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harbor seal
Seals are carnivorous pinnipeds while manatees and dugongs are called plant-eating sirenians. The difference between a sea lion and a seal is that the former had has flaps over its ears and large front flippers that allow it to maneuver around on land more easily. Seals and sea lions have many similarities with land carnivores like dogs and cats such as a long snout and sharp teeth and are thought to have developed from land mammals about 20 million to 25 million years ago. [Source: Roger Gentry, National Geographic, April 1987]

There are 34 species of seal, all of which mate and give birth on land and feed at sea. They fall into two categories: true seals and eared seals. True seals (phocids) have no external ear and use their fishtail-like rear flippers for swimming. They are not very mobile on land. They hunch along caterpillar fashion using their whole bodies. Their clawed foreflippers do not offer much help getting around on land.

Sea lions and fur seals are eared seal (otariids). They have visible earflaps and use all four limbs for walking on land and use their front flippers for swimming. Their rear flippers, which are short and turn forward and backward, are most useful for getting around on land. Eared seals can move much faster on land than seals. The creatures you see doing tricks in marine mammal shows are generally sea lions not seals. The name sea lion is derived from the male’s mane.

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Ross seal
A male seal is called a bull. A female is called a cow. Young are called pups. A group is called a herd, rookery or harem. Pinniped means “fin-footed.” Walruses (odobenids) are neither seals nor sea lions and have no external ear flap but can walk on all fours. Odobenids are the third grouping of pinnipeds.

The origin of seals and sea lions is a matter of some debate. Some say that they evolved separately, with sea lions descending from a common ancestor of weasel. Other say that seals and sea lions evolved together from otter-like creatures. In any case they descended from creatures that once lived full time on land. This is also believed to be the case with whales. Of seal lions and seals, sea lions — with their external ear flaps and more mobile limbs---are most closely related to their land-living ancestors.

Seal Characteristics

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Leopard seal
Seals are warm blooded creatures. What keeps them warm in cold water where many of them live is their thick layers of fat, called blubber, under their skin. Fur seals have a thick layer of fur which grows underneath coarse protective hair. But this hair does little to keep them warm when they dive deep in the water. To keep warm in deep seawater and on land seals and sea lions need blubber.

Sea lions have relatively thin layers of blubber but have more hair. Seals have a thin layer of coarse hair and more blubber. The blubber on seals that live in polar regions is typically between 1.6 and 2.4 inches thick.

Seals are not as fully adapted to sea life as whales. They still retain their legs and their heads are similar in shape to those of land mammals. Seals have four limbs as opposed to whales and dolphins which have two. The arm and leg bones of seals are relatively short and are contained within the body. The hand and foot bones are elongated and webbed and extend out of the body to form the flippers. The rear flippers can rotate, which allow the animals to move on land and swim with maximum efficiency.

Flippers allow pinnipeds to maneuver around in both water and land. Seals have flippers at the front and backs of their bodies. Those at the front mover freely and allow them to pull themselves out of the sea and get around, albeit slowly and laboriously, on land. Seals move on land like caterpillars by arching their back and drawing the hindquarter forward and then lunging forward with the front part of the body.

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Steller sea lion bull
There are two kinds of seals: those with forward-pointing back limbs, which allow them to get around better on land, and those with back limbs pointing backward, which are better adapted for swimming in the open sea.

Seals can close their eyes and ears underwater. In dim light, thanks to their large eyes, they can see underwater better than humans can on land. The seals’ sharp vision is essential for finding food at great depths. They can also discern higher pitched noises than humans and sometimes make clicking sounds and long sweeping trills. Some seals have very sensitive whiskers which they use to track prey.

Seals have fairly long life span; most live to 20 or more. A captive seal has lived to the age of 46. Sea lions can swim at speeds of 15 miles per hour.

Diving Seals

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Hawaiian monk seal
Scientists have long wondered exactly how dolphins, whales and seals can stay under water so long. The secret seems to be that they sink rather than swim downwards to conserve energy and do not use up oxygen unnecessarily. Seals stay as still as possible on the way down and use their energy to catch prey and swim back up. Whales, seals and dolphins all seem to use the same strategy. Scientists were able to observe this phenomena by strapping critter cam cameras on bottlenose dolphins, Wendell seals and even a blue whale.

When humans dive deep air taken into their lungs under pressure dissolves in the blood. When the human returns to the surface the air returns in the form of bubbles in the blood vessels, producing a condition called the bends, causing bleeding in vital organs, paralysis and even death. Seals avoid getting the bends because their lungs collapse during deep dives and force air into the windpipe, where nitrogen can't be absorbed into the blood.

The blood of some deep-diving seals contains 3½ times the hemoglobin of humans. Hemoglobin absorbs oxygen and transports it in the blood. Seals also have large amounts of another substance, myoglobin, within their muscles that absorbs oxygen in a similar way. Using the large qualities of hemoglobin and myoglobin they possess, seals can hold more oxygen in their blood and thus don’t need to store it in their lungs or take in more oxygen.

