The Weddell seal is an accomplished diver, able to reach depths of over 600 metres, ranging out to 5 kilometres from a breathing hole in a single dive, and spending up to 82 minutes underwater before surfacing to breathe (1) (2) (3) (4). Underwater vision is excellent, and a nictitating membrane protects the eyes from salt water and blowing snow (2) (4). Holes in the ice, for breathing and hauling out, are created and maintained by cutting and sawing at the ice with the teeth (1) (2) (4) (5). The Weddell seal feeds mainly on fish, including the large Antarctic cod, as well as on squid, octopus and crustaceans (2) (3) (4) (5) (9), and has also been recorded to occasionally take penguins (9). Although adult Weddell seals are relatively safe from predators, some, particularly younger individuals, are occasionally taken by killer whales and leopard seals (2) (5) (6) (7).
The female Weddell seal gives birth on the ice, usually within a small colony of other females, but the species is only loosely social, and individuals remain well spaced out. Births take place between September and November, and a single pup is usually born, or occasionally twins. The female remains with the pup for the first few weeks, feeding it on milk that is exceptionally high in fat and protein, so that the pup gains almost two kilograms in weight each day. The female does not forage during this time, and loses weight (2) (3) (4) (5) (7). After the first two weeks, the female starts to take the pup into the water, where it learns to swim (2) (4) (6). Weaning occurs at about six to seven weeks. After the last pups have been weaned, the fast ice breaks up, the adults disperse, and the pups are left to fend for themselves (2) (4) (7).
Subadult Weddell seals tend to congregate in large groups near the edge of the ice, away from the breeding colony, while adult males set up underwater territories beneath the ice, typically around ice holes used by the females. Each territory may cover an area up to 50 metres by 400 metres, and is defended by loud calling and sometimes bloody fights (2) (3) (4) (5). Mating takes place in the water, after the pups have been weaned, around mid-December. However, the implantation of the new embryo does not occur until January or February, allowing the female to moult, feed, and recover from lactation before the next young develops, and ensuring that the young will be born the following spring, after a total gestation period of about 11 months (2) (4) (5) (7). The female Weddell seal reaches maturity from about three to six years, and the male at seven to eight years. Lifespan may be up to 25 years (4) (5) (6) (7).