The seal is a diving machine wrapped in blubber.

The streamlined body

A seal is built from head to tail to move through water with as little resistance as possible. The body has the classic torpedo or spindle shape: narrow at both ends, wide in the middle, with no protruding parts to disturb the flow. In true seals (family Phocidae) — to which the common and grey seal belong — there are no external ear flaps: the ear is a simple opening in the skin that closes under water. Male genitalia are internal as well, and the females' nipples retract into the skin when not nursing.

The hind flippers point permanently backwards and cannot fold under the body. That makes a seal awkward on land — it shuffles and inches across the sandbar — but in the water it is the engine: by swinging its hindquarters from side to side in the horizontal plane, it generates thrust. The fore-flippers are used mainly for steering. With eared seals (Otariidae, like sea lions and fur seals) it's exactly the opposite: they swim with their fore-flippers and can rotate their hind flippers forwards, which is why they can "walk" on land.

The blubber

Beneath the skin lies a thick layer of white fat tissue, the blubber. In adult Dutch seals it is up to about five centimetres thick on the trunk, thinner on the head and flippers. The blubber serves three purposes at once: insulation against cold North Sea water, streamlining (smoothing out all cavities and muscle bundles), and — no less importantly — energy reserve.

That reserve is essential. During the annual moult and the nursing period, seals barely eat. A common-seal female loses up to a quarter of her body weight during the three to four weeks of nursing, almost entirely from her blubber. For pups it works the other way around: in the same period they build up a layer of fat thanks to the extremely rich mother's milk.

A seal stores oxygen not primarily in its lungs — which it empties before diving — but in its blood and muscles.

Muscles as a dive tank

A diving seal has a trick no scuba diver can match: it carries oxygen in its tissues themselves. The muscles contain a high concentration of myoglobin, an oxygen-binding protein related to haemoglobin. The blood, in turn, carries more haemoglobin than that of land mammals, and the blood volume per kilo of body weight is larger. As a result, despite its relatively small lungs, the seal can handle long dives: the average dive lasts a few minutes, but exceptional dives can reach around twenty minutes.

At the same time, the breathing reflex is weaker than ours. A seal that stays under for too long does not immediately get the panic signal to bolt for the surface. That sounds risky, but it's precisely what makes long dives possible. During a dive the heart rate also drops sharply — see the page on behaviour for the details of that dive reflex.

The senses

For an animal hunting in murky water, sometimes at night or in winter mud, the senses are not merely supportive — they are decisive.

• Whiskers (vibrissae)Follow hydrodynamic fish trails
• EyesLarge pupil, lens tuned for underwater
• HearingWorks above and below water
• NostrilsClose actively when diving
• Taste / smell above waterLimited role in hunting

The whiskers, or vibrissae, are the most striking sense organ. They are movable and connected to a large area of the brain. Experiments with blindfolded seals in laboratories show that with their vibrissae they can follow the hydrodynamic "trail" — the tiny eddies left behind by a fish — for minutes after the fish has passed. In the murky Waddenzee, where visibility is often less than half a metre, that's indispensable.

The eyes are conspicuously large for the body size. The pupil can dilate enormously in dim underwater light, and the lens is spherical to focus underwater — in humans the lens does its main work in air. Above water, seals see less sharply but well enough to recognise movement at a distance. Hearing works in both air and water, with separate adaptations for each medium. The nostrils are closed at rest and open only to draw an active breath — no muscular effort is needed to keep them shut, only to open them.

Fore- and hind flippers

A seal's flippers are, in evolutionary terms, fingers and toes. On an X-ray, the five finger bones are still perfectly recognisable; they are simply enclosed in a flap of skin. The fore-flippers carry claws — the seal uses them to scratch an itch, to push itself along the sandbar and sometimes to grip prey. The hind flippers have smaller nails and serve mainly as muscle propulsion. In the common seal the fore-flipper claws are substantial; in the grey seal at least as striking.

Coat and moulting

The coat consists of short, stiff guard hairs without a dense undercoat. The skin beneath is waterproof; the coat itself is not really, and acts more as a covering layer than as insulation — that role is filled by the blubber. Once a year, both species moult: the common seal usually between August and October, the grey seal mainly in spring. During moulting they spend a lot of time on the sandbar, because the skin receives extra blood flow to nourish the new hairs and cold water then drains away heat especially fast. Disturbance during this period is therefore more costly than people often realise; more on that on behaviour.

Phocidae versus Otariidae

The worldwide group of seals breaks down into two large families, plus the walrus. The difference is more than just the ear flaps.

The Netherlands is home only to Phocidae. Anyone who sees an animal in a zoo "walking" on four flippers with visible ear flaps is looking at an eared seal — often a Californian sea lion. The Dutch seals in the wild cannot do that. Anatomy thus directly determines what the animal can and cannot do: how it hunts, how it behaves on the sandbar, and how you tell the two species apart in the field.