What’s special about seals?
Seals are supremely adapted for living in the sea. Like other marine mammals such as whales and dolphins, they can dive to great depths for long periods of time in search of prey, their bodies are streamlined to move efficiently in water, and they have blubber to protect them against the cold. But unlike whales and dolphins, seals spend some time out of the sea. They give birth on land – their pups would drown if they did otherwise – and they shed their skin annually on land. Seals can also move about quite adequately, although rather inelegantly, on land.
Limbs and movement
A seal has much shorter limbs than most mammals: what appear to be the armpit and groin of a seal are, in fact, the equivalent of the wrist and the ankle. By comparison, the bones of their flippers are enormously long, and the skin between them forms a web which is used like a paddle to propel the seal along. They have long, sturdy claws on their front flippers which they use to help them move on land, especially when they need to grip onto rocks or ice.
When a seal swims quickly, it holds its front flippers tightly against its sides, and propels itself with its powerful hind flippers. Its lower body moves from side to side, rather like a fish, as it moves along. When the seal is swimming slowly, the front flippers are used as stabilisers and stick out to the sides.
On land, a seal moves with a ‘hitching’ action. It forces it weight onto its chest, and then stretches its back to swing its rear end forward. The weight is transferred to the pelvis, and the chest is thrown forward. It is an inefficient way to move, and has made them vulnerable to hunting by humans. On ice, however, the seals are far more limber. Ribbon seals and leopard seals, that live in the Arctic and the Antarctic respectively, can move faster than a human can run, by flailing their hind flippers vigorously.
Heat and cold
Most mammals need to maintain a body temperature of 37°C (98.6°F). The sea is much colder than this, and conducts heat away from a warm body much faster than air does. Seals have dealt with this in a number of ways. They have a hairy coat, which traps air and uses it as an extra insulating layer. However, when a seal dives, the pressure forces the air out of the fur, so this only works when the seal is on land or near the surface. Far more important is the layer of fatty tissue beneath the skin, called blubber.
Sometimes, seals are so well insulated from the cold, that they overheat. Seal watchers occasionally see the seals waving their flippers in the air when they are hauled out on the land. This is no idle movement. Flippers have a large peripheral blood supply that allows the heat to escape. The waving movement is to increase the heat loss and so cool the animal down. The inventive, intelligent seal often finds other ways of keeping cool, seeking the shade of a rock or cliff or lying in pools of water.
Sight: to be efficient underwater hunters, seals need to detect and catch prey. Since very little light penetrates at great depths, the eyes of seals are specially adapted to allow them to see underwater. The eyes are especially large – one of the endearing feature of pups – and the lens is structured to allow as much light in as possible. When on land, the eye is protected from bright sunlight by closing the pupil. Thus seals can see well both underwater and on land. Sight is probably more important on land than in the water, and anyone watching common seals will notice that they raise their heads regularly to look for danger.
Hearing: the ears of the seals are also adapted to allow them to hear underwater as well as on land. The bones of the middle ear are larger than in land mammals, and there are changes in the shape and size of other bones in the skull. Sensitivity to sound helps them to detect prey underwater. It has been suggested that seals echo-locate, like whales and bats. Common seals are known to make clicks and trills underwater. It could be, however, that they are simply talking to each other.
Touch: when water is especially dark or murky, seals cannot use their excellent eyesight to help catch their prey. They have, however, sensitive whiskers called ‘vibrissae’ that grow on either side of the snout, above the eyes, and on top of the nose and are thought to detect vibrations in the water caused by moving prey. There are cases of blind seals surviving for a number of years in the wild, suggesting that for fishing the whiskers are more important than sight.
Smell: a sense of smell does not work in water for seals. If you watch a seal, you will see that it closes its nostrils tightly before diving, to prevent seawater from irritating the delicate membranes in the nose. But the nose-bones in the seal are large and quite complex, suggesting that a sense of smell is important on land. As soon as a pup is born, it and its mother sniff at each other. Not only do they recognise each other by their individual call, but by their individual smell. Basking seals often raise their heads and sniff at the air.
Diving and feeding
Seals have been known to dive as deep as 4,100 metres (13,450 feet) and can remain submerged for up to an hour. Grey and common seals, however typically dive to a maximum depth of around 200 metres (655 feet), for periods of up to 15 minutes, although they can go deeper and for longer.
It is a pity that most people only see seals flopping about in their ungainly way on the shore. There is little that compares with the grace and speed of a swimming seal. Whether alone or in a group, the movements of a seal in its true environment are like an underwater ballet. It can swim at speeds of 20 knots (around 23 miles per hour) when it is pursuing its prey, although most of the time it cruises at about two or three knots.
Both grey and common seals eat a variety of prey – fish, shellfish, squid and octopus. They are opportunistic feeders, and will eat whatever is available, including cod, herring, flounder, sculpin, salmon, mackerel, sandeel, shrimp and whelk. Their fishy diet often brings them into conflict with Scottish fishermen. (Discussed under ‘Seals and Fishermen’)