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an order of aquatic mammals. The body is fusiform and, in the majority of pinnipeds, except for fur seals, covered with short, shiny hair without underfur. Members of the order have short tails and limbs with five digits. The limbs are modified into flippers, and the digits are not separated or are joined by webs. Pinnipeds have sweat and sebaceous glands in the skin and a layer of fat up to 8 cm thick under the skin. They have 18 (in walruses) to 38 (in eared seals) teeth; the tusks are well developed. The masticatory musculature is reduced (members of the order do not chew their food).
Pinnipeds are very mobile in the water, developing a speed of up to 30 km/hr, and clumsy on land. They feed on fish, mollusks, and crustaceans; the leopard seal feeds on penguins and other seals. The nostrils open only at the moment of exhalation and inhalation and are tightly closed the rest of the time. Pinnipeds are capable of holding their breath and diving for long periods (the Weddell seal up to 43 minutes, the Baikal seal up to 68 minutes). When pinnipeds dive, renal blood circulation, filtration of urine through the glomeruli, and discharge of urine cease; the pulse slows down by a factor of 10 to 20—from 55–180 beats per minute to 4–15 beats per minute; the blood flow is redistributed, as a result of which the brain, spinal cord, and the cardiac muscle are the first to receive oxygen. The decreased sensitivity of the respiratory center to the accumulation of CO2 in the blood makes it possible to utilize fully the oxygen stored by pinnipeds before diving. A large quantity of respiratory pigment (myoglobin) in the muscles ensures supplementary reserves of oxygen. The body temperature in pinnipeds is 36°-37°C. The brain is well developed, and the cerebral cortex has a large number of convolutions. Pinnipeds are quite sensitive to odors; they see well and hear excellently in water.
Pinnipeds spend most of their time in water, periodically surfacing to breathe; however, they come ashore or onto the ice to give birth, nurse their young, molt, mate, and rest. Female pinnipeds attain sexual maturity at three (seals and fur seals) to six (walruses) years, and males often a year later. The gestation period is eight to 12 months. Rut, especially in polygamous species (fur seals and Steller’s sea lions, for example), is accompanied by fights between males. In many species of pinnipeds there is a latent period of two to four months in the development of the fetus. Pinnipeds bear one or, rarely, two offspring. In different species lactation continues from three weeks (in Phocidae) to three months (in fur seals), and even more than one year (in the walrus). The milk is very nourishing: it contains 36–52 percent fat, 6–13 percent protein, and about 0.2 percent sugar. For this reason, the young weigh three to four times more than the newborn by the end of lactation and rapidly acquire a layer of subcutaneous fat. Pinnipeds may live up to 43 years (walrus, ringed seal); their age is determined by the layers of dentin in the teeth.
Pinnipeds are distributed mainly in cold and temperate seas of the northern and southern hemispheres (bipolar distribution). The Caspian Sea and Lake Baikal are each the unique habitat of a species of seal; a third species (the ringed seal) inhabits such lakes as Ladoga and Saimaa. The majority of pinnipeds are gregarious animals, forming rookeries on land or ice; they carry out regular seasonal migrations. Others live singly or in small groups. The 32 species of Pinnipedia are united in 20 genera, belonging to three families: Odobenidae (walruses), Otariidae (eared seals), and Phocidae (true seals). The total population of the Pinnipedia numbers approximately 20 million, of which half live in the southern hemisphere, including the antarctic seals— the leopard seal, crabeater seal, Weddell seal, Ross’s seal, and southern elephant seal. In some places in subtropical waters there are monk seals. Many species of pinnipeds are hunted commercially.
The ancestors of the Pinnipedia were apparently terrestrial predators of the Ursidae-Mustelidae group, who adapted to aquatic life in the Upper or Middle Eocene. Fossil remains of Otariidae have been found chiefly near the coast of the Northern Pacific (USA); fossil remains of Odobenidae and Phocidae have been found near the shores of the North Atlantic (Europe and USA).
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