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sea lion
For very deep dives seals conserve their oxygen stores by stopping or slowing blood circulation to all but critical organs. They can also slow their heart rate, sometimes to a mere 10 percent of their normal rate at the surface.

Seals’ muscles are almost black in color because myoglobin has a dark color. The black meat has a gamy flavor. Scientists at research stations in the Antarctic claim that seal brains are a real delicacy.

The larger a seal is generally the deeper it can dive. Animals weighing 100 pounds can go 600 feet and back in five minutes or less. Elephant seals that weigh several tons can dive down to 4,000 feet and stay submerged for two hours.

Seals Sleep As They Sink in the Sea

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leopard seal
Stéphan Reebs wrote Natural History magazine, “When northern elephant seals (Mirounga angustirostris) migrate between their breeding and foraging grounds, they spend as long as eight months at a time at sea. They’re almost always underwater, devoting only a few minutes to breathing at the surface between dives---hardly long enough for a nap. After a sip of air, they often sink quickly to 500 feet, then drift farther down in a shallow descent. Some experts have suggested that the drift is when the seals catch their Zs.” [Source: Stéphan Reebs. Natural History magazine. February 26, 2010]

“To find out, a team led by Yoko Mitani of Hokkaido University in Japan fitted six juvenile elephant seals with satellite transmitters and newfangled data loggers capable of recording such information as body position, flipper strokes, and the 3-D path of movement. They tracked the seals for up to eight days off the California coast. The resulting data revealed that drifting seals usually rolled over on their backs, stopped stroking, and spiraled peacefully down for a dozen minutes or so. (The team dubbed it the “falling-leaf phase” of the descent.)

“The belly-up position is consistent with slumber: ventral blubber tends to flip an unresponsive seal’s body. What’s more, a few animals that drifted in shallow areas hit the seafloor without reacting. The initial rapid descent is important, Mitani’s team points out. It takes the seals below the usual cruising depths of their main predators, killer whales and [white sharks. And their slow sinking thereafter makes for a relatively short ascent for air once they awake.”

Seal Behavior and Feeding

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California sea lion
Seals do not migrate but they wander as far as 1000 miles from their colonies in search of food. When they are on land seals prefer to stay on rocky island or points. They “haul out”, or come ashore, to rest. Sea lions express grief. Females have been observed wailing after witnessing their pups being eaten by killer whales.

Seals on average consume eight percent of their body weight in food every day. They eat a variety of fish and invertebrates, feeding on schools of small anchovy-like fish, squid, octopus and sometimes lobster. Some eat seabirds. Leopard seals and some sea lions eat seals. A critter cam attached to a monk seal in Hawaii revealed that it ate on the slopes of atolls not just in shallow reefs and was able to flip over boulders to get at fish that were underneath

Most females nurse their young for only short time, sometimes only ten days. Even in dense groups seals do not form social bonds. Each animal acts on its own behalf, and for its own benefit. Some seals are tolerant of humans, even snuggling up next to them, while males occasionally view a human as rival and charge. Bull fur seals weighing 350 pounds have been known to charge people. They have also been know to back off if shouted at loudly enough.

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male and female California sea lion
Seals can perform many of the same sophisticated learning tasks as dolphins and apes. After a seal “has learned words and signals for objects and actions," Roger Gentry wrote in National Geographic. "It can carry out very complex commands such as: Take the large white cube over to the small black ball."

Leo, an eight-year-old male South American sea lion at Hakkeijima Sea Paradise aquarium in Yokohama, Japan, grins and makes facial expression during training and can make the Chinese character for a rabbit and other Chinese zodiac symbols.

Male and Female Seals and Sea Lions

It is sometimes difficult to determine the sex of seal. The seal penis and testes are internal. Nipples can be retracted to reduce drag. Size is often the easiest way to tell males and females apart. Sea lion and fur seal males may be five times larger than females by weight. Greater size may give males an advantage in breeding by helping them remain on shore longer where most of the mating takes place. Larger males store more fat, which provides the fuel and water for long fasts. Males from some species of seal have been observed to go without food for 70 days.

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fur seals
With its big furry mane and loud belchlike roar you can see how a three-quarter-ton sea lion bull inspired the species name. The weight of an average male fur seal is about 400 pounds, but it can weigh as much as 800 pounds when it returns to land after spending 10 months at sea feeding. Bulls need the extra weight in order to do battle with rival males.

During the breeding season, dominant bulls arrive first at the seals’ land gathering area, or colony. They establish territories on sections of beach and keep others away. The dominant bulls force the young bachelor seals to form herds on the periphery of the breeding areas. When the females arrive they gather around the dominant bulls.

Male seals fight by charging each other. The fights can last for hours. Small males have little chance of winning and often retreat early to the edge of their territory or a neutral area at the head of the beach. The winners, surrounded by their harem, swagger arrogantly and bellow belligerently.

