Weddell Seal (marine mammals)


The Weddell seal has a circumpolar distribution around Antarctica. The seal hauls out through cracks in the fast ice formed from tidal action. This fast ice provides a stable platform for giving birth to pups, hauling out to avoid predators, and resting. Use of these traditional haul-out sites facilitated research on this species since the early 1960s. This species is the best studied of all Antarctic pinnipeds because the seals can be found reliably in breeding colonies at traditional sites. Several investigators maintained research programs near the U.S. bases at McMurdo Sound and Palmer Peninsula, the New Zealand Scott Base, and the Australian base at Davis Station.

I. Characters and Taxonomic Relationships

The Weddell seal belongs to the family Phocidae, subfamily Monacinae, and tribe Lobodontini. There is a single species within the genus Leptonychotes. The closest relatives are the other Antarctic seals in the tribe Lobodontini (i.e., crabeater, Lobodori carcinophaga; leopard, Hydriirga leptonyx; and Ross seals, Ommatophoca rossii) and the monk seals in the tribe Monachini (i.e., Caribbean, Mediterranean, and Hawaiian monk seals, Monachus spp.).

Weddell seals are black with grayish silver streaks, with individual variations. The adult fur does not have underfur. In pups, the lanugo is longer and thicker than adult fur and is shed by 44 days of age.

There is no dramatic sexual dimorphism in body size; males reach 2.5-2.9 m and females are 2.6-3.3 m in length. Weight ranges from 400 to 500 kg, with pregnant females attaining the greatest weights. At birth, pups weigh 22-29 kg and are about 1.5 m in length.

As with other phocids, the Weddell seal has a fusiform body shape and laboriously crawls on the belly or rolls to move on ice. It is not capable of upright stance or moving the hindlimbs forward. Under water, the Weddell seal propels itself easily by moving the hind flippers in a vertical plane. Manus are flippers and pes are fully webbed. The first metacarpal is noticeably larger than the others. Black claws on the front flippers are large and probably useful for gripping the ice or scratching. Claws on the hind flipper are reduced in size. The tail is distinct and free.

Its large brown eyes often have wet circles around them because of no lacrimal duct. It has a tapetum, which assists in seeing in low-light levels during the austral winter and deep dives. A nictitating membrane protects its eyes from blowing snow and allows its eyes to be open in salt water. The species has excellent vision under water. Some accounts report that the seal remains in familiar, shallow water areas during the dark austral winter.

The external ear is absent and nothing is known about hearing abilities. The nostrils are oriented vertically and normally are closed, but open when the seal needs to respire. There are seven rows of mystacial vibrissae and superciliary whiskers that are smooth, not beaded as in some pinnipeds. The tip of the tongue is notched. Testes are abdominal, the penis is retractable, and there is a baculum. The uterus is bipartite. Two mammary glands are present and the milk is exceptionally high in fat and protein content.

It has a simple stomach and eats food whole, fish heads down first. It does not drink water, rather obtains water meta-bolically through diet. It has reniculate kidneys adapted for conserving water and removing high salt loads. There are 34 chromosome pairs in this species.

The dental formula is 2/2, 1/1, 4/4, 1/1. The milk teeth disappear before or soon after birth. Cheek teeth have three points, but are not the exaggerated tricuspid structure of the crabeater and leopard seals. The outer incisors are larger and procumbent or project forward. The canines also are procumbent and. along with the last incisor, are used in “ice-sawing” behavior. The seal maintains circular breathing holes in the fast ice by turning within the hole and raking ice off the rim of the hole with its teeth. Such behavior allows the seals to maintain holes for breathing and hauling out throughout the year. However, ice-sawing behavior has prevented this species from being successfully maintained in captivity.

The seals stay warm through a heavy fur coat and thick layer of subcutaneous fat; their lower critical temperature is — 40°C. Weddell seals can vasodilate to dissipate heat or vasoconstrict to conserve body heat. On a warm Antarctic day, steam can be seen rising from their bodies.

II. Distribution and Ecology

The subfamily Monachinae evolved in the North Atlantic during the Miocene. A fossil, Homiphoca, found off the South African coast could be the intermediate form between monk seals and Antarctic seals.

No systematic, large-scale population census studies have been conducted, so estimates in the literature are approximate. The Weddell seal is abundant; the estimated range is from 500,000 to 1 million seals. Occasionally, single seals are seen at sub-Antarctic islands, including South Shetlands, the South Orkneys, and South Georgia. Single wandering Weddell seals have been found in remote locations such as Heard, Kerguelen, Macquarie, Auckland, Juan Fernandez, and Falkland Islands, as well as Australia, New Zealand, Patagonia, and Uruguay.

There is no predictable migration in this species, but because there is a trend to move toward the fast ice, seals move northward before the onset of winter as the ice expands and southward during the summer to find pockets of fast ice.

