London: Emperor penguins keep their body`s surface temperature lower than the surrounding air -- in an unusual phenomenon termed as `cold coat` -- to cope with the harsh conditions of the Antarctic, scientists say.
By forming this `cold coat`, Penguins avoid heat loss due to thermal radiation and maintain a body temperature of around 36.9 degrees Celsius, even in a climate where temperature can drop to minus 40 degrees Celsius.
Using thermal imaging technology on penguins, scientists from Glasgow`s Institute of Biodiversity, Animal Health and Comparative Medicine and the Universite de Strasbourg were able to determine that the surface temperature of the birds was four degrees cooler than the surrounding air.
The only parts of the penguins warmer than the air were the eyes, beaks and feet -- and only the eyes were warmer than freezing.
Moreover, as the plumage is colder than the surrounding air, computer simulations showed this `cold coat` may gain them back a little heat from the warmer air circulating around them.
Dr Dominic McCafferty of the University of Glasgow explained the phenomenon. "At first, we were very surprised by this discovery. But after analysing the data we realised the key to this is the temperature of the sky. The sky has a temperature that may be more than 20 degrees colder than the surrounding air," Dominic said.
The temperature of the plumage is therefore effectively influenced most strongly by the temperature of the sky rather than the surrounding air.
"A similar phenomenon can be observed if you park your car in the open on a cold night. Usually you will only find frost to have formed on the roof and windscreen but the sides do not `view` the sky and therefore are radiating to relatively warmer surroundings," the doctor explained.
The same phenomenon explains why fruit can be damaged by radiative frosts when the air doesn`t drop below freezing.
The difference is we didn`t think a warm-blooded animal could ever have an outer surface temperature that was colder than its surroundings.
"Paradoxically if the penguin plumage is colder than air then it should start to gain heat from its surroundings : think if you were to go into a warm room after being in the cold, your outer layers of clothing would start to warm up from the outside," said Dominic.
"However, Antarctic penguins are so well insulated; insulation equivalent of us wearing two ski suits, that heat will be transferred very slowly to the skin surface. In this way the penguin will remain warm on the inside but the outer surface of its plumage will be extremely cold," he added.
The findings are reported in the latest edition of the journal `Biology Letters`.