Sea lion males are especially large and ferocious. They have dagger-like canines and fight by slashing at their rival’s neck. Often large quantities of blood is spilt "The full roar of a 600-pound bull fur seal mating season call," John Blazar wrote in the Los Angeles Times, "exploded with a hydraulic vapor cloud, like smoke from a canon. The blast dimples one's eardrum."

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fur seals
Dominant bulls end up with a harem of five to 25 cows while defeated males wander the sea alone until the next breeding season when they try again. A particularly large bull may support a harem of 20 or 30 females who are about one third the male's size.

Seal Breeding and Sex

Unlike whales, seals and sea lions have not developed the ability to mate and give birth in the sea. Sea lions and fur seals bear their young on land. Finding a good place to give birth and raise young is not so easy. The place should be smooth and sandy and protected from large waves. The sea floor leading up to it should be sloped to make it easy to come ashore. It should also be safe from land predators either on an island that has no such predator or in an area protected by steep cliffs. Places that meet all these conditions are few and far between, which is why often hundreds or thousands of seals gather at such as places where they do exist.

The mating season for sea lions lasts for around two months, at which time the dominate bull is very busy and tired because he is expected to satisfy the sexual needs of all the members of his harem. Sex among sea lions sometimes looks rough and cruel because the male looks so much bigger than the female.

For the one or two month in the mating season, dominant males do nothing but guard their territory, bellow, roar and fight and try to mate with as many females as they can. They don’t go to sea and feed. In many cases they arrive at the beach huge and loaded with blubber and leave exhausted skeletons.

Dominate males jealously guard their harems. Male sea lion get sexually aroused watching other seals have sex. One reason for this is perhaps that so few males get the chance to really do it. Sometimes females mate within 48 hours of giving birth.

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California sea lions
Most true seals breed on ice rather land. There is always plenty of ice around and so it is not necessary to crowd into one place. Consequently their breeding habits are much different than those of sea lions. Rather than battle males for harems, seal males pursue females individually on the ice.

Crab-eater seals pair off on the ice during the breeding season, each next to a pup. They are not a happy family though with the male pitching in to help his mate protect their young. Rather it is a male waiting for the female to stop suckling their pup so he can mate with her again. The pup is typically born the previous year to a different father. The presence of the new seems to deter challenges. There are rarely battles between males.

Most seals copulate out of the water. Harbor seals mate in the water. Copulation takes place just below the water and often it lasts more than an hour. Groups of males wait for the female to enter the water and try to swim out to sea. Males swim near the female and start quivering their neck and producing a rumbling noise that gets louder and louder culminating with a loud crash. It is not clear whether this activity is intended to scare away other males or demonstrate to the female he is strong and powerful. In any case other male harbor seals often arrive and start singing along with the first-arriving male as if they were part of an a capella group. It is not known why they do this because the first-arriving make usually establishes himself as the dominant male. What the other males get out of it is perplexing.

Seal Births

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Weddell seal baby and mother
Sea lion females arrive on the beach a week or two after the males. They are pregnant with young conceived a year earlier. Within a few days they give birth. Soon after that they become sexually receptive again and are pounced upon by dominate males. Afterwards the females go to sea and feed and return from time to time to suckle their pups. Each female above the age of three or so usually gives birth to one pup each season.

The females do not get pregnant right away. The fertilized egg remains dormant for three months before development begins. The gestation period is nine months. This way female seals can synchronize mating with giving birth, freeing both males and females to spend the rest of the year at sea gorging themselves. The females stay close to the colony where they nurse their young.

Often the cows give birth to a single pup a few weeks after the bulls return from the open sea. The young seal lion are delivered in the territory of the dominant male. A week or so later the male begins mating with females in his harem.

The area where females give birth to their pups is very specific. Some produce pups within ten meters of the same spot year after year, which often times is where they themselves were born. Usually only one pup is born, probably because the young must be fairly large to survive in cold ocean water. Most seals can swim on the day they are born.

Young Seals

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baby Weddell seal
Hooded seal milk is 61 percent fat. An eight ounce glass of it contains about 1,400 calories. Seal pups nurse for only four days but gain 45 pounds in that time and build up a layer of fat for protection in cold water. In contrast, rhino milk has two thirds the calories of human milk and is only 0.2 percent fat, one of the lowest of any species. Young rhinos and horses get their energy from sugars rather than fat.

After pups are born mothers spend much of their time at sea feeding to support themselves and produce enough milk for their young. Mother seals who return from fishing can tell their pups among the hundreds of other by smell.

After about two of three months on land the young are taught how to swim by their mothers. A mother nuzzles her pup into the water and lets it struggle until it become exhausted and then puts her flipper under it and to let the pup rest. After about two weeks the pups learn to swim. After that the mothers teach them how to hunt. At the end of the summer the pups head off to sea with their mothers and the bulls go their own way.

On average one in every four seals born in Cape Cross Namibia dies. Sometimes they starve to death when their mother is eaten by a shark. They also die after being born prematurely or trampled to death.

Image Sources: Wikimedia Commons, National Oceanic and Atmospheric Administration (NOAA)

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

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© 2008 Jeffrey Hays

Last updated January 2012

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