Seals belong to a group of mammals known as "pinnipeds". All pinnipeds have streamlined bodies and limbs modified into flippers. Pinnipeds are divided into three families: walruses, true seals and eared seals. True seals do not have external ears, cannot turn their hind flippers forward (therefore can not walk on them), and have fur on both surfaces of their flippers. New Zealand examples include the leopard seal and southern elephant seal.
Eared seals include fur seals and sea lions. They have external ears, hind flippers they can turn forward under the body and walk on and no fur on the under side of their flippers. New Zealand examples are the New Zealand fur seal and New Zealand sea lion.
Pinnipeds are typically sleek-bodied and barrel-shaped. Their bodies are well adapted to the aquatic habitat where they spend most of their lives. Their limbs consist of short, wide, flat flippers. The smallest pinniped, the Baikal seal, weighs about 70 kg on average when full-grown and is 1.3 m long; the largest, the male southern elephant seal, is over 4 metres long and weighs up to 4,000 kilograms.
Fur seals have both blubber and a specially adapted fur coat, including outer guard hairs that repel water and a layer of insulating underfur. For this reason they were particularly prized by sealers. Many species were nearly hunted to extinction.
For most pinniped species, molting is an annual process of replacing worn fur (and in some cases, skin) that temporarily grounds them. Molting can compromise thermoregulation, so some species, such as elephant seals, fast and remain onshore for a month or more.
In many species, pups are born with a natal coat of a different length, texture and/or colour than adults. This coat is adapted for the terrestrial, preweaning period, either a thick pelage to keep them warm in arctic environments, or a thin layer of fur to keep them cool on summer sands. During their first molt (about 11 days after birth for harp seals), the pups replace this with an adult coat better suited to life at sea. Until this age, pups risk hypothermia and drowning if they spend too much time in the ocean.
Pinnipeds use several strategies to conserve body heat while foraging in cold waters. Most primarily rely on a thick layer of blubber (fat) under their skin, which also provides buoyancy, hydrodynamic shape, and stores energy. Some young seals have a thick fur coat as well as blubber. Additionally, the pinniped circulatory system is uniquely adapted to redirect blood away from body surface areas to prevent heat loss.
Pinnipeds living in warmer climes, such as Galapagos or Australian sea lions, must keep cool when they haul out onto land to rest, breed, and nurse their pups. Strategies include resting in the shade or in tide pools, or covering themselves in a thin layer of sand ("sand-flipping"). They can also shunt blood to the surface of their flippers for rapid cooling by waving or dipping in pools
Pinnipeds have unique lungs with airways highly reinforced with cartilage and smooth muscle, and alveoli that completely collapse during deeper dives.
While overemptying of the lungs in humans classifies as a lung disease and terrestrial mammals are in general unable to empty their lungs, pinnipeds are able to reinflate their lungs even after complete respiratory collapse.
The hydrostatic pressure pinnipeds experience during diving is hypothesized to have acted as an external selection force during evolution for adaptations in their pulmonary surfactant system to meet the challenges of extreme hydrostatic pressure and regularly collapsing lungs.
Pinniped eyes are well adapted for seeing both above and below the water surface. The animal has a clear membrane that covers and protects its eyes underwater. In addition, its nostrils close and blood circulation stops to most of its organs when diving. Testicles and mammary glands are located in slits under the skin to maintain the pinniped’s streamlined shape. They also have whiskers to help navigate, and sensors in their skull to absorb sounds underwater and transmit them to the cochlea.
A large group of seals during breeding is called a harem. Adult males are called bulls and females are called cows, while a young seal is a pup. Immature males are sometimes called SAMs (sub-adult males) or bachelors.
Males of some species, including elephant seals, South American sea lions, and northern fur seals, aggressively defend groups of specific females, referred to as harems. Males of other species, including most sea lions and brown fur seals, defend territories on reproductive rookeries while females move freely between them. Occasionally, violent competition for females or territories is an integral part of male breeding strategy among most pinnipeds. Otariids, which are generally more land-adapted, form major colonies in the summer months on beaches or rocky outcrops. Consequently, their reproductive behavior is easier to observe and relatively well-studied. Walruses and many phocids, on the other hand, form smaller colonies, often in remote locations or on ice, and copulate in the water. Their reproductive behavior is less well known.
Females have a postpartum estrus that allows them to mate soon after giving birth. Delayed embryo implantation (embryonic diapause) obviates the need to come ashore (haul out) to twice a year, once to give birth and again to mate. After giving birth, mothers suckle their young. To compensate for the short lactation period, the fat content of phocid milk (45–60% fat) is higher than in any other marine mammal species. After lactation, most females migrate to feeding grounds for intensive foraging to recoup energy reserves. Protracted nursing also leads to the formation of social bonds.After the female returns from her first feeding trip, the most important task for her is to find her own pup out of the mass of other pups. Feeding a different mother's pup is a significant waste of energy, as milk production has a high parental cost. The seal overcomes this complication by voice recognition. The mother and pup must learn each other's voices in the first few days after the pup is born, before the dam goes on her next hunt. On returning, the mother seal calls out to her pup, causing the pup to call back. This enables the mother to find her own pup and not waste any energy feeding and taking care of the wrong pup, as this would only decrease her own fitness.
Pinnipeds are carnivorous, eating fish, shellfish, squid, penguins, and other marine creatures. Most are generalist feeders, but some specialize. For example, Ross seals and southern elephant seals mainly feed on squid. Crabeater seals eat mostly krill, and ringed seals almost exclusively consume crustaceans. The walrus consumes its molluscan prey by sucking the soft parts from the shell.
Some seals eat warm-blooded prey, including other seals. The leopard seal, which is probably the most carnivorous and predatory pinniped, eats penguins as well as crabeater and Ross seals. The South American sea lion also eats penguins, flying seabirds and young South American fur seals. Steller sea lions eat northern fur seal pups, common seal pups, and birds.
Almost all pinnipeds are potential prey for orcas and larger sharks. Arctic species are an important component of polar bear diets. Antarctic seals, however, are not preyed upon by terrestial predators.
Pinnipeds can hold their breath for nearly two hours underwater by conserving oxygen. When the animal starts diving, its heart rate slows to about one-tenth of its normal rate. The arteries squeeze shut and the sense organs and nervous system are the only organs to receive normal blood flow. They are able to resist more pain and fatigue caused by lactic acid accumulation than other mammals. However, once they return to the surface, they need time to recover and normalize their body chemistry.
Pinnipeds spend many months at a time at sea and so they must sleep in the water. Scientists have recorded them sleeping for minutes at a time while slowing drifting downward belly up.
The image above shows the largest species of seal, the Elephant Seal.
Smallest species of seal, the Ringed Seal.