Why Are Seals Semi-Aquatic Mammals?


Seals spend some of their lives in the water and some on land and are classed as semi-aquatic mammals. I was recently on a seal-watching trip, and they seem at home lounging on a rock as they do swimming in the water.

Seals are semi-aquatic because they are adapted to spend time on water and land. They are excellent swimmers and divers and have remarkable adaptations to the cold water. Seals can often be seen in haul-out spots resting their large bodies on land.

Seals have a streamlined shape which they need to live in the water, deal with the cold, and swim efficiently. Seals have changes to the layout of their bodies that allow them to spend a lot of their time below the surface and diving while also allowing them access to land.

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Seal

Life on land

Unlike many aquatic animals such as whales and dolphins, seals come onto land, making them semi-aquatic. Whereas whales and dolphins give birth and feed their young while in the water, seals cannot. Seals have to come on to land to give birth and feed their young.

Seals will give birth on rocks, ice, or sand, and it is for this reason that they are semi-aquatic.

The harbour seal will give birth on land such as sand or rocks, but the pups need to be ready to swim within a few hours of delivery. Harbour seals are born with fully developed hind flippers and adult coats. Suckling occurs on land but can also occur in the water, and they spend a lot of time in the water in the first few weeks.

While harbour seals are happy to spend time in the water so quickly, grey seals are born with little fat and white fur, which becomes wet quickly. They will have trebled their size within the first few weeks, putting on enough layers of fat that they are ready for the cold waters.

Seals will spend time on land to give birth, suckle their young, and for other reasons. Seals will return to their breeding grounds and find other lands to rest. If they are tired, then seals can often be seen lying around on rocks called haul-out sites.

Grey seals moult three or four months after breeding, and their skin can become sensitive while the old fur is shed and the new coat is being grown. It has been suggested that seals will stay ashore in this period to keep heat loss at a minimum and to increase their comfort.

Seals can sleep at sea, but they use haul-out spots to rest. When sleeping at sea, they sink, so they have to swim to the surface to breathe, so resting on land helps them conserve their energy. When I went seal watching recently, I noted that they all seemed to be laid out on the shadiest sides of the rocks. This would stop them from getting too hot while resting.

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Swimming

Seals are excellent swimmers, and their flippers allow them to propel themselves forward quickly. Although seals are heavy and contain a lot of fat, a seal’s body is streamlined and efficient.

The body is streamlined using fat, which rounds out their cylinder shape and makes them hydrodynamic.

Because they are cylinder-shaped, they have no external protuberances to cause drag while swimming. Phocid seals have no external ears, and otariid seals have tiny flaps. The mammary glands and nipples are inverted, as are the male genitals. The nipples protrude when pups nurse while on land.

Although they are all excellent swimmers, the two types of seals swim in different ways. Phocid seals use their hind flippers to create strong sideways movements to push themselves forward. They push themselves from the rear, pushing against the water with their back end while using their fore flippers for steering.

Otariid seals use their fore flippers to force themselves forward and drive from the front. Although excellent swimmers, seals can look ungainly when on land. However, they can reach good speed on land, especially when going downhill or over slippery rocks. Don’t get too close to one on land because they can soon bowl you over before you know what is happening.

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Seal

How seals cope with the cold

Seals are exceptionally well adapted to living in cold water, and they have some modifications from land-dwelling mammals.

Heat is lost from the body surface in contact with cold water, so seals have evolved a couple of ways to keep the heat loss as low as possible. Because seals are so large, they reduce heat loss by having a favourable ratio of the outer surface area to the inner volume. Being rotund and streamlined with few protruding limbs can minimise heat loss.

Seals also have a large amount of insulation. The layer of fat or blubber is beneath the skin and can be up to 10cm (4in) thick, enough to keep them warm in arctic waters. While the layer of fat is generally the same thickness around the body, the limbs and head typically contain a thinner layer.

Seals have a layer of fur, with fur seals containing a thick fur coat but lower amounts of fat. Fur doesn’t insulate the seal as well as blubber, especially in dives over a few feet. As the fur flattens against the skin, its insulation is less, whereas the blubber doesn’t change.

Diving

To survive, seals need to catch fish, so they need to dive to catch them. Grey seals at sea can spend 80% of their time below the surface. If you spot a grey seal at the top of the water, it will usually be for less than a minute before it dives again. They typically dive for between four to eight minutes, and they do this by taking large amounts of oxygen with them. Seals cannot breathe underwater.

Seals do not carry oxygen in gas form as this would cause problems with buoyancy and could also cause problems when resurfacing. The bends are caused when bubbles of nitrogen form in the blood as the pressure declines towards the surface.

Instead, seals carry their oxygen attached to chemical compounds called pigments. Haemoglobin and myoglobin in the blood carry oxygen, giving them enough to survive while making their dives. Because seals have more blood than many terrestrial animals, they have an ample supply of oxygen.

As they only spend less than a minute at the surface, they need to get rid of the carbon dioxide quickly to make room for more oxygen. When seals surface, their heart rates are almost triple the rate when diving. A seal’s heart rate at the surface can be up to 120 beats per minute, while diving is about 40 bpm.

These large oxygen stores and increased heart rate equip seals for many short dives for hours. However, seals can also complete longer, deeper dives, and to do this, they have to make some physiological changes.

Seals can reduce their heart rates to just 4 or 5 bpm when needed if they think they will be diving for a long time or when frightened. Grey and harbour seals can stay underwater for up to 30 minutes in these circumstances, but they need extra time on the surface to make up for these extended dives.

Seals seem to be able to dive as deep as they need to. The northern elephant seal can dive to over 3,288 (7,835 feet) for 60 minutes.

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Bones

Although a seal’s skeleton contains the same essential bones as most other mammals, it also reflects their need for swimming. The skull has a large cranium, with huge eye sockets and a snout shaped according to their needs.

The spine is similar to other mammals, although they only have twenty vertebrae. They have a long chest section and a short tail. Seals have short and flat limbs, with their flippers and tails being about the same length.

These are enclosed within the body, so the working parts we can see are only from the wrist and ankle. The hand bones are covered in a hair-covered, fleshy pad, while the hind flippers have strong skin to help propel them forward.

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