Species of Arctic dwelling marine mammal
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The bearded seal (Erignathus barbatus), also called the square flipper seal, is a medium-sized pinniped that is found in and near to the Arctic Ocean. It gets its generic name from two Greek words ("eri" and "gnathos") that refer to its heavy jaw. The other part of its Linnaean name means bearded and refers to its most characteristic feature, the conspicuous and very abundant whiskers. When dry, these whiskers curl very elegantly, giving the bearded seal a "raffish" look.
Bearded seals are the largest northern phocid. They have been found to weigh as much as with the females being the largest. However, male and female bearded seals are not very dimorphic.
The only member of the genus Erignathus, the bearded seal is unique in that it is an intermediate. Bearded seals belong to the family Phocidae which contains two subfamilies: Phocinae and Monachinae. The bearded seal possesses characteristics of both of these subfamilies.
Fossils first described in 2002 indicate that, during the Pleistocene epoch, bearded seals ranged as far south as South Carolina.
Description.
Distinguishing features of this earless seal include square fore flippers and thick bristles on its muzzle. Adults are greyish-brown in colour, darker on the back; rarely with a few faint spots on the back or dark spots on the sides. Occasionally the face and neck are reddish brown. Bearded seal pups are born with a greyish-brown natal fur with scattered patches of white on the back and head. The bearded seal is unique in the subfamily Phocinae in having two pairs of teats, a feature it shares with monk seals.
Bearded seals reach about in nose-to-tail length and from in weight. The female seal is larger than the male, meaning that they are sexually dimorphic.
Bearded seals, along with ringed seals, are a major food source for polar bears. They are also an important food source for the Inuit of the Arctic coast. The Inuit language name for the seal is "ugjuk" (plural: "ugjuit") or "oogrook" or "oogruk". The Inuit preferred the ringed seal for food and light; the meat would be eaten and the blubber burnt in the "kudlik" (stone lamp). The skin of the bearded seal is tougher than regular seal and was used to make shoes, whips, dog sled harnesses, to cover a wooden frame boat, the "Umiak" and in constructing summer tents known as "tupiq".
The body fat content of a bearded seal is about 25–40%.
Distribution.
Bearded seals are extant in Arctic and subarctic regions. In the Pacific region, they extend from the Chukchi Sea in the Arctic, south into the Bering Sea where they span from Bristol Bay on the Alaskan coast to the Sea of Okhotsk on the Russian coast, up to but not including the northern coast of Japan. In the Arctic Ocean, they are found along the northern coasts of Russia, Norway, Canada, and Alaska, including the Norwegian Archipelago of Svalbard and Canadian Arctic Archipelago. In the Atlantic, Bearded seals are found along the northern coast of Iceland, the east and west coasts of Greenland and the Canadian mainland as far south as Labrador.
Although the range typically only extends down into subarctic areas bearded seals have been seen in Japan and China as well as extremely far south of their range in Germany, Netherlands, United Kingdom, France, Spain and Portugal.
Hunting and diet.
Primarily benthic, bearded seals feed on a variety of small prey found along the ocean floor, including clams, squid, and fish. Their whiskers serve as feelers in the soft bottom sediments. Adults tend not to dive very deep, favoring shallow coastal areas no more than deep. Pups up to one year old, however, will venture much deeper, diving as deep as . In a study conducted during the summer months, the seals have been found to feed on invertebrates such as anemones, sea cucumbers, and polychaete worms. The same study found that sculpins and Arctic cod made up most of their summer diet. Sculpin were also found to be the largest fish consumed by the seals. Bearded seals are capable of preying on pelagic and demersal fish in addition to their benthic prey.
Reproduction and lifecycle.
Bearded seals give birth in the spring. In the Canadian Arctic, seal pupping occurs in May. In Svalbard, bearded seals reach sexual maturity at 5 or 6 years of age. Further south, in Alaska, most pups are born in late April. Pups are born on small drifting ice floes in shallow waters, usually weighing around . They enter the water only hours after they are born, and quickly become proficient divers. Mothers care for the pups for 18–24 days, during which time the pups grow at an average rate of per day. During this time, pups consume an average of of milk a day. By the time they are weaned, the pups have grown to about .
