The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.
All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.
About Seals and Sea Lions
Seals and sea lions belong to a group of mammals called pinnipeds, a Latin term meaning "wing-footed," a reference to their flippers. There are three groups of pinnipeds: the earless seals of the family Phocidae; the eared seals of the family Otariidae; and the walrus of the family Odobenidae. The limbs of all pinnipeds have evolved into flippers because all species are aquatic, that is, they spend virtually all of their lives in and around water whether it be oceans, seas, and in one case, lakes. The Weddell seal is a phocid seal.
Adapted for Water
Pinnipeds are not the only mammals adapted to an aquatic existence, they are not even the most aquatic. Whales, dolphins, manatees, and dugongs spend all their lives actually in the water—their young are even born in the water—whereas all pinnipeds spend part of their lives out of the water either on solid land or ice, where they give birth.
Pinnipeds are carnivores, but unlike many other carnivores pinnipeds get virtually all their food from an aquatic environment and the diet can be quite varied. Many species are generalists, feeding on a variety of fish, squid, and other large organisms while others are more specialized and eat smaller prey. Some, like the Antarctic fur seal, concentrate on shrimp-like crustaceans called krill, while walruses specialize on bottom-dwelling clams and cockles. Weddell seals eat a variety of fish, squid, and have even been reported eating penguins.
Pinnipeds range in size from the huge walrus (2,000 kg) to the relatively tiny ringed seal (about 50 kg). While they exhibit quite a range in size, pinnipeds' body shapes are quite similar—long, sleek, tapered bodies; not much of a neck; tiny external ears (if any at all); fore- and hind-limbs modified into flippers; short, dense fur; not very graceful on land, but very mobile in the water. These are adaptations for moving and hunting in water.
While phocids and otariids have generally similar adaptations for locomotion in the water, they go about it in somewhat different ways. Otariids (the sea lion group) primarily use their fore-limbs to generate propulsion, appearing to "fly" through the water in much the same way penguins do. On the other hand, phocids (true/earless seals) hold their fore-limbs next to the body when swimming and generate propulsion with a wavelike movement of the hind-body and flippers. This difference in locomotory styles is also reflected in the arrangement of muscle mass in the two groups: otariids have heavy muscle mass in the neck and shoulders (to power the fore-limbs), whereas phocids concentrate muscle in the hind end. Weddell seals are large, bulky seals (400-500 kg), with small flippers and a long barrel shape.
Built for Their Environment
Water is a challenging thermal environment to live in because it conducts heat from the body. Pinnipeds exhibit a number of adaptations to reduce heat loss, the most extreme of these occurring in species living in frigid arctic and Antarctic conditions. For starters, pinnipeds have short limbs and compact bodies that reduce the ratio of their surface area to their volume—this essentially reduces the amount of surface area from which body heat can "radiate". Dense, waterproof fur and a thick layer of fat, or blubber, also reduces heat transfer to the environment. Thick fur has the disadvantage of increasing buoyancy (because it traps air) which is fine for species, like fur seals, that usually forage near the surface, but not so good for species that must forage at greater depths. These latter species, like the Weddell seal, have essentially forsaken a fur coat in favor of a thick layer of blubber whose physical characteristics permit deeper diving. Blubber is not only used for insulation, it is also an energy store that can be drawn upon when food is less available or during times when seals must fast.
For an aquatic predator to be successful it must find prey in the water, and the sensory system of pinnipeds is well adapted to this purpose. Pinniped eyes are relatively large, and well adapted to low light, and they have excellent hearing and have even evolved modifications to the bones of the skull that permit them to localize sound underwater. The well-developed and sensitive whiskers so characteristic of many pinnipeds probably function to detect movement of prey underwater as well as during social encounters. Pinnipeds are very vocal animals and they can emit a variety of sounds in the air and underwater ranging from roars and barks (in the air) to chirps, clicks, trills, and squeaks (in the water).