Master diver of the oceans

If the beautiful Sedna had not resisted her father’s wish to marry her off to a black raven, there would be no seals in the seas today. The gruesome details of this mythical origin story of the seals are still told by the Inuit in their frosty hunter camps on the pack ice.

According to legend, the vain girl Sedna spurned every suitor until her father bitterly ordered that the next hunter to come to the settlement was to be wed. Soon Sedna found herself, against her will, in the kayak of a well-dressed stranger with a veiled face, who took her to him across the sea.

The new home, however, turned out to be a sheer cliff – and the new spouse a real raven. Sedna screamed and cried so loudly and heartbreakingly that even her father could still hear the wailing. He immediately set out to rescue his unfortunate daughter, loaded her into his kayak and paddled back across the sea for days. The raven, duly enraged at the abduction of his wife, flew after the two.

With his wings he stirred up the Arctic Ocean – a great storm broke, the water seemed to boil. The father, stricken with fear, pushed his daughter out of the boat and shouted into the roaring wind: “Don’t hurt me and take her!”. Sedna clung to the edge of the kayak with her already icy cold fingers. Panicked, her horrified father swatted at her fingers with the paddle. Sedna’s fingers, almost frozen, splintered under the force of the strap and fell into the raging sea. Influenced by the raven husband’s gruesome power, Sedna’s fingers, as they sank gently down into the sea, turned into seals.

The Inuit girl Sedna, however, did not drown. She became the goddess of the sea and the mother of sea creatures. And even today the hunters of the far north between Greenland and Alaska pay her great respect. Once an Inuit has captured a seal, he dribbles some seawater down the animal’s gullet to thank Sedna for her kindness and for allowing him to feed his family.

Four legged ancestors

However, today’s science has a whole lot of sober facts to counter this eerily beautiful Inuit legend. The modern story of the origin of seals goes like this: Not from Sedna’s fingers, but from a bear-like, carnivorous land predator, the first (eared) seal formed in the North Pacific about 25 million years ago. Their relatives, the true seals, evolved a few million years later, in the middle Miocene. Its predecessor was an otter-like creature from the northern Atlantic.

In zoology, seals are found in the order of pinnipeds (Pinnipedia). As mammals of the sea, they share their wet habitat with other marine mammals such as whales, dolphins, manatees and otters. With over thirty representatives, seals make up almost a third of all marine mammal species.

But the devil is in the details – and anyone who has photographed a “seal” on a polar voyage would do well to pick out the picture and correctly identify the animal. Because seal or sea lion are terms that easily roll off the tongue and remind you of your first childhood encounters with seals in zoos and circuses. But in the Arctic or the Antarctic, these two representatives of the seals are not to be found at all (if we generously disregard the sea-lions at the coasts of South America and the Falkland-Islands)…

But species identification is not that difficult if we proceed step by step. Ears first! Can you see them, or are the outer auricles not there at all? This inconspicuous characteristic helps us to divide the seals already into two of its altogether three families – into the eared seals (with sea-lions, fur seals) and into the true seals, that do not possess visible ear-cups. They include the Antarctic crabeater seal and Weddell seal, as well as the leopard seal, elephant seal, and Arctic bearded seal, ringed seal, and harbor (common) seals.

The third family, that of the walruses, stands apart a bit. This family consists of only one species, the walrus. Although it has no visible ears, the walrus is closer to the eared seals because it moves on land on all four limbs.

Which brings us to another feature in species identification: locomotion on land. Eared seals and the walrus don’t support their body only on the front-flippers, they can hit also the rear-flippers forward under the body at the same time and go quadrupedally. True seals cannot do this; they “crawl” across the beach, making relatively slow progress.

For visitors to polar regions, the combination of seal, pinna and common beach section therefore calls for increased attention – because eared seals such as the Antarctic fur seal can quickly and “light-footedly” pursue unsuspecting beach walkers. Not to mention the jaws of this predator of the seas….

Nine months “at sea

The change between sea and land, this amphibian way of life of the seals, goes back on the land-inhabiting ancestors and remained a characteristic of the seals until today. Although they must come to the ice or land to give birth to the young or to change the thick coat, seals are excellently adapted to a life in and under water. And that’s exactly where they spend most of their lives, although the elephant seal, for example, is a bit extreme: if it’s in the sea, it spends up to 90 percent of its time underwater – it usually dives to depths of between 200 and 400 metres, sometimes even going as deep as 1500 metres.

Although we are used to seeing the seal – the marine mammal – on land or on ice, the “schedule” gives much more space to life in the water. For example, the northern fur seal stays on land for only one month per year – the rest of the time it spends in the sea. Elephant seal bulls are known to spend more than three months on land (defending their harem…), but for three quarters of the year they are far from any coast.

