A layer of fat, called blubber, lies just und er the skin, surrounding the main body. The fat near the skin may be very cold, but it forms an insulating barrier that keeps the inner body, like the heart and other major organs, warm. Flippers do not have blubber, and may be quite cold. Seals do not lose much heat from their flippers because the warm blood going out to the flippers loses its heat to the cold blood returning to the body. This is called counter current exchange.
Seals are carnivores and eat a lot of food for energy to keep warm and to build blubber.
Diets vary from different kinds of fish to squid and crabs, and even penguins in the case of the leopard seal. While hunting, a seal is able to close off the back of its throat, allowing it to open its mouth and chase fish without swallowing water. Sharp teeth are made for grabbing and tearing but not chewing prey. Small fish are swallowed whole, head first so that the scales slip easily down the throat. The teeth of the crabeater seal are more specialized, though.