The Weddell seal is large seal with a bulky body, and a relatively small head with a short, wide snout. Adults are brown to dark gray with light and dark patches on the back and silvery white on the belly. The front flippers are smallish relative to body size. The fur color of pups is light gray or sometimes golden.
Weddell seals inhabit Antarctic and sub-Antarctic waters. They do not migrate and winter are spent under the ice of the Antarctic Sea, emerging through blowholes and cracks to breathe. They congregate in large groups on pack ice. They are found in areas further south than all other mammals, 1,287 km (800 miles) distant from the South Pole in McMurdo Sound waters.
Weddell seals form small groups around holes and cracks in the ice. This is its lifeline, as the hole is essential for both resurfacing for air and diving for food. When on ice, a seal will rarely travel more than three meters away from its hole. Most of its activities happen at night under water. The seals can dive down as far a 600 m, and stay under water for as long as an hour. They are especially noisy underwater, with a constant barrage of sounds. Vocalizations made by the male are more complex than the female's noises and can be heard as far as 20 miles away. Such sounds could be for territorial defense. The Weddell seal is not very social, but will associate with others at comfortable distances on the ice.
The diet is mostly fish and squid, favorites being the emerald rock-cod and the Antarctic silverfish. In summer, Weddell seals hunt a little more at night and it seems that they eat their food underwater. They get enough water from metabolizing sea water or from their food but sometimes individuals will eat snow.
Male-male fighting occurs, which suggests that mating systems are polygynous. Reproductive behavior occurs underwater. Birthing is from September to December. The gestation period is 9-10 months. A single pup is usually born but sometimes twins are produced. The mother and pup then stay together. The pups first enter the water around 1-2 weeks old. Breeding males stay in the water under the ice, defending territory and a breathing hole until pups are weaned at 6 weeks old. The pup is then left to learn to hunt. The mother goes off to mate again. Weddell Seal females are mature sexually as early as 3 years old, but both females and males don't usually breed until they're about 7.
Weddell seals face no immediate or significant threats. Potential threats are climate change and global warming. Weddell seal numbers may decline with increasing temperatures if Antarctic sea ice is significantly reduced. Disturbance by vessel noise and close approach by people may also harm this seal.
According to IUCN, the global population of Weddell seals has been variously estimated at 200,000 to 1,000,000 individuals. The ICUN classifies the Weddell seal as "Least Concern".