Modern research techniques shed light on seal behaviour and reveal that the crabeater seal, for example, regularly switches between land/ice and sea in the period February to December, i.e. in the southern winter. The longest stay out of the water was 19 hours (mean just under 8 hours), the longest time in the sea was measured at around 86 hours (mean around 15 hours). Young crabeaters spent about twice as long in the fresh air on an ice floe or on the beach as adults. It is speculated that the young feel safer outside the water because they are better protected from enemies there.

Another Antarctic seal, the Weddell seal, provides us with a different approach. Their readings from the time of the coat change indicate that few Weddell seals rest out of the water in the morning. Their big hour strikes between 14. 30 o’clock and 17 o’clock, when most Weddell seals sit on a floe or on land.

In the Arctic, among other things, the behaviour of the ringed seal was studied in more detail. During the Arctic winter between November and January, adult ringed seals spent just 4 percent of their time per month on the ice, while pups spent about 16 percent of their time out of the water. From the end of March onwards, the ringed seals constantly stayed a little longer on the ice, and in June they spent more than half of a day lolling under the Arctic sun.

Ideal body shape

As ponderous and clumsy as a seal may seem on land or on an ice floe, in the water even the fattest elephant seal becomes a graceful ballerina. Until it could come evolution-technically so far, the seals had to go through quite a few adaptations in its body-construction over itself – like all other once land-living mammals, that went back to the life in the water.

The body took the shape of a spindle to reduce water resistance in the sea. In order to eliminate also the very last possibility of disturbing vortex-formation with the swimming and diving, the seals put on itself a well trained subcutaneous-fat-tissue. This layer of fat is called blubber; it unifies the contours of the body, serves as heat insulation, stabilizes when swimming and makes the surface of the body elastic, which brings nothing but advantages when diving deep in icy cold water.

In meager times, the blubber must also serve as food-reserve, because four fifths of this tissue consists of pure fat. Seals are so abundant with their blubber that it can make up as much as 40 percent of their body weight.

Waterproof and cold resistant

Isolation is good. But what good is it if water can penetrate the body? The seals have also developed an optimal solution to this problem: their body openings can be closed. On the one hand this is done actively by muscle power, on the other hand also passively, for example by the immense water pressure. For example, the water pressure closes the nostrils and the ear openings once an appropriate diving depth has been reached. This allows the seal to conveniently save muscle power and consequently oxygen.

In their water habitat, seals are exposed to constant cold stress because water conducts heat 24 times better than air. In other words, you cool down 24 times faster in the water than in the fresh air. Blubber is the name of the solution when it comes to insulation, as we have already seen. But what about the (few) extremities of a seal, the fore and hind flippers? Isn’t a lot of heat lost there?

Like many other animals, seals rely on a special system of their blood vessels for this issue: The arteries that carry warm blood from the heart to the flippers branch en route with the veins that carry just-cooled blood from the extremities to the heart. There is an exchange of heat in the countercurrent principle: the arterial blood gradually becomes colder (so that only little heat can be lost through the flippers), and the venous blood, on its way towards the body core, warms up (so that the inside of the seal cannot cool down). For the same reason, birds resting on an ice floe, for example, do not freeze to the floe.

To keep warm, seals use another simple trick: lolling around in the sun! In the process, the animals store solar energy. In windy weather, they seek sheltered spots or lie close to each other. When it gets extremely cold, it is most comfortable in seawater, because its temperature cannot drop further than about -1.9° degrees.

Airless lungs

Like all diving animals, seals must be economical with their oxygen supply, because the heart and the central nervous system need to be supplied with sufficient oxygen during the dive. Most of this is therefore directed to the two most important organs, the brain and the heart.

And it goes like this: Before a seal or other marine mammal dives, the animal exhales. So it takes minimal or no air supply with it underwater. In a Weddell seal from Antarctica, the lungs collapse completely from a depth of 25 meters. This radical process reduces buoyancy and prevents gas exchange with the blood and thus the development of the diver’s disease. The little bit of residual air remains in the upper airways, from which the air cannot pass into the blood. The real magic ingredient that allows marine mammals to dive deep without worry is called myoglobin. This protein is found in the cells of skeletal muscles and the heart, including humans. It is the actual storage place for oxygen par excellence and makes it clear why seals can dive for a long time and to great depths without an ounce of air in their lungs: The metabolism simply makes use of the stored oxygen in myoglobin. Deep divers among marine mammals have 10 to 15 times more myoglobin than a human.

