Weddell seal - Leptonychotes weddellii

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Weddell seals are large heavy bodied phocids. The chest and abdomen of the fusiform body are long and thick. The neck is short and the head small, rounded, and flattened on top. The eyes are forward facing and large. The muzzle is short and somewhat narrow with a sparse number of inconspicuous short vibrissae. Taken together, the head and muzzle shape, large eyes, and upturned mouth line impart a cat-like appearance to the face. Weddell seal fore flippers are short, relatively narrow, and owing to the short neck originate in the forward third of the body.

In freshly molted pelage, adult Weddell seals are generally dark gray above with silver to light gray highlights, and pale gray to off-white below. There is a variable amount of light and dark spotting, streaking, and blotching. These markings are fewer or absent on the top of the back, and become heavier on the sides and ventral surface, where the markings can fuse into irregular shapes. Dorsal color fades from a rich very dark gray just after the molt to a dull brownish gray over the 11 to 12 months prior to the start of the next molt. The pelage on the end and sides of the muzzle has light gray to whitish highlights, and there are similar crescentric-shaped highlights above and behind the eyes. Pups are born in a woolly silver gray coat, with a darker swath along the mid-line of the back. This lanugo coat is molted for the adult pelage starting one to four weeks after birth, and the molt is usually completed in two to three weeks.

Adult males reach 2.9 m in length, females 3.3 m. Adults in their prime weigh 400 to 450 kg, with females somewhat heavier than males, but not enough so as to permit separation of the sexes in the field based on weight or length. Adult female weight fluctuates dramatically during the year with dramatic weight loss occurring after birth and during lactation. Newborns are about 1.5 m long and average 29 kg.

The dental formula is I 2/2, C1/1, PC 5/5. A special adaptation of the second upper incisors and the upper canines is that they are procumbent, or forward facing.

Four phocids share the Weddell seal's range. Of these, Ross and crabeater seals are the most similar, and leopard and southern elephant seals are more readily separated from Weddell seals. Ross seals differ in having a proportionately larger and wider neck and head, very small muzzle, that taken with the neck and head size impart a blunt profile to the face, and stripes on the neck and sides. Crabeater seals have a longer head and muzzle, with an upturned appearance to the end of the muzzle. They are also paler dorsally and on their sides, with fewer spots than are found on Weddell seals. Also, Weddell seal fore flippers are shorter and set more forward on the body than those on crabeater seals.

Circumpolar and widespread in the Southern Hemisphere, Weddell seals occur in large numbers on fast ice, right up to the Antarctic continent. They also occur offshore in the pack ice zone north to the seasonally shifting limits of the Antarctic Convergence. Weddell seals are present at many islands along the Antarctic Peninsula that are seasonally ice-free. Vagrants have been recorded in many northern areas including South America, New Zealand and southern Australia.

Weddell seal pups are born from September through November and nursed for seven to eight weeks. Those animals in lower latitudes pup earlier than animals living at higher latitudes. Males set up territories in the water around access holes in the ice used by females to enter and leave the water. The only copulation that has been observed occurred underwater. The behavior of animals breeding in the pack ice is not well known.

Weddell seals are not very social when out of the water, avoiding physical contact most of the time. When in the shore-fast habitat they tend to congregate in groups along recurrent cracks, leads, and near access holes to the water. There is debate over whether or not this species is migratory. Some individuals remain in residence year round in the fast ice at latitudes as high as 78°S in McMurdo Sound. Others, particularly newly weaned and subadult animals, move north from the continent and spend the winter in the pack ice.

Weddell seals are prodigious divers reaching over 600 m, and have the capability of undertaking 82 minute dives. Deep dives are regularly used for foraging and long dives for searching for new breathing holes, cracks and leads. Seals living in fast ice areas, or facing freezing over of access holes and leads, abrade and grind the ice to maintain access to and from the water. They bite at the ice and then rapidly swing the head from side to side to grind away the ice with their teeth. Predators include killer whales and leopard seals.

The diving abilities of this species are helpful in finding new breathing holes and obtaining important prey, such as the huge Antarctic cod. In addition to Antarctic cod, the diet of Weddell seals primarily consists of Notothenid fishes. Squid and other invertebrates are also taken as small percentages of the diet.

Weddell seals served as an important source of food for men and dogs throughout the heroic period of Antarctic exploration. They continued to be taken to feed sled dogs at one base into the 1980s. Local populations of seals no doubt suffered declines from these harvests, but in the case of the population in McMurdo Sound the population has recovered in the 20 years since the harvest ended. The population of Weddell seals has been variously estimated at 500,000 to one million or more. This is a widespread species and, as with other Antarctic seals inhabiting the pack ice, population assessments are very difficult and expensive to conduct and therefore infrequently undertaken. At present there are no immediate threats to the Weddell seal, and they are protected by the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals.

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Burns, J.M. and M.A. Castellini. 1998. Dive data from satellite tags and time-depth recorders: a comparison in Weddell seal pups. Marine Mammal Science 14(4):750-764.

Burns, J.M. and G.L. Kooyman. 2001. Habitat use by Weddell seals and emperor penguins foraging in the Ross Sea, Antarctica. American Zoologist 41:90-98.

Green, K. and H.R. Burton. 1987. Seasonal and geographical variation in the food of Weddell seals, Leptonychotes weddelii, in Antarctica. Australian Wildlife Research 14:475-89.

Kooyman, G.L. 1981. Weddell seal, consummate diver. Cambridge University Press. 135 pp.

Kooyman, G.L. 1981. Weddell seal - Leptonychotes weddelli Lesson, 1826. pp. 275-296 in S.H. Ridgway and R. Harrison (eds.), Handbook of Marine Mammals, Vol. 2: Seals. Academic Press.

Testa, J.W., D.B. Siniff, J.P. Croxall and H.R. Burton. 1990. A comparison of reproductive parameters among three populations of Weddell Seals (Leptonychotes weddellii). Journal of Animal Ecology 59:1165-1175.

Thomas, J.L. 2002. Leopard seal Leptonychotes weddellii. pp. 1300-1302 in W.F. Perrin, B. Würsig and J.G.M. Thiewissen (eds.), Encyclopedia of Marine Mammals. Academic Press.