Weddell seal
The Weddell seal (Leptonychotes weddellii), also known as Weddell's seal, is one of 19 species of marine mammals in the family of True seals. Together with the families of Eared seals and Walruses, True seals form the group of marine mammals known as pinnipeds.
The seal is named after its discoverer James Weddell, the captain of a sealing ship who brought a specimen back from the South Orkney Islands during early exploration of the Antarctic in 1823. It is the most southerly breeding mammal.
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Physical Description
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Scientific Classification Kingdom: Animalia (Animals) |
Adult Weddell seals have a dark gray coat that is marked with black and lighter gray areas. Males are 2.5 to 2.9 meter in length and females reach up to 3.5 meters. They weigh between 400 and 600 kilograms (kg).
Reproduction
Reproductive behavior occurs underwater. The female is mounted by the male from behind while her sides are held by his foreflippers. Quite often she is bitten on her neck while copulation occurs. Male-male fighting occurs, which suggests that mating systems are polygynous. From mid-September to late December active spermatogenesis occurs. In late November to mid-December females are impregnated and about mid-January implantation occurs, with ensuing gestation lasting nine to ten months.
Birth occurs onto the sea ice, which often results in a change of external temperature in newborn pups. Pups are usually born singly, and the time of birth usually varies with latitude from early September at latitude 60 degrees south to late October at latitude 78 degrees south. Weddell seal pups weigh about 29 kg at birth. They have a gray lanugo, which after three to four weeks turns to a dark coat. Young pups are born with their permanent dentition. Weaning takes place at six weeks of age and maturity at three years. First breeding of females may be denied for one or two years under some population conditions and males usually don't mate until six to eight years of age because of social pressures.
Behavior
The majority of the behavior occurs under water during the night. Weddell seals move in slow humping motion on land as well as on ice. They swim at a speed of about five to seven knots, using their fore and hind flippers. Diving has been measured at depths of 600 m, and they can remain under for up to an hour.
Water consumption requirements are met by ingesting sea water as well as well as snow; eating occurs chiefly underwater.
Fighting consists of continual chest contact, but is ceased when striking of chest, neck, or other areas occurs. Play fighting occurs commonly with subadults. Weddell seals sleep in the same position on the stomach or back for hours, but lying on their sides is most common. To maximize the sun's thermal benefits, seals often lie perpendicular to the sun.
The nails of the foreflipper are used for grooming the head, neck, upper sides, and chest. Other areas are groomed by rubbing back and forth against the ice. The eyes of Weddell seals are well developed for low light visibility. This is an adaptive feature of this creature which assists it in locating breathing holes in the ice. Vocalization occurs underwater for communication. Overlapped calls are longer than solitary calls, which constitute the varied repertoire of vocal communication in the Weddell seal.
Distribution
Weddell seals are found on fast ice along the coast of the Antarctic continent as well as on offshore islands. Vagrant Weddell seals have been recorded as far north as South America, New Zealand and southern Australia. A small population of Weddell seals lives year-around on the Southern Hemisphere small island group, South Georgia. The species is the most southerly breeding mammal.
Habitat
Weddell seals live in Antarctic regions on fast ice areas and in the sea. They don't migrate and local movements are caused by changes in ice conditions. Underwater swimming occurs under natural ice cracks or under ice areas thin enough so that the seals can chew breathing holes using canine teeth. Ice areas where these seals dwell are usually flat icy plains.
Predators
Weddell's seals can be preyed on by Orcas (Orca orcinus) or, occasionally, Leopard seals (Hydrurga leptonyx). These seals are heavily infested with worms and often reguritate them as a means of expulsion. The louse, Antarctophthirus ogmorhini, attacks the hind quarters as well as the penile orifice of these seals. Lice also infest subadults.
Food Habits
The diet of Weddell seals consist of Notothenidae fishes, squids, and crustaceans, although they have been witnessed attacking Dissostichus mawsoni (Antarctic toothfish) as large as 54 kg in weight. These seals can dive up to 600 meters in search of food and are stealthy hunters, able to sneak attack fish from close range. They also use a method of disturbing fish from ice cracks by blowing bubbles into them and preying on the fish that emerge.
Conservation Status
Weddell seals exist in considerable numbers (500,000 to 1,000,000) and breed over an expansive geographic area. There are no major threats to the species and it is considered a species of "Least Concern."
Economic Importance for Humans
Weddell seals are often killed and used as dog food. Their dead bodies are also of benefit to those studying worms and parasite infestation since these occur so often in this species. Study of their vocal abilities has advanced our attempt to communicate with animals, similar to the vocal communication studies performed with dolphins.
Further Reading
- Leptonychotes weddellii (Lesson, 1826) Encyclopedia of Life (accessed April 15, 2009)
- Leptonychotes weddellii, Bayi, O., 2000, Animal Diversity Web (accessed April 15, 2009)
- Weddell seal, Seal Conservation Society (accessed April 15, 2009)
- Leptonychotes weddellii, IUCN Red List of Threatened Species (accessed April 15, 2009)
- The Pinnipeds: Seals, Sea Lions, and Walruses, Marianne Riedman, University of California Press, 1991 ISBN: 0520064984
- Encyclopedia of Marine Mammals, Bernd Wursig, Academic Press, 2002 ISBN: 0125513402
- Marine Mammal Research: Conservation beyond Crisis, edited by John E. Reynolds III, William F. Perrin, Randall R. Reeves, Suzanne Montgomery and Timothy J. Ragen, Johns Hopkins University Press, 2005 ISBN: 0801882559
- Walker's Mammals of the World, Ronald M. Nowak, Johns Hopkins University Press, 1999 ISBN: 0801857899
- Weddell seal, MarineBio.org (accessed April 15, 2009)