Stored oxygen

Conveniently, those seal muscles that are responsible for swimming accumulate markedly more oxygen than the other muscles. While we landlubbers can store about 8 millilitres of oxygen in every kilo of our muscle, a seal can store 70 millilitres. On the one hand, this has to do with myoglobin, of which the seals simply have more than we do. But to be fair it has to be said that seals are the better divers, because of course they have much more blood in their veins than we humans do. The body of a petite, 30-kilogram female seal pulsates with about 4.5 liters of blood – almost the same amount as a 70-kilogram male. Seal blood is also more viscous and has a particularly large number of red blood cells, which are even larger than ours.

Recent research on hooded seals, a North Atlantic seal species, has brought to light another tactic seals use to protect themselves from the cold and allow themselves to dive safely: they stop shivering. While seals, like other mammals, shiver to keep warm before diving, this reflex stops once they have descended. In this way, they save oxygen that would otherwise have to be spent on muscle tremors. At the same time, this also reduces the body temperature of the animals by up to 3° degrees, which also reduces the oxygen demand of the remaining tissues.

Polar Night Diver

So much for the observations during the summer months. But what do these animals do in winter, when it is bitterly cold, the sea is frozen over hundreds, even thousands of kilometres and the polar night brings months of darkness? Small satellite transmitters attached to the animals give us minute information about how a seal lives its real life.

(Almost) nothing remains hidden anymore – even diving depths and times, the position of the body under water and even the nature of the water, speeds or distances covered can be explored thanks to these mini-transmitters. For example, we now know that a Weddell seal covered 1840 metres underwater during its 15-minute dive in Antarctica and reached a depth of 401 metres (its record: 750 metres deep and 73 minutes underwater!).

Or the leopard seals: As soon as the energy-sapping hair change is over at the end of summer and the Southern Ocean begins to freeze, the leopards head out to sea. They remain in the open water at the edge of the pack ice that tightly encloses the continent of Antarctica throughout the southern winter, rarely using the ice as a resting place. A few make larger trips to the subantarctic islands like Kerguelen, Heard or Macquarie.

Those leopard seals that visit South Georgia have recently been known to show up more frequently during winters with lower water temperatures. This is probably because leopard seals are pagophilic, as biologists say – ice-loving. The largest daily distance was 150 kilometres. While these seals still dive rather at night for food in the autumn and early winter, so they prefer the light day-times for its forays from mid-winter. Most of the measured dives were shorter than 5 minutes and ranged in depth from 10 to 50 meters, one time it went down to 304 meters.

A rather unusual collaboration between German and Danish zoologists and astronomers recently revealed that the common seal, a seal species of the northern European coasts, can recognize the stars in the night firmament for orientation. Now researchers are speculating whether astronavigation may be even more advanced in seals and other marine mammals than in migratory birds.

When the sea makes you tired

Because seals spend eight months of the year or more in the open ocean, we inevitably ask the question: how do they sleep? Earthbound as we humans are, we find it hard to imagine that as a seal you don’t necessarily have to rest your head on a pillow to sleep comfortably. The seal’s pillow is the sea – and it’s a great place to sleep, even underwater.

However, when sleeping underwater, seals wake up very frequently and regularly to come to the surface and take a few deep breaths. Scientists suspect that seals that sleep at depth feel safer than at the water’s surface because there are fewer enemies deep down.

One also recognized that eared seals put a completely different sleep-behavior to the day (better: to the night) than true seals. In eared seals, only one hemisphere of the brain sleeps – an ability that has also been found in birds. So when an eared seal, half asleep, briefly emerges to breathe with the help of one awake half of the brain, the other half of the brain quietly dozes on.

Walruses, seals and sometimes single elephant seals have been seen in a very unusual sleeping position in the middle of the sea: They sleep on the surface of the water. But you can only just see the head with the big nostrils sticking out of the water. The rest of the seal’s body drifts almost vertically in the water like the cork of a wine bottle. “Bottling” is what this sleeping method is called, from the English word for bottle.

At least in the case of elephant seals, thanks to the satellite transmitters mentioned above, we now know how these animals normally sleep in open water. The largest seal species in the world spends months at sea, covering migratory distances that can be several thousand kilometres. Of course, this makes them tired: when sleep overcomes them, elephant seals turn onto their backs and let themselves fall for up to 16 minutes.

Like a big, fat leaf weighing tons, they trundle in spirals toward the ocean floor. One would almost be tempted to think of the fingers of Sedna, the goddess of the sea, as they gently sink down into the sea, and to which all the seals of the world owe their existence…

By Peter Balwin (text)