Seals are large carnivorous marine mammals in the order Pinnipedia that feed on fish, squid, and shell-fish; some even feed on penguins. They are aquatic animals that spend time on shores and ice floes. Seals have streamlined bodies and webbed digits, with the forelimbs acting as flippers, while the hind limbs are backwardly directed in swimming and act as a propulsive tail. A small tail is also present. There are three families of pinnipeds: the Otariidae (sea lions), the Odobenidae (the walrus), and the Phocidae (the true seals). The "earless" seals of the Phocidae, such as the monk seal and the ringed seal, lack external ear flaps, while the seals with external ears include the walrus, sea lions, and fur seals.
Seals are air-breathing mammals, with fur, placental development, and lactation of the newborns. Moreover, seals are endotherms, maintaining a constant internal temperature of about 97.7–99.5°F (36.5–37.5°C) regardless of the outside temperature.
All seals are carnivores, eating fish, crustaceans, and krill (shrimp-like animals). Seals are related to terrestrial carnivores such as dogs and cats; they breed and rest on land, but are equally comfortable on land or in water. The thick layer of fatty blubber underneath the skin of seals serves to insulate the animal, to assist with buoyancy, and as an energy reserve when food is scarce.
Seals regulate their body temperature in several ways. In cold temperatures, the peripheral blood vessels constrict, conserving heat by keeping the warm blood away from the external environment, while insulating blubber reduces heat loss. The hind flippers have numerous superficial blood vessels close to the skin and only a few deep blood vessels. When cold, seals press the hind flippers together, in effect "pooling" the heat contained in the numerous superficial vessels. The superficial vessels then conduct this heat to the deeper vessels, which keeps the internal organs warm and functioning properly.
Another unusual feature of the seal's digestive tract is the stomach, which contains stones, some of them quite large. Small stones are probably swallowed accidentally, but some of the large stones might be deliberately swallowed. It is thought that these stones help seals to eject fish bones from the stomach, and may assist in breaking up big chunks of food, since seals do not chew their food but swallow all items in one piece. Another interesting theory is that the stones might act as balance, stabilizing the seal body and preventing the seal from tipping or rolling in the water.
Seal senses include touch, smell, taste, sight, hearing, and perhaps echolocation. Hearing in seals is especially keen, while smell is not well developed. Seal vision is remarkable in that vision underwater is about the same as a cat' s vision on land. Seal researchers have observed evidence of echolocation, in which an animal navigates by sensing the echo of sounds it emits that then bounce off of objects. Underwater, seals do indeed make clicks and similar sounds that suggest echolocation, but so far no definitive evidence has emerged that establishes the presence of this sense in seals.
Half of a seal's life is spent on land, the other half in water. Seals are diving mammals, and have evolved the ability to stay underwater for long periods of time. The reproductive behavior of seals also demonstrates the "double life" of seals. Some seals migrate to long distances across the oceans to breed or feed.
Seals are accomplished divers, and have evolved a number of adaptations that allow them to survive underwater. Some seals, such as the Weddell seal, can stay underwater for over an hour. In order for an air-breathing animal such as a seal to remain submerged for such a long period of time, it must have a means of conserving oxygen. Another crucial diving adaptation is adjustment Northern elephant seals (Mirounga angustirostris) at the Ano Nueva Reserve, California. This species takes its name from its great size and overhanging snout: bulls weigh several tons and may be up to 20 ft (6.1 m) in length. Photograph by Robert J. Huffman. Field Mark Publications. Reproduced by permission. to the high pressure of the water at great depths. Pressure increases by one atmosphere for every 33 ft (10 m) of water, and at great depths, there is a danger that the weight of the water will crush an animal. Some seals, however, can dive to great depths and remain unaffected by the extremely high water pressure. Similarly, seals that dive to these depths have evolved a way to deal with decompression sickness. When a human comes to the surface rapidly after a deep dive, the swift change in pressure forces nitrogen out of the blood. The nitrogen bubbles that form in the blood vessels cause decompression sickness—the painful condition known as "the bends," named for the fact that people in this condition typically bend over in pain. If the nitrogen bubbles are numerous, they can block blood vessels, and if this happens in the brain it leads to a stroke and possibly death. Humans can prevent the bends by rising to the surface slowly. Seals, on the other hand, have evolved a way to avoid decompression altogether.
A diving seal uses oxygen with great efficiency. Seals have about twice as much blood per unit of volume as humans (in seals, blood takes up 12% of the total body weight; in humans, it takes up 7%). Blood carries oxygen from the lungs to other body tissues, so the high volume of blood in a seal makes it an efficient transporter of oxygen. In addition, the red blood cells of a seal contain a lot of hemoglobin. Hemoglobin transports oxygen in red blood cells, binding oxygen in the lungs and then releasing it into the body tissues. The high amount of hemoglobin in a seal's blood allows a high amount of oxygen to be ferried to the seal's tissues. The Northern fur seal with his harem of female cows. Photograph by Eva Momatiuk & John Eastcott/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.
muscles of a seal also contain oxygen stores, bound to myoglobin, a protein similar in structure to hemoglobin.