The short answer is that a penguin cannot allow its feet to freeze, says Mike Toms.
When the ambient temperature falls below 0°C a penguin increases the circulation of warm blood to its feet. But, while this prevents the feet from freezing, it also increases the heat loss.
Balancing the need to keep the feet warm against the problem of heat (and therefore energy) loss from the body is a key issue in penguin thermoregulation.
These birds are well insulated, with a well-defined fat layer and a waterproof coat of specialised feathers.
They also have a highly developed vascular system. complete with counter current heat-exchange systems for their extremities. Both the flippers and feet are a major source of heat loss, so the birds heat-exchange systems work by passing warm arterial blood from the body through fine arteries in close contact with veins returning from the feet and flippers. The warm arterial blood is cooled before it reaches the extremities, while the returning blood is warmed before it can chill the body.
Though this combination of adaptations works well in the cold high latitudes, it could cause penguins to overheat where temperatures are higher. This is why species in warmer places, such as South Africa, have large flippers and bare areas on the face.
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