the water gained by the frogs exceeded that lost to evaporation caused by activity.
The amount of water gained was also related to the temperature difference between the frog and its hollow: the greater the difference the more dew formed.
"In the dry months the frogs tend to congregate in larger tree hollows that remain warm and humid over night, while during the wet season, when they don't need to make their own water, smaller tree hollows that don't have the same thermal performance tend to be used more frequently."
Other species?
He says the green tree frogs are the only species known to use this strategy."The next step is to work out just how important this method is to other frog species as well," says Tracy. "Obviously in the wet tropics the humidity and lower temperature differential means this strategy may not be used."
"However in drier areas, where this may be the only source of water in the dry season, changes to habitat that result from either local land use practices or global climate change may make getting water by this method impossible.
"This could potentially have a big impact on the distribution of even a relatively hardy species like the green tree frog."
Rachel Sullivan
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