Seal (mammal) - biology.

« remote lakes by swimming thousands of kilometers up rivers from the Arctic Ocean.

A few other species such as ringed seals and harbor seals have been found livingyear-round in lakes and rivers near the coasts of Russia, Canada, and Alaska. IV DIET OF SEALS Most seals eat fish and sometimes squid.

The leopard seal, an Antarctic species, may have the most diverse diet of all, commonly hunting penguins and other seabirds,smaller seals, as well as fish, squid, krill (small shrimplike crustaceans), and other invertebrates, as well as feeding on carcasses of dead whales.

Leopard sealssometimes hunt humans, lunging onto ice floes to chase people who are on foot, and also threatening scuba divers underwater. Sea lions and walruses may occasionally kill and eat other seals, although more commonly sea lions eat fish, and walruses dive to the bottom and dig for clams, worms,crustaceans, and other organisms from the mud.

Walruses have a large “mustache” of especially sensitive whiskers, or vibrissae, that help them detect their food on thedark sea floor.

A big walrus can eat about 45 kg (about 100 lb) of shellfish in one day. Crabeater seals of the Antarctic are known to eat fish but are unusual in that they feed primarily on krill using unique branching teeth.

The seal’s upper and lower teethmesh together, forming an efficient sieve that acts similarly to the giant filtering plates of baleen found in krill-eating whales. V REPRODUCTION Most seals mate on land or ice and, in all species, females give birth out of the water.

In most land-breeding seals, such as elephant seals and sea lions, densetemporary colonies called rookeries form in the breeding season.

In these species, males tend to grow larger than females, and a harem system prevails in which thestrongest males mate with several females and guard them from other males.

This harem system ensures that only the strongest males pass their genes to the nextgeneration.

A study of northern elephant seals found that only 9 percent of males succeed in mating. Seals that breed on ice, by contrast, do not typically organize into large harems.

These species, including ribbon seals, harp seals, and Weddell seals, form colonies thatspread out over wide areas.

Most ice-breeding species are monogamous, and the males and females are nearly the same in size and appearance.

An exception is thewalrus, which breeds on ice islands where large dominant males gather sizeable harems.

Walruses use their tusks in threatening displays toward rival males and incourtship rituals with females.

Males have large throat pouches that produce bell-like sounds to attract receptive females. Most females breed every year and on average bear a single pup (or calf in the walrus) 12 months after mating.

The seal embryo does not begin development for 3 to5 months after conception, thus ensuring that young are born when food is plentiful.

Newborn seals vary in size according to species—a newborn ringed seal weighsonly 12 kg (25 lb) while a newborn walrus can weigh 63 kg (140 lb).

Infant seals grow extremely fast and rapidly build up a layer of blubber that is generally lacking atbirth.

Females nurse young for a few days to two years, depending on the species.

The hooded seal nurses for only three to five days and the pups nearly double theirweight from 22 to 43 kg (49 to 95 lb) in this time.

At the other extreme are sea lions, fur seals, and walruses, which continue to nurse their young for one to two years. Seal milk is extremely rich in fat and protein.

True seals have the highest milk fat levels, averaging 40 to 50 percent, while sea lions and fur seals pack 10 to 14 percentprotein into their milk.

Elephant seal milk tastes bland and waxy and physically resembles melted vanilla ice cream. VI SEAL ORIGINS Seals evolved from bearlike carnivores about 25 to 30 million years ago.

Early seal fossils are found in Europe’s North Atlantic and Mediterranean regions.

By 8 to 10million years ago seals were well established in the northern hemisphere—numerous seal fossils have been discovered in the Chesapeake Bay region of the UnitedStates dating from this time period.

Found with these fossils are giant teeth resembling those of the great white shark, perhaps then, as now, an important sealpredator. Scientists debate whether all seals evolved from a single land ancestor, or whether true seals developed independently of the eared seals and the walrus.

Recentmolecular evidence from deoxyribonucleic acid (DNA), the genetic material found in all living organisms, suggests that true seals, eared seals, and the walrus are allmore closely related to each other than to any other mammal.

This indicates they all had the same land ancestor. Seals moved into the southern hemisphere only in the last few million years—long after they had become common and diverse in the north.

In Antarctic waters, theyevolved into unique species such as the Weddell, crabeater, and leopard seals. The monk seals remained in the tropics, perhaps lagging behind as their relatives crossed the equator into southern latitudes.

Scientists consider the Hawaiian monkseal to be a living fossil.

While modern seals have fused tibia and fibula bones in their flippers, the Hawaiian monk seal still has separate tibia and fibula bones in its hindflippers—a condition seen in the earliest fossil seals.

Even the structure of the large vena cava blood vessel in the Hawaiian monk seal resembles that of bears and dogsmore than that of other seals. VII ENDANGERED SEALS Natural predators of seals include large sharks, especially the great white shark, the orca, or killer whale, and other seals such as the leopard seal.

Polar bears kill sealson land and ice in the Arctic.

There are even reports of eagle attacks on baby Caspian seals. Perhaps the greatest menace to seals are humans, who have long hunted seals for food and seal skins, used for clothing and even housing and small boats.

Commercialsealing developed into a profitable business in Europe and colonial America by the 1700s.

Like whales, seals were hunted for their blubber, which was converted to oilfor fuel, lubrication, and tanning.

Fur seals were especially valued for their luxuriant pelts.

Today seals are still killed for their skins in some countries, such as Canadaand Russia. After only fifty years of commercial hunting in the 1800s, the northern elephant seal became nearly extinct.

Hawaiian and Mediterranean monk seals live along warmshores and are easily approachable, making them particularly vulnerable to human hunters.

Both monk seal species are currently endangered.

A third monk seal, onceabundant throughout the Caribbean Sea, became extinct by about 1950. One of the first formal attempts to protect seal populations occurred in 1911, when the United States, Canada, Russia, and Japan agreed to ban open-sea hunting ofseals.

After Mexico banned seal hunting in 1922, the northern elephant seal population began to grow.

In 1910, only 100 seals were found on Isla de Guadalupe; todaythis species numbers around 150,000 and has recolonized much of its former range from northern Mexico to southern Alaska.

However, another species, the Guadalupefur seal, hunted during the same period, has not recovered its former abundance along the California and Mexican coasts. Some countries have enacted laws to protect seals and other marine mammals.

Sadly, these laws came too late to save the Caribbean monk seal.

Even though thehunting of seals is now much less intense than in the past, threats from pollution, especially oil spills, and the accumulation of marine debris such as lost or discarded. »