The Weddell Seal is a common sight around the coastline and surrounding sea ice of Antarctica, and is well adapted for life throughout the Antarctic year. Weddells live on flat areas of ice attached to the Antarctic mainland, and on freer-floating areas of pack ice, and they dive beneath the ice to feed on fish, crustaceans and molluscs.
During the Antarctic summer the extent of the sea ice shrinks to about 2.6 million km², but in winter it grows to about 14.2 million km², and becomes thicker. In the summer the temperature around the Antarctic coast and the sea ice ranges between 0°C and -4°C, and in winter between -10°C and -15°C. These temperature ranges are mild in comparison to the brutal extremes of the inner Antarctic continent, where temperatures plummet to -70°C or worse in the winter. But life on the Antarctic coast and sea ice presents great challenges to a mammal, which must be able to cope with the cold air on land, the much greater chilling effect of swimming in the cold ocean, and the challenge of finding food through the barrier of the sea ice. Weddell Seals, however, have a range of physical and behavioural adaptations that allow them to cope well with the cold air, the cold water and the sea ice.
Their bodies are insulated by a thick layer of blubber, which also aids in buoyancy. They have a streamlined, slug-like shape which gives them the smallest possible surface area in relation to their body mass, which means that they lose less heat to their surroundings. They also conserve heat in the way that blood circulates around their body: the arteries that carry warm blood out from the heart run very close to the veins that bring cool blood back from the body extremities, and much of the heat is transferred from arteries to veins, so minimising the heat that is lost at the skin. When basking in the sun, Weddells have also been observed to lie at right-angles to the sun, to maximise the warmth they receive.
Adults have a thin fur layer, which provides insulation on land and in the water, though the insulating properties of fur are more effective when dry. Pups have a thicker layer of fur, which insulates them well from the cold air as they spend the first stage of their life on land, being fattened by their mothers’ rich milk, until they have grown and built up a blubber layer sufficient to insulate them in the water. At this stage they shed their pup fur.
The fat content of Weddell Seal milk is extremely rich, in common with all mammals that need to develop a thick layer of blubber. The Weddell pup is able to rapidly pack on a thick blubber layer which will insulate it when it takes to the sea, and both add to its body mass and streamline its body shape to maximise its ability to retain body heat. (Penguin chicks are able to pile on the weight in an analogous way, as the regurgitated fish supplied by their mothers is very rich in fish oils.)
Weddells need access to gaps in the sea ice, so that they can dive down to hunt and re-emerge to breathe. This becomes a greater challenge as the sea ice grows and thickens in the winter, but Weddells can take advantage of natural gaps in the ice, either cracks or natural openings called polynyas, which are kept open by winds and water currents. They also create and maintain breathing holes in the ice, keeping them open by gnawing at them with their canine teeth.
The cold Antarctic seas are surprisingly rich in life. Cold water contains much more oxygen than warm water, and the polar oceans are also rich in other nutrients essential for life, such as nitrates, silicates and phosphates. Winds and waves have the effect of mixing the upper layer of the ocean, which helps to distribute these nutrients. These provide the conditions for vast quantities of microscopic organisms called plankton to survive and thrive, and these plankton are the basis of a rich marine ecosystem. And it’s not just the sea which is rich in microscopic life – the sea ice itself may look devoid of life, but it can contain a thick layer of algae, as rich in plant matter as an especially lush field of grass!
Plant plankton harness the sun’s energy, especially in the polar summer, to provide the first stage of the food chain. They in turn may be eaten by carnivorous plankton, and these in turn by a kind of shrimp called krill, which is enormously abundant in Antarctic waters. These krill then provide food for a whole marine food web, including many species of fish well adapted to such cold waters, squid, whales, penguins, seabirds, and a range of seal species including the Weddell.
Article for OU Module S175 The Frozen Planet © Andrew Murray