Seals are large carnivorous marine mammals in the suborder Pinnipedia that feed on fish, squid, and shellfish; 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. 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.
The body of a typical seal is long and streamlined. Each seal has four flippers, two in front and two in back. The hair covering the seal’s entire body is of two types: soft underfur which insulates the seal against cold when on land, and coarser guard hairs above the underfur, which form the first line of protection against cold air temperatures. Whiskers, located on either side of the mouth, over the eyes, and around the nose, serve as tactile organs that help seals locate food and alert the seal to predators.
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.
A few species of seals are found in warmer climates. When seals get too hot, they lie in the surf, seek shade, or remain inactive. When the heat becomes extreme, they enter the water to cool off. Sea lions and fur seals are particularly sensitive to heat. When the outside temperature reaches 86°F (30°C), they are unable to maintain a stable internal temperature; in this condition, they stay immobile, or seek water if the temperature rises. The inability to dissipate heat makes these seals vulnerable to heat-related illness.
The small intestine of a seal is extremely long—an unusual feature for carnivores, which generally have short intestines. Long intestines are usually found in plant-eating animals, which need a long intestine to process the tough woody stems and fibers in their diet. Several theories have been proposed to explain the unusually long seal intestine. One theory holds that the high metabolic rate of seals makes a long intestine necessary. Another theory suggests that the heavy infestations of parasitic worms found in seals compromise normal intestinal function, and the greater length compensates for low-functioning areas of the intestine.
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.
The nervous system of a seal consists of the brain and spinal cord, along with a branching tree of nerves. Seal brains are relatively large in relation to their body weight: the brain accounts for about 35% of total body weight. This percentage is considerable when compared to the percentage of brain weight to total body weight in most terrestrial mammals. The spinal cord is quite short in seals, compared to other mammals.
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 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 muscles of a seal also contain oxygen stores, bound to myoglobin, a protein similar in structure to hemoglobin.
Before a seal dives, it usually exhales. Only a small amount of oxygen is left behind in the body, and what little oxygen is left is used to its best advantage due to the oxygen-conserving adaptations. If a seal dives for an extraordinarily long period of time—such as an hour or more—body functions that do not actually require oxygen to work start to function anaerobically (without oxygen). The heart rate also slows, further conserving oxygen.
Avoiding decompression and dealing with water pressure
Decompression sickness occurs because nitrogen leaks out from the blood as water pressure changes. Since seals do not have a lot of gaseous air within their bodies at the start of a dive, the problem of decompression is avoided—there is not as much air for nitrogen to leak out of. Exhaling most of its oxygen at the start of a dive also helps the seal withstand water pressure. Human divers without a breathing apparatus are affected by high water pressures because they need air to supply oxygen underwater, and this air in the lungs is compressed underwater. Seals, which do not have this pool of compressible air, are unaffected by water pressure. Seals close their outside orifices before a dive, making then watertight and incompressible and allowing dives to depths of 200 ft (60 m) or more.
Seal pups are born on land in the spring and summer. To take advantage of warmer seasonal environments and plentiful food, some seal species are migratory, feeding in one spot in the summer and early autumn, and then traveling to a warmer spot in the autumn and winter to give birth and mate shortly afterwards. Seals can give birth in large groups, in which a crowd of seals have returned to a particular spot to breed, or they can give birth alone. Migratory seals usually give birth in groups, after which they mate with males and conceive another pup.
Another way to ensure that a pup is born at an optimal time is to delay implantation of the embryo inside the uterus. In seals, fertilization (the meeting of egg and sperm) may take place in April, but the embryo might not implant in the mother’s uterus until October. This phenomenon of delayed implantation also occurs in roe deer, armadillos, and badgers. The total gestation period (the time it takes for the pup to develop inside its mother) is 9-15 months, depending on the species. The average active gestation period (the time from implantation to birth) is probably about 3-5 months.
There are 19 species of earless seals, 9 species of sea lions, 5 species of fur seals, and 1 species of walrus.
Of the earless seals, some of the more familiar are the harbor seals (Phoca vitulina ) that are found in the North Atlantic and Pacific Oceans. These seals position themselves on rocks or sandbars uncovered by low tides, swimming only when the high tide reaches them and threatens their perch. The seals that both entertain and annoy residents of San Francisco Bay with their loud barks and enormous appetites are harbor seals.
Another earless seal is the elephant seal (Mirounga angustirostris ), which can weigh up to four tons. The largest of all pinnipeds, the male elephant seal has a characteristic inflatable proboscis (nose) reminiscent of an elephant’s trunk.
The harp seal (Pagophilus groenlandicus ) was at one time one of the most endangered of the earless seals, since the pure white coat of the harp seal pup was prized by the fur industry. Harp seals are migratory animals and are found in the Arctic Atlantic Ocean.
Among the eared seals, the long-tusked walrus is one of the most familiar. Walruses use their tusks to lever themselves out of the water; at one time it was thought that they also used them to dig up food. Walruses can weigh up to two tons, feeding on mol-lusks, which they delicately suck out of the shell before spitting it out. Like all eared seals, walruses have front flippers that can be rotated forward, allowing them to walk and run on land, walk backward, and rest upright on their front flippers.
Sea lions are eared seals, commonly seen performing tricks in zoological parks. They lack the thick underfur seen in the earless seals, and so have not been hunted heavily for their pelts. In contrast, the fur seals are eared seals that have almost vanished completely due to intense hunting but are now protected: in 1972, the United States passed the Marine Mammal Protection Act, which outlaws the killing of seals for their fur and other products and restricts the selling of these products within the United States.
King, Judith. Seals of the World. Ithaca, NY: Cornell University Press, 1983.
Kooyman, Gerald L. Weddell Seal: Consummate Diver. Cambridge: Cambridge University Press, 1981.
Riedman, Marianne. The Pinnipeds: Seals, Sea Lions, and Walruses. Berkeley: University of California Press, 1990.
Allen, Sarah G., et al. “Red-pelaged Harbor Seals of the San Francisco Bay Region.” Journal of Mammology 74 (August 1993): 588-93.
Campagna, Claudio. “Super Seals.” Wildlife Conservation 95 (July-August 1992): 22-27.
Golden, Frederic. “Hot-Blooded Divers.” Sea Frontiers 38 (October 1992): 92-99.
Monastersky, Richard. “The Cold Facts of Life: Tracking the Species That Thrive in the Harsh Antarctic.” Science News 143 (24 April 1993): 269-271.
Zimmer, Carl. “Portrait in Blubber.” Discover 13 (March 1992): 86-89.