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. Genetic evidence suggests that Weddell seal population numbers may have increased during the Pleistocene. Because of its abundance, relative accessibility, and ease of approach by humans, it is the best-studied of the Antarctic seals. An estimated 800,000 individuals remain today. A genetic survey did not detect evidence of a recent, sustained genetic bottleneck in this species, which suggests that populations do not appear to have suffered a substantial and sustained decline in the recent past. Weddell seal pups leave their mothers at a few months of age. In those months, they are fed by their mothers' warming and fat-rich milk. They 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. 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 ft 2 in–11 ft 6 in) long and weigh 400–600 kg (880–1,320 lb). Males weigh less than females, usually about 500 kg (1,100 lb) or less. Male and female Weddell seals are generally about the same length, though females can be slightly larger. However, the male seal tends to have a thicker neck and a broader head and muzzle than the female. A molecular genetic based technique has been established to confirm the sex of individuals in the laboratory. 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 its 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 3–4 weeks; later, they turn a darker color. The pups reach maturity at three years of age. The pups are around half the length of their mother at birth, and weigh 25–30 kg (55–66 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).
Weddell seals are commonly found on fast ice, or ice fastened to land, and gather in small groups around cracks and holes within the ice. In the winter, they stay in the water to avoid blizzards, with only their heads poking through breathing holes in the ice. These seals are often observed lying on their sides when on land. They are very docile, placid animals that can be easily approached.
Weddell seals are non-migratory phocids that move regionally to follow the distribution of breathing holes in the ice between seasons. Weddell seals dive to forage for food, maintain breathing holes in fast ice, and explore to find more ice holes. These seals exhibit a diurnal haul-out pattern. A higher frequency of seals haul out during the afternoon, usually around 4:00 PM, because of warmer air temperatures.
Weddell seals return to fast ice colonies during the spring for birthing and breeding. Weddell seal populations will often return to the same breeding sites over consecutive breeding seasons. 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. Weddell seals usually give birth to one pup per year, however the Weddell seal is one of the only species of seals that can give birth to twin pups. Birth of the pup only takes around one to four minutes. Newborn pups weigh about 29 kg and grow to two times their weight within their first week of life. The pups take their first swim around one to two weeks old. During the first two weeks mother Weddell seals distinguish their pups through olfactory smells, specialized vocalizations, and stay in the same spatial area. 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 average lifespan of a Weddell seal is about 30 years.
Weddell seals have a high potential for polygyny. Males do not participate in raising newborn pups and instead focus on mating as much as possible during the breeding season. Additionally, fast ice breeding grounds causes females to cluster in large aggregations, making it easier for males to take control over his own harem.
The mating season occurs during austral spring between late November and December after pups are weaned and females begin ovulating. During the mating season, Weddell seals make noises loud enough to be felt through the ice. Males defend underwater territories during the breeding season and have been observed to fight.Copulation has only been observed to occur under water, where the female submits to the male as he approaches her dorsal side. The female is often bitten on the neck by her partner if she tries to escape or terminate copulation. The seals are normally around six to eight years old when they first breed, but this can be much earlier for some females.
Weddell seals undergo delayed implantation. The embryo is not embedded into the uterus until the beginning of austral summer, between mid-January and mid-February, allowing for birth under more favorable environmental conditions.
Weddell seals have been observed to dive as deep as 600 m for up to an hour Such deep dives involve foraging sessions, as well as searching for cracks in the ice sheets that can lead to new breathing holes. Weddell seals exhibit a diel dive pattern, diving deeper and longer during the day than at night. After dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 m, allowing them to dive with little effort as they make a “meandering descent". 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, so another way to compensate for functioning with a lack of oxygen over an extended period of time must exist. Seals, unlike other terrestrial mammals such as humans, can undergo anaerobic metabolism for these extended dives, which causes a build-up of lactic acid in the muscles. The seals can also release oxygenated blood from their spleens into the rest of their bodies, acting as an oxygen reserve.
Male and female Weddell seals communicate through a variety of sounds, males specifically use “trills” to communicate sometimes. Weddell seals are also able to communicate to each other through different mediums. Weddell seals on ice are able to hear the calls of Weddell seals in the water as long as noise level on land is low and they are in close proximity of one another. Sound waves can be transmitted either through the ice itself or from water to breathing holes where female Weddell seals usually are when breeding.
There have been recordings of Weddell seal vocalizations that are described as songs. Their songs consist of repetitive sequences of the same vocal elements, and they only vary slightly over time. Individual Weddell seals can each produce their own unique song, but singing behavior is not common when observing them.
Vocalizations are also important in mother-pup Weddell seal interactions. Mother Weddell seals use vocalizations to call their pups from further distances when smell can no longer be used efficiently. Pups also use higher, more urgent vocalizations when hungry to alert their mothers to feed.
Diet and predation
Weddell seals are top predators in the Antarctic. They eat an array of fish, bottom-feeding prawns, cephalopods and crustaceans. A sedentary adult eats around 10 kg (22 lb) a day, while an active adult eats over 50 kg (110 lb) a day.Cod icefish constitute the majority of their diet. Cephalopods are common prey, and crustacean remains are sometimes found in Weddell seal scat, but at much lower rates than other prey species. They are opportunistic feeders that hunt in different parts of the water column depending on prey availability. Weddell seals hunt in both pelagic and benthic-demersal habitats.
Although seabirds are not part of their diet, there have been sightings of them chasing and killing penguins in the wild. Other Antarctic phocids are known to be seabird predators, resulting in implications that penguin hunting is a learned behavior. There are recordings of four different penguin species being attacked by Weddell seals: Gentoo penguins, an emperor penguin, an Adélie penguin, and a chinstrap penguin. It has not been confirmed, however, if the penguins were consumed after being killed.
Scientists believe Weddell seals rely mainly on eyesight to hunt for food when there is light. However, during the Antarctic winter darkness, when there is no light under the ice where the seals forage, they rely on other senses, primarily the sense of touch from their 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 are circumpolar and widely distributed throughout the Southern Hemisphere where individuals inhabit areas of both pack ice and fast ice. Large numbers of individuals can be found most abundantly on fast ice that continues on until the Antarctic shoreline, and occasionally in pack ice regions located offshore—near the limits of the Antarctic Convergence. A small population has also been observed year-round in Larsen Harbor, South Georgia. Individuals have been reported wandering north of Antarctica in South America, New Zealand and south of Australia.
Physical factors, such as glacial movement and tidal action, also increase fluctuations in distributions. For example, pupping colonies are highly variable due to these factors as they provide necessary areas for breathing. Weddell seals are not migratory but move as the distribution of breathing holes and exit cracks within the ice change throughout the winter. 
Throughout the early periods of the Antarctic exploration, Weddell seals suffered dramatic declines as they were hunted for food and oil. Fortunately, populations have since recovered after the elimination of commercial sealing in the 1950s.
The effects of global climate change on Antarctic seals are still to be fully determined however, research estimates seal populations may decline as the availability of their habitat is extremely temperature sensitive thus making them potentially vulnerable.
There are no immediate threats to the Weddell seal, and the species is not listed as endangered or threatened. The Weddell seal is protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals.
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