Weddell seals are extremely good divers, commonly diving about 600 m, for up to 82 min (20 min is typical), ranging out to 5 km from a breadiing hole and returning on a single dive. A great deal about their diving behavior has been documented using satellite tags with time-depth recorders and video cameras attached to the seal’s back. Before a dive, the seal exhales and die nostrils and mouth are closed. Seals usually make a series of short shallow dives before commencing on a longer dive. Swim speed is 8-12 km/hr and die descent rate for a dive is 35 m/min. During a dive, die Weddell seal collapses the lungs, compresses the trachea, shunts blood to extremities, drops the metabolic rate to 20% of the resting rate, and undergoes bradycardia from about 85 to 16 beats/min. The blood and muscles have a three to five times greater oxygen-carrying capacity than that of humans.

III. Behavior and Life History

Modier Weddell seals return to tidal cracks each austral spring and give birth to pups from late September to early November. Typically, a single pup is born, but twins have been observed and even an albino pup was reported. A few mothers have pups in isolation from odier seals, but most give birth in colonies of up to 50 mothers with pups. Mothers and pups maintain individual spacing when hauled out on ice (Fig. 1). When disturbed, modi-ers vocalize to intruders. Pups nurse for a 7- to 8-week period, gaining almost 2 kg per day. At weaning, pups weigh about 125 kg and mothers have lost nearly that amount of weight because they do not forage during the nursing period. Pups are first enticed into the water by their modier’s calling at 10-14 days of age. Pups struggle to swim and stay under the water, popping to the ice surface. Some pups die in die breathing holes, not being able to crawl out of the slippery, steep hole. Modiers and older pups go into the water progressively more often as the pup grows. They exhibit a distinct diel pattern of haul-out, with most seals hauling out in the colony for several hours around midday.

Adult males are not tolerated on the ice in maternal colonies. Only occasionally is a nonbreeding, adult male seen in die periphery of the colony. Soon after females give birth, adult males arrive and compete for underwater territories aligned beneath the tidal cracks. Males patrol using loud (up to 193 dB re 1 |xPa) trills to advertise and defend their underwater territories, which cover an area 15-50 m wide by 50-400 m long. Sometimes bloody fights occur between males under water and even continue onto the ice. with the loser evicted from the territory. The underwater repertoire of Weddell seals is elaborate, including some 34 sound types near McMiudo Sound. Geographic variations in the repertoire were documented among McMurdo Sound, Palmer Peninsula, and Davis Station seals. Their sounds are some of the longest among marine mammals, ranging up to 70.0 sec, with 75 calls given per minute by a colony during die height of the breeding season. Mating takes place in water, with about 80% of females becoming pregnant, at least in the McMurdo Sound area. Once die pups wean, the mother enters the water to feed and mating occurs within a male’s territory. Adult seals molt after mating. Implantation of the blastocyst is delayed until mid-January. Because of delayed implantation, pups are bornm the following spring at approximately the same time of year. Delayed implantation allows the mother to molt, feed, and recover from the dramatic weight loss associated with lactation before another fetus starts to develop.

A mother Weddell seal calls to her pup to maintain spacing within a breeding column.

Figure 1 A mother Weddell seal calls to her pup to maintain spacing within a breeding column.

About the time the last pup in the colony weans in mid-December the fast ice breaks up, adults disperse, and newly weaned pups are left to fend for themselves. Some data taken from pups with transmitters indicated pups feed near shore on small fish such as Pleurograma spp.

Subadult seals are rarely seen in breeding colonies. They tend to congregate in large groups near the ice edge. Subadult Weddell seals do not exhibit the scars from leopard seal pre-dation that are seen oil crabeater seals. However, killer whales are known to take Weddell seals of all ages. When killer whales (Orchitis orca) and leopard seals move into areas of Weddell seals, Leptonychotes suddenly and dramatically stops calling underwater, perhaps to avoid detection.

An adult seal was seen to surface in a breathing hole with a large, 100-pound, Antarctic cod (Dissostichus mmvsoni) in its mouth. Video footage from a camera mounted oil a seal’s back recorded an episode of capture of this large prey. If a fish hides in a crevice, the seal blows bubbles into the water to chase the fish into an open area.

IV. Interactions with Humans

The seals have little fear of humans because there are no natural land predators like the (Arctic) polar bears. James Weddell reported seeing and hearing this seal during his Antarctic expeditions and the seal was named after him. A detailed description of this species was recorded in 1907 by E. A. Wilson while he spent a 2-year period at Hut Point on Captain R. F. Scott’s first expedition to Antarctica. The expedition reportedly took seals to feed their sled dogs. In 1934-1935, A. A. Lindsey studied Weddell seals at the Bay of Whales as part of Admiral R. E. Byrd’s second Antarctic expedition and specimens now are deposited in the Field Museum of Natural History in Chicago. In 1940, G. C. L. Bertram studied the species as part of the British expedition to the Antarctic Peninsula. J. Sapin-Jaloustre (1952) studied the seal at Terre Adelie Land as part of a French expedition. Some early specimens were collected by the Little America explorers. In 1957. the International Geophysical Year established the importance of conservation and research in the Antarctic. The Antarctic treaty was signed in 1961, establishing the protection of the species and its habitat. During the 1970s and 1980s, inhabitants of the New Zealand station in McMurdo Sound took Weddell seals to leed their sled dogs over the winter. This practice has stopped. Otherwise. there is no record of extensive harvest of this species.

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