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Around the world, seals and sea lions represent different things to different people.

To marine theme park visitors, they are cute and talented performers able to balance balls and walk on their flippers; to environmentalists, they are defenceless pups slaughtered for their pristine fur; to commercial fishermen they are a threat to fish stocks; and to wild-life enthusiasts they are among the most spectacular creatures to watch at play in the wild.

Throughout history, seals have played a substantial role in many cultures, providing food, fuel and clothing for indigenous tribes in the Northern Hemisphere's frozen regions. Because of their expressive faces, these marine mammals have also been the focus of many legends, ranging from the 'selchie' stories of north-western Europe (in which seals are believed to be women and children condemned to a life where neither land nor sea provide a permanent home) to the superstitions that it is bad luck to kill a seal because they embody the souls of dead sailors.1

Seals, sea lions and walruses are grouped together under the name 'pinnipeds' (from Latin: 'wing-footed' or 'fin-footed').

Pinnipeds are divided into three families: the Phocidae, Otariidae and Odobenidae.

The Phocidae are known in scientific circles as 'true seals'. They are fantastic swimmers, using their hind flippers to propel themselves through the water while their small front flippers act as rudders or stabilizers. They lack external ears, hearing through small holes on either side of their head (see How they hear without ears below).

The Otariidae are known as 'eared seals' because, as the name suggests, they have (small) external ears. They also have long front flippers (measuring up to one-third of their body length), which propel them when swimming, while their hind flippers serve mainly as a rudder for steering. These animals are commonly referred to as sea lions.

All members of the Odobenidae family are walruses, and they combine features of both other families: they lack ears, but use both front flippers and hind flippers to move in the water.

It's easy to remember the difference between 'true seals' and 'sea lions': true seals (no external ears) have small front flippers and can only move awkwardly on land by shuffling their large bodies, while the eared sea lions not only have the larger front flippers on which they can walk, but they are also able to turn their hind flippers forward under their body, thus enabling 'four-footed' locomotion. The relative agility of eared seals on land helps to explain why the Californian sea lion is the most commonly used pinniped in marine theme parks.

Baby sea lions and true seals are called 'pups' and baby walruses 'calves'. For simplicity, we will refer to all pinnipeds as 'seals' in this article.

Seals are most abundant in cold circumpolar seas, where there are 14 species of eared seals (including fur seals) and 19 species of true seals (including harp, leopard, elephant and harbour seals). Exceptions to this include the Hawaiian monk seal, Californian sea lion, and species in the Mediterranean and Galápagos, all found in warmer climates.

For most seals, a good meal consists of fish, cuttlefish, octopuses and crustaceans, although several seals are more predatory, with the Australian sea lion and leopard seal feeding largely on penguins.2 The crabeater seal, the most abundant seal in the world, mostly lives on krill, a prawn-like crustacean abundant in polar waters.2

Seals have a number of amazing design features that make them perfectly suited to their life on both land and sea. Many live and hunt their food in the ocean, but come ashore to rest, mate and bear their young. The harbour seal (a 'true seal') can be found sleeping belly-up on top of the water,3 or can even lie asleep on the sea bed for up to an hour. Although eared seals spend less time in the water than 'true seals', they too can sleep at sea while 'sailing' in the 'tea cup' position, i.e. floating belly-up with one front flipper tucked under the other.4

At birth, a newborn pup can travel on land and swim, although it takes a few weeks for them to develop enough blubber to keep them floating and insulate them against the cold. During this time, their mother's milk is low in water content and high in fat, encouraging rapid growth. This also helps the mother in habitats where water conservation is important.5

The seal's body shape not only helps it move efficiently in the water, but also effectively retains heat as the mammals move in and out of often freezing water. Along with whales and dolphins, seals have a circulatory system that allows arteries carrying warm blood to transfer heat to veins carrying cooler blood from the body surface, helping to stop warmth being lost.6

In addition to these amazing features, seals have sophisticated methods of insulation involving an outer coat of protective guard hairs, dense water-repelling undercoat, and a special layer of blubber. During their deep diving feats (see Effortless divers in the deep below), this fat layer does not compress, thus maintaining its heat-retaining properties.

Seals have good sight underwater, even at night, and their eyes are able to adjust quickly to changing light, both when emerging from dark water into bright daylight,7 and when descending to the gloomy depths.8 Sensitive whiskers help the animal locate prey, particularly in those species feeding on the bottom of the ocean.2

Most people would recognize the sound of a seal 'barking'. This vocalization may play a role in navigation, social behaviour and foraging. Males may bark to show dominance and defend territory, while females may use the communication to locate their young on returning from feeding in the sea. Seals can also be heard to roar, honk, chirp, bleat, grunt or cough.7

One of the most engaging sights in the wild is a large number of seals swimming close together and waving their fore-flippers in the air. This is not, of course, a trick to entertain tourists, but a way for pinnipeds such as fur seals and sea lions to cool down; their insulation is effective in the cold, but can lead to over-heating problems when the temperature rises, and this action provides relief. On land, elephant seals tackle the problem by throwing sand over themselves with their flippers, keeping the sun off their skin.9

