Penguins are warm blooded animals and they maintain internal body temperatures similar to warm blooded animals in any other climate zone – that is about 35–42 °C (95–107 °F). Tropical animals with more variable body temperatures such as reptiles and amphibians can warm up by basking in the sun if they cool down and they never cool down that much. A large (bigger than a small insect) Antarctic animal will never get enough energy from the surroundings to become active if it allows itself to cool, so they have to stay warm to be active.

Warm blooded animals in cold climates are pretty large. Even the smallest Antarctic birds are on the large side and the smallest Antarctic penguin, the Rockhopper is 2.5kg. The Adelie and Emperor penguins of the deep south are larger still. Adult weights are 5kg for the Adelie and 30kg for the Emperor – a similar size to an overweight 10 year old child, but with a man–sized chest measurement. The larger the animal, the smaller the surface-area volume ratio and hence a less relative area of its body is exposed. This helps the penguins to stay warm.