Weddell Seal
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McMurdo Sound
The ice-clogged waters of Antarctica's McMurdo Sound extend about 55 km long and wide. The sound opens into the Ross Sea to the north. The Royal Society Range rises from sea level to 13,205 feet on the western shoreline. The nearby McMurdo Ice Shelf scribes McMurdo Sound's southern boundary...
(at 77°S). It is the only species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Leptonychotes, and the only member of the Antarctic tribe
Tribe (biology)
In biology, a tribe is a taxonomic rank between family and genus. It is sometimes subdivided into subtribes.Some examples include the tribes: Canini, Acalypheae, Hominini, Bombini, and Antidesmeae.-See also:* Biological classification* Rank...
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
James Weddell
James Weddell was a British sailor, navigator and seal hunter who in the early Spring of 1823 sailed to latitude of 74°15' S and into a region of the Southern Ocean that later became known as the Weddell Sea.-Early life:He entered the merchant service very...
, the British sealing captain, to the parts of the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...
now known as the Weddell Sea
Weddell Sea
The Weddell Sea is part of the Southern Ocean and contains the Weddell Gyre. Its land boundaries are defined by the bay formed from the coasts of Coats Land and the Antarctic Peninsula. The easternmost point is Cape Norvegia at Princess Martha Coast, Queen Maud Land. To the east of Cape Norvegia is...
. 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 sealCrabeater Seal
The crabeater seal, Lobodon carcinophagus, is a true seal with a circumpolar distribution around the coast of Antarctica. They are medium to large-sized , relatively slender and pale-colored, found primarily on the free floating pack ice that extends seasonally out from the Antarctic coast, which...
(Lobodon carcinophaga), the Ross seal
Ross Seal
The Ross seal is a true seal with a range confined entirely to the pack ice of Antarctica. It is the only species of the genus Ommatophoca. First described during James Clark Ross' British Antarctic Expedition in 1841, it is the smallest, least abundant and least well known of the Antarctic...
(Ommatophoca rossii), and the leopard seal
Leopard Seal
The leopard seal , also referred to as the sea leopard, is the second largest species of seal in the Antarctic...
(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
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, the Mirounga (elephant seals) in the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
to early Pliocene
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
, when they migrated southward and diversified rapidly to form four distinct genera
Genera
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...
in relative isolation around Antarctica.
Physical appearance
Male Weddell seals weigh less than female Weddell seals. So males usually weigh about 500 kg (1100 lbs) or less.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 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 that 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 scar
Scar
Scars are areas of fibrous tissue that replace normal skin after injury. A scar results from the biological process of wound repair in the skin and other tissues of the body. Thus, scarring is a natural part of the healing process. With the exception of very minor lesions, every wound results in...
s, 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 (55.1 to 66.1 lb). They gain around 2 kg (4.4 lb) a day, and by 6–7 weeks old they can weigh around 100 kg (220.5 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
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
it inhabits, this marine mammal
Marine mammal
Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...
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 (328.1 ft). After six to seven weeks they are weaned and begin to hunt independently.
Diving
The Weddell seal is known for its very deep dives, which may reach some 700 m (2,296.6 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
Foraging
- Definitions and significance of foraging behavior :Foraging is the act of searching for and exploiting food resources. It affects an animal's fitness because it plays an important role in an animal's ability to survive and reproduce...
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
Myoglobin
Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
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
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
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
Spleen
The spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
into the rest of their body acting as an oxygen reserve. Muscle cells also contain more myoglobin
Myoglobin
Myoglobin is an iron- and oxygen-binding protein found in the muscle tissue of vertebrates in general and in almost all mammals. It is related to hemoglobin, which is the iron- and oxygen-binding protein in blood, specifically in the red blood cells. The only time myoglobin is found in the...
, which has a high affinity for oxygen.
Other circulatory adjustments include reducing their heart rate, and blood buffering, which prevents the pH of the 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 (often times 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 fishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, krill
Krill
Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...
, squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, 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 (110.2 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
Fast ice
Fast ice is sea ice that has frozen along coasts along the shoals, or to the sea floor over shallow parts of the continental shelf, and extends out from land into sea. In Antarctica, fast ice may also extend between grounded icebergs...
. 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
Convention for the Conservation of Antarctic Seals
The Convention for the Conservation of Antarctic Seals is part of the Antarctic Treaty System. It was signed at the conclusion of a multilateral conference in London on February 11, 1972...
.