While there are many seals in the wild, several types are endangered, such as monk seals (likely named for their preference for solitude or the loose skin around their necks, which resembles the hood of a monk's robe). The total number of Hawaiian monk seals is estimated at between 1,200 and 1,500, the Mediterranean monk seal is rarely seen, and the Caribbean monk seal, last seen in 1952, is considered extinct.10

Evolutionists tend to disagree on the 'natural history' of seals, sea lions and walruses, but agree that the 'earliest' fossil records — supposedly 20 million years old — reveal seals that look very much like those alive today.11 So much so that monk seals are often referred to as 'living fossils' because 'they have remained virtually unchanged for 15 million years'.10

The fact that the three families share a number of characteristics creates lively debate among evolutionist scientists. One theory suggests sea lions, fur seals (Otariids) and walruses evolved from a bear-like ancestor on the shores of the Pacific, while the true seals (Phocids) arose 'more recently' from an otter-like ancestor around the Atlantic.11 There is no fossil evidence to justify that claim,12 or any other theory on the evolution of pinnipeds.

Claims of 'convergent' evolution — that two types of animals could evolve similar adaptations and features separately — are also without evidence, and, in fact, lack credibility, given the highly specialized features all three families possess (for heat retention and diving, as mentioned above).

There is no mystery if we accept that these specialized features are not the result of some chance succession of evolutionary flukes, but were perfectly designed by the Creator, who not only created animals perfectly suited to their environment and life, but made creatures that are beautiful to watch and study.

References and notes

  1. Miller, D., Seals & Sea Lions, Voyager Press, Stillwater, MN, USA, p. 7, 1998.
  2. The New Encyclopaedia Britannica, 15th Edition, 23:423, 1992.
  3. Harbor Seal, <http://www.letsfindout.com/subjects/undersea/rfiharse.html> May 5, 2000.
  4. Conversation with Sea World, Gold Coast, Australia, May 5, 2000.
  5. Ref. 2, p. 424.
  6. Ref. 1, p. 31.
  7. Ref. 2, p. 424.
  8. New Scientist163(2199):23, July 31, 1999.
  9. Ref. 1, p. 32.
  10. Hawaiian Monk Seals, <http://leahi.kcc. hawaii.edu/~et/wlcurric/seals.html> February 8, 2000.
  11. Ref. 1, p. 9.
  12. Flynn, J.J., Ancestry of sea mammals and Wyss, A.R., Evidence from flipper structure for a single origin of pinnipeds,

Effortless divers in the deep

Using sonic transmitters, researchers have been astounded to find that some seals are able to dive repeatedly to depths of more than 100 m (330 ft) in search of food. Weddell seals — with recorded deep-water dive times of up to 73 minutes1 — are capable of diving to almost 600 m (2,000 ft). A northern bull elephant seal was recorded diving to depths below 1,500 m (4,900 ft), deeper than some whales are known to venture.2 When diving, a muscular reflex not only closes the nostrils, but also the larynx and oesophagus, so seals can open their mouth to catch prey without swallowing water.3

Before submerging, seals do not take in a large breath of air. This would create difficulties with buoyancy and could lead to 'bends' when they resurface (caused by dissolved nitrogen forming bubbles in the blood). Instead, seals breathe out first, and carry the oxygen they need attached to special pigments in their blood and muscle tissue. Seals have 10 times the amount of oxygen-carrying myoglobin (a protein pigment) than humans in their muscles. Their lungs are designed to collapse under pressure, so what little air there is, is forced back into the windpipe, where nitrogen cannot be absorbed into the blood.3,4 The increasing lung collapse, as the pressure increases with depth, changes the animal's volume, decreasing its buoyancy. This makes for effortless deep diving, involving little or no expenditure of energy.5

Seals also reduce their heart rate by 10 to 20% while diving, at the same time diverting blood from parts of the body where it is less needed, such as the liver, to essential organs such as the brain.2

References

  1. Miller, D., Seals & Sea Lions, Voyager Press, Stillwater, MN, USA, p. 36, 1998.
  2. Ref. 1, p. 36.
  3. Ref. 1, p. 35.
  4. New Scientist166(2233):73, April 8, 2000.
  5. Camera catches secrets of the deep divers,

How they hear without ears

How are earless seals (and dolphins) able to hear? With only pinprick holes for ears, somehow marine mammals collect sounds and conduct them to the middle and inner ears, deep in their heads. Scientists suspected facial fat might be involved, but they couldn't explain how the fat conducted sound.

Now sophisticated technology has revealed that bundles of fat in the lower jaws and ear canals conduct sound extraordinarily well. 'These fats have a shape like an ear trumpet,' says one researcher, describing seals and dolphins as 'acoustic fatheads'.1

Reference

  1. How dolphins hear without ears, New Scientist, 164(2218):17, December 25, 1999 / January 1, 2000.

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