A Weddell seal can grow 3 meters long and weigh between 400-600 kg. Although female Weddell seals can be slightly larger, both male and female are generally about the same size. . However, the male seals have a thicker neck, and a broader head and muzzle than the females. Weddell seals have a smiling appearance because of their up turned mouth. Their face can be compared to a cat due to their short mouth line, and similarities with their nose and whiskers. .
The Weddell seal grows a thin fur coat around their whole body except for small areas around their flippers, the colour and pattern of this coat varies. As the fur ages, it often fades to a duller colour. . This coat moults around the beginning of summer. . Adults are brown, lighter ventrally, and mottled with large darker and lighter patches, those on the belly being silvery white. Adult males usually bear scars, most of them around the genital region. Young Weddell Seals have a gray pelage for the first 3 to 4 weeks; later they turn a darker color. The pups reach maturity at 3 years of age. The pups are born at around half the length of their mother, and weigh 25 to 30 kg (55 to 66 lb). They gain around 2 kg (4.4 lb) a day, and by 6"?7 weeks old they can weigh around 100 kg (220 lb). .
Depending on the latitude it inhabits, this marine mammal gives birth from early September through November, with those living at lower latitudes giving birth earlier. During the mating season the Weddell seals make noises that can be felt through the ice, they are that strong.. Copulation has only been observed to occur underwater, where the female is often bitten on the neck by her partner. The seals are normally around 6-8yrs old when they first breed (but this can be much earlier for some females). . The Weddell seal is one of the only breeds of seals that can give birth to twin pups. . Birth of the pup only takes around 1"?4 minutes. The pups take their first swim at around 1"?2 weeks old. They can hold their breath for five minutes enabling them to be able to dive to lengths of 100 m (330 ft). After 6"?7 weeks they are weaned off their mother and can basically take care of themselves. .
Weddell seals eat an array of fish, krill, squid, bottom-feeding prawns, cephalopods, crustaceans and sometimes penguins. . A sedentary adult will need to eat around 10 kg (22 lb) a day, while an active adult will eat over 50 kg (110 lb) a day. 
Weddell seals gather in small groups around cracks and holes in the ice. This animal can also be found in large groups on ice attached to the continent and can be easily approached by humans. In the winter months they stay in the water to avoid blizzards, with only their heads poking through breathing holes in the ice. . They are very docile and placid animals..
Weddell seals have no natural predators as they live on fast ice. If they are caught on pack ice they become prey for leopard seals and killer whales, this can often happen to pups and sub adults out exploring. . The Weddell Seal is protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals.
The Weddell Seal is known for its very deep dives, which may reach some 700 m (2,300 ft). It can also stay underwater for approximately 80 minutes. Such deep dives involve foraging sessions, as well as searching for cracks in the ice sheets that can lead to new breathing holes. The seal can remain submerged for such long periods of time because of high concentrations of myoglobin in their muscles.
The Weddell Seal has a shorter lifespan than most other pinnipeds. On average, the Weddell Seal lives for 20 years, compared to an average life expectancy of 40 years for most other seals. This is because the Weddell seals lives under the Antarctic sea ice in the winter adjacent to continental Antarctica where it must constantly maintain breathing holes by scraping the ice with its teeth. This has the effect of wearing down its teeth over time. Once a Weddell Seal's teeth have worn down to a certain level, the seal is unable to eat and eventually starves to death.
The Weddell Seal lives farther south than any other mammal, inhabiting the waters of McMurdo Sound, 1,300 km (810 mi) from the South Pole.