The Weddell seal, Leptonychotes weddellii, is a relatively large and abundant true seal (family: Phocidae) with a circumpolar distribution surrounding Antarctica. Weddell seals have the most southerly distribution of any mammal, with a habitat that extends as far south as McMurdo Sound (at 77° S). It is the only species in the genus Leptonychotes, and the only member of the Antarctic tribe of lobodontine seals to prefer in-shore habitats on shore-fast ice over free-floating pack ice. Because of its abundance, relative accessibility, and ease of approach by humans, it is the best studied of the Antarctic seals. It is estimated that there are approximately 800,000 individuals today. Weddell Seal pups leave their mothers at the age of a few months. In those months they get fed by their mothers fat and warming milk. They soon leave when they are ready to hunt and are fat enough to survive in the harsh weather
The Weddell seal was discovered and named in the 1820s during expeditions led by James Weddell, the British sealing captain, to the parts of the Southern Ocean now known as the Weddell Sea. However, it is found in relatively uniform densities around the entire Antarctic continent.
Taxonomy and evolution
The Weddell seal shares a common recent ancestor with the other Antarctic seals, which are together known as the lobodontine seals. These include the crabeater seal (Lobodon carcinophaga), the Ross seal (Ommatophoca rossii), and the leopard seal (Hydrurga leptonyx). These species share teeth adaptations including lobes and cusps useful for straining smaller prey items out of the water column. The ancestral Lobodontini likely diverged from its sister clade, the Mirounga (elephant seals) in the late Miocene to early Pliocene, when they migrated southward and diversified rapidly to form four distinct genera in relative isolation around Antarctica.
Weddell seals measure about 2.5-3.5 m (8.2-11.5 ft) long and weigh between 400–600 kg (880-1360 lbs). Male Weddell seals weigh less than female Weddell seals. So males usually weigh about 500 kg (1100 lbs) or less. Male and female Weddell seals are generally about the same size, though females can be slightly larger. However, male seals tend to have a thicker neck and a broader head and muzzle than the females. 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. Their upturned mouths give them the appearance of smiling.
The Weddell seal grows a thin fur coat around their whole body except for small areas around the flippers. The colour and pattern of the coat varies, often fading to a duller colour as the seal ages. This coat moults around the beginning of summer. Adults are generally brown, with lighter ventral (belly) pelage. They are 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 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 around half the length of their mother at birth, 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).
Behavior and breeding
Weddell seals gather in small groups around cracks and holes in the ice. These animals can also be found in large groups on ice attached to the continent. In the winter months, they stay in the water to avoid blizzards, with only their heads poking through breathing holes in the ice. This seal is often observed lying on its side, when on land. They are very docile and placid animals and can be approached easily.
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, Weddell seals make noises that are loud enough to be felt through the ice. 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 six to eight years 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 one to four minutes. The pups take their first swim at around one to two weeks old. They can hold their breath for five minutes, enabling them to dive to depths of 100 m (330 ft). After six to seven weeks they are weaned and begin to hunt independently.
The Weddell seal is known for its very deep dives, which may reach some 750 m (2,460 ft). After dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 meters, allowing them to dive with little effort as they make a “meandering descent.
They 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 seals can remain submerged for such long periods of time because of high concentrations of myoglobin in their muscles.
Weddell seals' metabolism is relatively constant during deep water dives, meaning there must be another way to account for functioning with a lack of oxygen over an extended period of time. Seals, unlike other mammals (such as humans) can undergo anaerobic metabolism for these extended dives, which causes a build up of lactic acid in the muscles. The lactic acid does not enter the bloodstream, however, until after the animal has surfaced. This is done by constricting the capillaries going from the muscles to the veins. This requires a longer recovery time though, which in the long run, may be less efficient than quicker, aerobic metabolic dives.
These seals also compensate for prolonged lack of available oxygen by increasing their oxygen carrying capacity. This is done by having more red blood cells per unit volume of blood, as well as more having more blood relative to other mammals. Typical oxygen concentration levels in human blood at sea level are about 15cc/kg, where as Weddell Seals can have 60cc/kg. They can also release oxygenated blood from their spleen into the rest of their body acting as an oxygen reserve. Muscle cells also contain more myoglobin, which has a high affinity for oxygen.
Other circulatory adjustments include reducing their heart rate, and blood buffering, which prevents the pH of the blood from decreasing too much. Low pH in the blood sends a signal to the brain that it needs oxygen before it actually needs oxygen. This can be observed after holding ones breath for a long time, and then exhaling, relieving the need to breath for a few more seconds.
The seals do not send blood to where it is not needed while diving. Essential structures like the brain still receive blood, while the gut and lungs may not (oftentimes they will collapse their lungs at great depths due to such high pressure). It is important to understand that diving mammals are not able to dive for long periods of time because of a higher lung capacity, but more importantly, because of increased oxygen carrying capacity in the blood and muscles.
Diet and predation
Weddell seals eat an array of fish, krill, squid, bottom-feeding prawns, cephalopods, crustaceans and sometimes penguins. A sedentary adult will eat around 10 kg (22 lb) a day, while an active adult will eat over 50 kg (110 lb) a day.
Scientists believe Weddell seals rely mainly on eyesight to hunt for food when light is available. However, during the Antarctic winter darkness, when there is no light under the ice where the seals forage, they rely on other senses, primarily vibrissae or whiskers, which are not just hairs but very complicated sense organs with more than 500 nerve endings that attach to the animal’s snout. The hairs allow the seals to detect the wake of swimming fish and use that to capture prey.
Weddell seals have no natural predators when on fast ice. At sea or on pack ice, they become prey for killer whales and leopard seals, which prey primarily on juveniles and pups. The Weddell seal is protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals.
The Weddell Seal habitat generally extends no further than a concentric ring around the Antarctic shoreline. They spend most of their lives in the water rather than on the ice sheets or dry land.
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|This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (May 2008)|
- ADW: Leptonychotes weddellii
- Adaptions of Diving Mammals
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