Grey seal moves in for a close up Photo: Jane Morgan Seals often play with divers fins Photo: Jane Morgan A harbour seal at Loch Carron Photo: Sue Scott Grey seals have long straight noses Photo: Jane Morgan Greys can grown upto 2m long Photo: Jane Morgan Grey seal nips at divers ankle weights Photo: Jane Morgan Grey seal at the surface Photo: Jane Morgan A Harbour seal basks in the sun Photo: Sue Scott

Halichoerus grypus & Phoca vitulina

Seals are warm-blooded mammals that have taken up a largely aquatic life. Their distinctive blubber-slug shape is their own version of a diving suit - a thick layer of insulating fat to keep them warm in the sea. So well does this work, that seals hauled out on a sunny day are in danger of overheating, and often wave their flippers, which have a rich blood supply, to cool down. Their fur has little insulating effect underwater, but helps to keep them warm on land by trapping a layer of air, and the colour and pattern helps to disguise them - a seal ashore does an excellent impression of a boulder! Seals moult their fur and skin once a year, which gets rid of external parasites, and they usually haul out until their new hair has grown.

Like other marine mammals, a seal's flippers are legs modified for swimming. However, unlike whales and dolphins, seals return to the shore to breed, so they still need to be able to move on land. They do this in a most ungainly way, and apparently with great effort, heaving their heavy bodies along, with fore and hind flippers like bloated caterpillars. Long claws help to grip slippery, seaweed-covered rocks. Their clumsiness on land contrasts with supreme ease and elegance underwater, and it's obvious where they are most at home.

Seals come ashore once a year to give birth to a single pup, and also to mate for the next season's brood. Seal breeding grounds can be chaotic and noisy places. Mothers must learn to recognise their newborn pups by smell and sound in the first few minutes after birth, or they may abandon them. Pups are at risk of being trampled by bull seals intent on fighting rivals and mating, and in a crowded breeding site, 15 per cent of pups may die. Common (or harbour) seal pups can swim with their mothers only a few hours after birth, and the mother suckles her pup on fat-rich milk for up to six weeks. Grey seal pups are far more helpless and the mother has to remain ashore with them, suckling them intensively for about three weeks. During this time the pup gains around 30kg, while its mother loses around 65kg and is eventually forced back to sea to feed. Unlike common seals, grey seal pups are born with pure-white coats, thought to be a hangover from the time when they were born on ice, when white was the best camouflage against predators.

Seals catch their food, mainly fish, by diving, often spending five to ten minutes underwater, although they can manage at least half an hour. Seals hyperventilate before diving, but expel most of the air from their lungs before submerging, so they don't suffer from the bends even when diving to great depths. Seals carry a much greater supply of oxygen in their blood than land mammals. As well as having proportionally much more blood, they also have more haemoglobin, so can carry between three and five times as much oxygen per unit of body weight as humans. Seal muscle also has much more oxygen-carrying protein. There are also huge modifications to their system of blood vessels. When on a prolonged dive, blood flows mainly between the heart and brain, and the seal's heart rate falls from around 60 beats per minute to fewer than 15, so their supply of oxygen is used far more economically. Sure beats lugging a heavy twin-set!

KNOWN HAUNTS
Grey seals prefer remoter, more exposed coasts, and live mainly around western and northern coasts and islands. Harbour seals prefer more sheltered rocky shores, and sand banks in the east coast estuaries.

BEST PLACE TO SEE
At the Farne Islands in Northumberland grey seals are particularly tolerant of divers. Both grey and harbour seals are often seen on dives off the west coast of Scotland.

LIKELY TO APPEAR
Harbour seals inhabit the same general areas of coast all year round. Grey seals are best seen near their breeding sites in autumn to early winter, and while moulting a month or so later.

DISTINGUISHING FEATURES
Seals are easily distinguished from other marine mammals by their size, shape, and dog-like faces. Harbour seals have a generally more delicate, Labrador-like face, with small nostrils in a V shape. Grey seals have longer, straight or convex noses, and much larger nostrils that are nearly parallel.

SIZE
Grey seal bulls grow up to 2m long and 230kg in weight, and cows to 180cm and 150kg. Harbour seals grow to 170cm and 120kg, with cows only slightly smaller than bulls. Seals of both species often live for more than 30 years.