Lead author Dr Dominic McCafferty admitted, "We didn't think a warm blooded animal could ever have an outer surface temperature that was colder than its surroundings".
A team of researchers from Scotland and France took thermal images of penguins at the breeding colony in Terre Adélie, Antarctica. During clear sky conditions most outer surfaces of the body were colder than surrounding sub-zero air due to radiative cooling.
Dr McCafferty, from the University of Glasgow, explained, "This occurs because emperor penguins are losing very large amounts of heat by radiation to the clear sky (the clear atmosphere above us). The sky has a temperature that may be more than 20°C colder than surrounding air and therefore the outer plumage temperature is effectively influenced most strongly by the cold sky rather than the surrounding air.
"A similar phenomenon can be observed if you park your car in the open on a cold night. Often you will only find frost to have formed on the roof and windscreen as these areas 'view' the night sky unlike the car's sides that are radiating to warmer surroundings.
"Paradoxically if penguin plumage is colder than air then it should start to gain heat from the warmer air by convection . However, for Antarctic penguins their plumage is so well insulated that any heat gained will not be transferred to the skin surface. In this way the penguin will remain warm on the inside but the outer surface of its plumage will be freezing.
"The extreme radiative cooling doesn't keep them warm but the insulation provided by their thick plumage does and the fact that they have only small areas of the body that are not covered in feathers means that there is only a small area for heat to escape. The pure physics of heat transfer mean that Emperor penguins literally have no choice other than to be the coolest birds on the planet."
McCfferty, D. et al. Emperor penguin body surfaces cool below air temperature, Biology Letters.
Provided by The Royal Society
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