The Weddell seal (Leptonychotes weddellii) is a relatively large and abundant true seal with a circumpolar distribution surrounding Antarctica. The Weddell seal was discovered and named in the 1820s during expeditions led by British sealing captain James Weddell to the area of the Southern Ocean now known as the Weddell Sea. The life history of this species is well documented since it occupies fast ice environments close to the Antarctic continent and often adjacent to Antarctic bases. It is the only species in the genus Leptonychotes.
Description
Weddell seals measure about long and weigh . They are amongst the largest seals, with a rather bulky body and short fore flippers relative to their body length. Males weigh less than females, usually about or less. Male and female Weddell seals are generally about the same length, though females can be slightly larger. However, the male seal tends to have a thicker neck and a broader head and muzzle than the female. A molecular genetic-based technique has been established to confirm the sex of individuals in the laboratory. The Weddell seal face has been compared to that of a cat due to a short mouth line and similarities in the structure of the nose and whiskers.
The Weddell seal grows a thin fur coat around its whole body except for small areas around the flippers. The colour and pattern of the coat vary, often fading to a duller colour as the seal ages. This coat moults around the beginning of summer. Adults show a counter-shaded colouration that varies from bluish-black to dark grey dorsally and to light grey/silver ventrally. Coats may change to shades of brown before the annual moult. Adult males usually bear scars, most of them around the genital region. Weddell seal pups are born with a lanugo of similar colouration and they moult after 3–4 weeks; later, they turn a darker colour similar to that of adults. The pups are around half the length of their mother at birth and weigh . They gain around a day, and by 6–7 weeks old they can weigh around .
Behaviour
Movements
Weddell seals are commonly found on fast ice, or ice fastened to land, and gather in small groups around cracks and holes within the ice. In the winter, they stay in the water to avoid blizzards, with only their heads poking through breathing holes in the ice. These seals are often observed lying on their sides when on land. Weddell seals are non-migratory phocids that move regionally to follow the distribution of breathing holes and exit cracks within the ice changes between seasons. The species is primarily restricted to Antarctic waters: physical factors, such as glacial movement and tidal action, may increase fluctuations in distributions.
Foraging
Weddell seals dive to forage for food, maintain breathing holes in fast ice, and explore to find more ice holes. They have been observed to dive as deep as 600 m for up to an hour. These seals exhibit a diel dive pattern, diving deeper and longer during the day than at night. After dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 m, allowing them to dive with little effort.
Weddell seals are top predators in the Antarctic. They eat an array of fish, bottom-feeding prawns, cephalopods and crustaceans. A sedentary adult eats around 10 kg (22 lb) a day, while an active adult eats over 50 kg (110 lb) a day. Antarctic cod and silverfish constitute the majority of their diet. Cephalopods are common prey, and crustacean remains are sometimes found in Weddell seal scat, but at much lower rates than other prey species. They are opportunistic feeders that hunt in different parts of the water column depending on prey availability. Weddell seals hunt in both pelagic and benthic-demersal habitats.