The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.
All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.
Weddell Seal Fact Sheet
Species: Leptonychotes weddelli
Adult Weddell seals have dark backs, mottled sides, and mottled and white undersides. Females are generally larger than males. The largest individuals are nearly 11 feet (3.3 meters long). Weights vary considerably from season to season and at different stages of reproduction.
Weddell seals inhabit the fast ice (sea ice attached to the shore or connecting two icebergs) all around the Antarctic and nearby islands and as far north as the Falkland Islands. The first scientific specimen of this species was collected in the South Orkney Islands .
Ecology and Behavior
The Weddell seal lives on fast ice and is not migratory. Local movements may be stimulated by changing location of prey or changing condition of the ice on which they haul out. Weddell seals haul out near natural cracks in the fast ice where the ice is thin enough to maintain suitable breathing holes. During the winter, seals chew at the ice with their slightly forward-pointing canine and incisor teeth to keep breathing holes open. As a result, Weddell seal teeth show considerable wear and the rate of wear may affect longevity. Pupping localities are largely determined by the presence of adequate breathing holes.
Weddell seals dive through breathing holes to forage below the ice. Their eyes are well developed for low-light conditions and they are able to hold their breath for an extended period of 20 minutes or more. They can dive quite rapidly at a rate that may exceed 120 meters per minute. Research conducted with acoustic tracking equipment has shown that most dives are less than 20 minutes and that there are two dive levels: 0-160 meters, occurring primarily during the night, and 340-450 meters, primarily during the day. Underwater navigation appears to be excellent but the exact mechanism by which they negotiate a dark underwater environment and are able to return to breathing-holes is not known.
During the mating season, males defend access to breathing holes and attempt to exclude other adult and sub-adult males. Mating occurs in the water. During the pupping season, individuals space themselves more widely on the ice and become somewhat aggressive toward intruders.
Mating occurs mid-November through December. There is then a short period delayed implantation followed by a 9-10 month gestation. Usually, a single pup is born but twins have been reported. Newborns weigh about 30 kg at birth, are covered with soft, gray pelage that is molted in four to six weeks to the adult pelage. At birth, young have a full compliment of adult teeth. Pup survival appears to increase with age and experience of the mother with the highest pup mortality occurring to first-time mothers.
Weddell seal mothers remain with their pups and fast for at least part of lactation and lose considerable body mass during lactation. When lactating females resume foraging, they may dive deeper and longer to catch in an effort to restore their own energy stores, which have been depleted during lactation. Pups are weaned at about six weeks and reach full adult size in about three years. Reproduction generally appears to be delayed until age four to five for females and six to seven for males.
After weaning, Weddell seal pups leave their natal area and move along the Antarctic continent shoreline gradually perfecting their foraging skills. They will sometimes use pack ice for hauling out, but prefer to remain closer to the coastline than adults do.
The exact life span is not known but may average around 12 to 15 years in the wild; the oldest female recorded over a 40-year study monitoring Weddell seals at McMurdo Station was 27 years old and the oldest male 24 years.
The diet is primarily fish and squid. The Antarctic silverfish and the emerald rock-cod are preferred species. In the summer, Weddell seals forage slightly more at night than during the day and they apparently eat their food underwater. In the summer and winter, when there are few environmental time cues, Weddell seals may use tidal movements to determine the best hunting opportunities. While Weddell seals may get all the water they need from their food or from metabolizing sea water, individuals have occasionally been seen eating snow.
When sleeping and resting, Weddell seals may remain in the same spot for hours melting a hollow in the ice underneath them with their own body heat. Weddell seals have also been observed sleeping under the ice, but it is not known how common this phenomenon is and for how long individuals can remain sleeping underwater.
Weddell seals have a variety of underwater vocalizations that are apparently made with the vocal cords and larynx. Most calls are made at a depth of ten to 35 meters in parts of the water column where light penetrates—where the vocalizer can also be seen.
Weddell seals groom parts of their bodies they can reach with nails on their fore-flippers; they roll and rub themselves on the ice to groom areas the flippers cannot reach.