Cool frogs collect water, according to articles in ScienceNOW 2 September 2011 and ABC News in Science 9 September 2011. Green tree frogs living in northern Australia are able to survive long dry spells by hiding away in tree hollows or burrows and going into a torpid state. Australian scientists, who were conducting a radio tracking study, were surprised to find the frogs were going out at night and sitting for long periods on branches and termite mounds. It can get quite chilly in the Australian savannah regions at night, which meant the cold-blooded frogs could only function at a low metabolic rate, so they could not be active enough to catch any prey.

The scientists decided to test a theory that amphibians were collecting water as they moved back into their warm humid hollows, using the same principle that results in water collecting on a cold drink can when it is taken out of a fridge. The researchers chilled some frogs and then placed them into a tree hollow. When the cool frogs were returned to their warm, moist hideaway the researchers saw that water droplets formed on the frogs’ skin. Amphibians have porous skin so they can absorb water directly through their skin. To check that the frogs had actually gained water by this process the scientists weighed them before and after, and found they had gained more water than they would have lost from crawling out to get cool. The hollows can be 10 degrees Celsius warmer than the outside temperature and can reach relative humidity of 90 to 100 percent. The bigger the difference in temperature and humidity inside the hollow compared with the outside environment the more water the frogs can collect. Christopher Tracey, one of the research team commented: “In the dry months the frogs tend to congregate in larger tree hollows that remain warm and humid overnight, 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”.


Editorial Comment: Here is a very nice application of the laws of physics and chemistry, but how did the frogs work it out? This kind of behaviour needs some pre-programming to work. The frogs have to know when to go back to their warm burrows before they become paralysed by the cold, but not before they are cool enough to capture water back in their humid frog hollow. By deliberately allowing their body temperatures to go down and becoming slow in their movements they are making themselves more vulnerable to predators. Therefore, it is not worth doing unless they know they are going to get some benefit.

This method of getting water would have worked well in the original very good world, where the air was moist and the earth was watered by dew and mist. Frogs could cool down at night, stay out of the sun until the environment warmed up and then move out into a warm place and collect the moisture forming on their skins. In a very good world there were no predators. The frogs get away with such behaviour now simply because in the Australian savannah, there aren’t many predators. We wonder how many frogs have died trying to do this in other environments where there are more predators? (Ref. amphibians, dew point, hydration)

Evidence News 16 November 2011