Thirsty on a hot day? Most of us would reach for a glass of water to gulp down. But what if you could drink through your skin? And what if there isn't any water lying around because it's the dry season in the North Australian tropical savannah?
The green tree frog Litoria caerulea has figured out a clever way to virtually squeeze water out of thin air. Researchers from Australia's Charles Darwin University led by Christopher Tracy were intrigued by how frogs could still remain active even on cool nights, when their body temperatures (being cold-blooded animals, or poikilotherms) could plummet to as low as 12.5ºC. At those temperatures they are sluggish and so are unlikely to be hunting for food. Instead, the scientists thought they might be seeking out water instead.
Pockets of warm air remain in places like tree trunk hollows. Cool objects placed in warm air will tend to condense the water vapour that remains in the air, which is precisely why glasses of cold drinks "sweat" on the outside. They placed some of these chilled frogs into hollows where they also measured the temperature and humidity. Just to be sure, they also used artificial hollows, essentially chambers filled with humidified air.
By weighing the frogs before and after, they found a small (on the order of several tenths or hundredths of a gram) but perceptible gain in mass, which was on average about half a percent of the frog's mass. It doesn't sound like much, but a cup of water is about a quarter of a percent of my total body mass. Droplets of condensed water could also be observed on the frogs after some time.
For more information (and pictures of the frogs), have a look at the press release that went with the paper.
CR Tracy, N Laurence, KA Christian. 2011. Condensation onto the skin as a means for water gain by tree frogs in tropical Australia. The American Naturalist 178 (4): 553-558.