A seal’s fur consists of two kinds of hair: protective hairs on top and smaller, much softer hairs underneath. Water does not easily seep through onto the soft hair underneath, which keeps the skin dry and the cold out. The make-up of seals’ fur differs between the species. For instance, fur seals have a much fuller undercoat, which forms an air bubble around their bodies when they dive. Alternatively, common seals have a lanuga, a long, woolly, white fur, which they shed in the womb before being born. Grey seals, on the other hand, keep their white baby fur during the first weeks of their lives, which makes it impossible for them to swim. It is not until they exchange their baby fur for their adult pelt, that they become good swimmers.
During their first year, young seals have softer fur because their hairs are thinner. After their first moult, their hairs become thicker, making their fur bristlier.
Seals exist all over the world: From the cold Antarctica to the Mediterranean Sea. It is important therefore that seals can withstand extreme temperatures. Their fur only has so much insulating power and even under water this is limited. Especially cold temperatures demand huge adjustments of the body. Therefore, seals have a layer of fat (blubber), which is up to a couple of centimetres thick, to keep them warm. This blubber surrounds the entire body, except for the (protruding) front and back flippers.
Seals use their protruding limbs as thermal regulators. Especially the back flippers are useful for this, because they can be spread out in such a way that the webbed flippers are exposed to the air. So if seals get too warm, they can send more blood to their back flippers, spread them and the wind or water will cool the blood . When they feel the cold, they curl up their back flippers, making them as small as possible.
Seals lose a lot of warmth through the cold water, but thanks to their body, they can adjust their temperature. The blood supply to the protruding limbs is fitted out with a ‘heat regulator’, which works like this: Blood circulating to the flipper is cooled by blood circulating back in the opposite direction. So, blood that goes into the cold flipper is already cooler and saves energy. Also, cold blood that leaves the flipper is warmed up by warm blood travelling in the opposite direction. Seals always have cold feet, which is why they try to hold their flippers above the surface as soon as the sandbanks begin to fall dry: They lose less body heat and warm up their flippers in the sun.