Just before the pups are weaned, a new mating cycle takes place. Females ovulate at the end of their lactation period, but remain close to their pups, ready to defend them if necessary. During the mating season, male seals will "sing", emitting a long-drawn-out warbling note that ends in a sort of moan or sigh. This sound may attract females, or may be used by the males to proclaim their territory or their readiness for breeding. Males occupy the same areas from one year to the next.
Like many Arctic mammals, bearded seals employ a reproductive strategy known as delayed implantation. This means that the blastocyst is not implanted for two months after fertilization, most often becoming implanted in July. Thus, the seal's total gestation period is around eleven months, though its active gestation period is nine months.
Natural predators of the bearded seal include polar bears, who rely on these seals as a major food source. Killer whales also prey on these seals, sometimes overturning ice floes to reach them. Walruses also eat these seals, mainly pups, but such predation is rare.
Bearded seals are believed to live up to 31 years.
Vocalization.
The vocalizations produced by the bearded seal are very unique, possibly because their trachea is different from that of other Northern Pacific phocids. A majority of the rings in the trachea are incomplete with only a membrane attaching the two ends.
The sounds of the bearded seal usually consist of a long oscillating trill lasting for a minute or more followed by a short, deep moan. This "song" is often repeated frequently. The number of call types within a population can vary geographically, with four types found in Svalbard and eleven in the Western Canadian Arctic. The most frequent sounds are trills, moans, and sweeps. A sweep can be compared to a short trill.
Bearded seals produce distinct trills from late March to late June, with a decline in rhythmicity in late May and June. This timeline coincides with their breeding and pupping season, which is from April to May. The repetitive and transmittable nature of bearded seal trills leads researchers to believe that they are utilized for communication, likely during courtship and breeding. Males use these sounds to establish mating territories and communicate their fitness, but it is likely that females produce these sounds as well.
Underwater, bearded seal trills can be heard from a distance of over , with some types of sounds traveling farther than others. This makes it possible for one animal to communicate with another animal that is far away, although acoustic degradation does occur as the sound passes through the environment. A seal must produce a trill with a sound-pressure of at least 100 dB at 1 m in order for the sound to propagate 30 km, meaning that bearded seals can likely produce sounds at this level.
Conservation status.
On March 28, 2008, the U.S. National Marine Fisheries Service initiated a status review under the Endangered Species Act (ESA) to determine if listing this species under the ESA is warranted. All bearded seals are protected under the Marine Mammal Protection Act, and determined by the IUCN to be classified as a "least concern" for extinction. This classification was determined due to various factors including, widespread distribution, stable population size, as well as alternating prey resources. NOAA determined that the factors influencing any change in conservancy status of the bearded seal may include: loss of sea ice by climate change, bycatch from commercial fishing gear, and hunting. Their main predators include polar bears; however, typically pups around age 2 are attacked within birthing lairs, leaving older juveniles and adults commonly unharmed. Due to climate change, factors such as loss of sea ice, as well as decrease in prey population may create negative results on the bearded seal population in the future. Therefore, monitoring of the species as well as influences of human activity, will be vital to ensure species stability.
Subspecies.
There are two recognized subspecies of this seal:
While the validity of these subspecies has been questioned, and is not yet supported by any molecular data, analysis of the animals' calls does indicate a differentiation between different populations.
Evolutionary history.
Bearded seal fossils have been found to be as old as the early to mid Pleistocene. These early fossils were found in northern regions like England, Alaska, and Sweden, as well as the North Sea and the Champlain Sea.
Bearded seals, like all true seals, belong to the family Phocidae which is one of the three families in the clade Pinnipedia, along with Otariidae and Odobenidae. Pinnipeds are thought to have originated 27 to 25 million years ago during the late Oligocene period. One hypothesis for the evolution of pinnipeds is that pinnipeds are a diphyletic group and otariids and odobenids are more closely related to bears, and phocids are more closely related to mustelids like weasels. Another hypothesis suggests that pinnipeds are a monophyletic group that descended from a single ancestor. This has been more supported by phylogenetic analysis than the diphylectic hypothesis. One such study suggests that phocids are sister taxa to the common ancestor to both otariids and odobenids.
Bearded seals belong to the subfamily Phocinae (Northern Hemisphere seals). Phocinae can be further divided into three clades: Erignathini (bearded seals), Cystophorini (hooded seals), and Phocini (all remaining genera). Bearded seals are the sister taxa to the common ancestor of Cystophorini and Phocini.
References.
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Further reading.
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