Seals and Walrus

Seals are more at home in the water than they are on land, even although they may spend long hours lying on the rocks or on the beaches. To get their food they must be capable of deep dives, and even a baby seal, on its first dive, may go down as much as 70 feet, Adult seals can go much deeper, to 300 feet or more, and seals have been caught in nets as deep as 600 feet. This means they must be able to stay under water for up to 20 minutes. They are enabled to do this as a result of several things. First, they can carry a high proportion of oxygen in their blood. Secondly, they can carry a large amount of carbon dioxide without inducing them to breathe: in land animals, including ourselves, it is not shortage of oxygen but excess of carbon dioxide that makes us gasp for breath. Thirdly, seals have a mechanism for cutting down the supply of blood, while underwater, to all but the essential organs, such as the brain, heart and flippers. At the beginning of a dive a seal's blood contains 20 per cent oxygen, and the animal does not need to re-surface until this has been dropped to 2 per cent. While underwater its heart beats at the rate of 10 a minute, but on the return to the surface the heart beat rises to 150 a minute, and then a few deep breaths wash out the excess carbon dioxide.

All this affects a seal's behaviour on land as well, because its respiratory system is geared to deep diving. So on land it may go for up to ten minutes without breathing and this is followed by periods of 3 to 4 minutes during which only twenty short breaths may be taken. In effect, then, a seal must be, in a manner of speaking, less than half awake when underwater, and able to slow down its faculties while on land. If we were experiencing these conditions we would probably say we were half-asleep.

Remembering these things, it is not surprising to find that seals sleep soundly. A seal pup can be lifted up without waking it, and one scientist found he could lie down on a sleeping elephant seal without waking it. Gray, or Atlantic, seals can be seen asleep in the water, hanging vertically with just the tip of the nose out of water. Or they may sleep underwater, rising periodically, while still asleep apparently, to take a breath and then sinking again to continue their slumber. The northern fur seals in the Pacific Ocean come to islands off Alaska to breed. After that they spread out over the northern Pacific to spend most of the year at sea, hundreds of miles from land. How they behave there is difficult to study but there can be little doubt that they sleep like gray seals, either suspended at the surface with only the nose exposed or by sleeping underwater and coming to the surface in their sleep to take a breath.

SLEEPING WITHOUT BREATHING

Elephant seals in the subantarctic regions spend much of their time on land. They gather in muddy depressions in the ground, where a number of them will lie on top of each other in a pyramid to sleep. These wallows fill with water at times and then those underneath will be sleeping underwater. From time to time they must struggle out from under the pyramid for air, and there are times when one or more may fail to do this and will be killed by suffocation. These habits enable us to see how long a sea-elephant can sleep without taking breath. It may be submerged in water and under a pile of the heavy bodies of its fellows for as much as a quarter of an hour before coming up for air.

LIFE UNDER THE ICE SHEET

The Weddell seal, about 10 feet long with a disproportionately small head, lives all round the shore of the continent of Antarctica and the adjacent islands. It feeds on small fish and squids; and it does not migrate, and it spends much of its time in water, only occasionally hauling out onto ice floes, even during the southern summer. So during the winter, when the seas around Antarctica are covered with a continuous sheet of ice it must feed and sleep below the ice. To breathe it visits cracks in the ice, pushing its nose above water. Should these become frozen in the Weddell seal opens breathing holes in them, by sawing with its teeth.

Until a few years ago it was not known how they slept. Then British scientists in the Antarctic found that they slept at the breathing holes, hanging more or less vertically in the water, with the tip of the snout just above the water-line.

There is still much to be learned about the habits of this seal, but we can reasonably suppose they are capable, like the elephant seal, dealt with in the previous article, of holding their breath, so to speak, for long periods of time, long enough to enable them to hunt beneath the ice yet reach a breathing hole before the air in their lungs is exhausted.

BALLOON-NECKED SLEEPER

Another of the fin-footed marine animals with peculiar sleeping habits is the walrus, which is something of a 'mystery' animal although it is so well-known. We would like, for example, to know more about how it uses its tusks for digging up food from the seabed. But direct observation of a large animal underwater in cold seas is not easy. Then again, it is always said confidently that a walrus uses its tusks to haul itself onto ice floes, but this is founded on very few records and it may eventually be found that the animal does so only occasionally.

Walruses are not easily kept in captivity, and this accounts for some of the gaps in our knowledge. The first living walrus was brought to England in 1608, but this lived only a few days. Not until 1853 was another brought here, to the Zoological Gardens in London; and that lived only a few days. They have been kept more successfully in zoos in Denmark and the United States, enabling studies to be made on their growth and development. From these we know that a walrus sometimes sleeps vertically in the water with its nose in the air, and that when it does this its neck seems to be swollen. In the opinion of one scientist this is because it is able to blow its gullet out like a balloon, which is almost as good as wearing a lifebelt round its neck. If this is the case, it can only be supposed that the walrus does this by deliberately swallowing air. But we have to wait for further research for the answer to the walrus' method of taking its slumber.