Weddell Seal

The Weddell Seal is a large, true seal species found further south than any other mammal except humans, inhabiting the fast ice around Antarctica. It is renowned for its ability to dive to extreme depths and remain submerged for over an hour while foraging under thick ice. Highly adapted to a life in extreme cold, it maintains breathing holes in the ice using its specialized teeth. It is one of the best-studied Antarctic seals due to its accessibility near research stations.

Antarctica - coastal waters and fast ice surrounding the continent

Circumpolar distribution around the Antarctic continent, including the Ross Sea, Weddell Sea, McMurdo Sound, and various sub-Antarctic islands such as South Georgia and South Orkney Islands.

Altitude and depth

Dives regularly to 200-400m, with maximum recorded depths exceeding 700m and dive durations up to 80 minutes.

Seasonal movements

Breeding occurs in spring (October-November in Antarctic seasons), with molting following in the Antarctic summer (December-February); seals largely remain in the same region year-round rather than migrating long distances.

Primarily found on and under fast ice (sea ice attached to the shore) rather than pack ice, as well as in coastal fjords and bays. They rely on stable ice with access to breathing holes and haul-out sites for resting and breeding.

Adaptations to its environment

Possesses large, forward-projecting canine and incisor teeth used to ream and maintain breathing holes in ice. Exhibits extreme diving physiology including high blood volume, elevated myoglobin levels, and bradycardia to conserve oxygen during long dives. Thick blubber layer provides insulation against frigid Antarctic waters.

Listed as Least Concern by the IUCN due to a large, stable population and lack of significant threats; the species has not undergone commercial exploitation on a large scale.

Estimated population

Estimated at 800,000 individuals globally, with populations considered stable, though comprehensive circumpolar surveys are challenging.

Potential threats include climate change affecting sea ice stability, changes in prey availability due to shifting ocean conditions, and increased human activity/research disturbance in Antarctica.

Diseases and parasites

Known to host various endoparasites including nematodes and cestodes, as well as ectoparasites like lice; specific disease studies are limited due to the remote habitat.

Protected under the Antarctic Treaty System and the Convention for the Conservation of Antarctic Seals (CCAS), which regulates any potential harvesting and disturbance in Antarctic waters.

Recommended actions

Continued long-term population monitoring, research into climate change impacts on sea ice habitat, and maintenance of strict protections under international Antarctic agreements.

Body coloration is dark grey to black on the back, fading to lighter grey or silver on the belly, often with irregular light and dark blotches and spots. They have a small, rounded head with a cat-like face and relatively short, dense fur.

Size

Adults typically measure 2.5 to 3.5 meters in length.

Weight

Weight ranges from 400 to 600 kg, with females generally slightly larger than males.

Distinctive features

Distinguished from other Antarctic seals by their small heads relative to body size, distinctive mottled coat pattern, and specialized worn/reamed teeth used for ice maintenance.

Differences between males and females

Females are slightly larger than males, which is unusual among mammals but common among phocid seals; there is no notable difference in coloration between sexes.

Generally solitary or found in small, loose aggregations, though they form larger groups at breeding colonies on fast ice. Social structure at breeding sites involves male competition for underwater territories near breathing holes.

When it is active

Cathemeral (active both day and night), with activity patterns often dictated by the extreme Antarctic photoperiod featuring continuous daylight or darkness for months at a time.

Diet

Primarily piscivorous, feeding on Antarctic silverfish, other fish species, and cephalopods; also consumes some krill and benthic invertebrates.

How it feeds and hunts

Hunts by diving to great depths under sea ice, using echolocation-like vocalizations and visual cues in dim light to locate prey; captures prey with strong jaws and conical teeth.

Communication

Highly vocal underwater, producing a wide range of complex trills, chirps, and whistles used for communication, territory defense, and possibly echolocation-like functions.

Interactions with other species

Natural predators include killer whales (orcas) and leopard seals, particularly targeting pups; they compete with other seal species for fish resources but generally occupy distinct habitat niches.

Role in its ecosystem

Functions as a mid-to-upper trophic level predator in the Antarctic marine ecosystem, helping regulate fish and cephalopod populations while serving as prey for top predators like orcas.

Breeding occurs in spring (October-November), with a single pup born per year after an 11-month gestation period including delayed implantation; pups are weaned after approximately 6-7 weeks.

Raising the young

Mothers provide intensive care, nursing pups with extremely fat-rich milk to promote rapid growth; pups are weaned quickly and mothers do not forage far from their pups during this period.

Maturity and lifespan

Sexual maturity is typically reached around 3-6 years of age; lifespan in the wild can reach up to 20-30 years.

Classification

Class Mammalia > Order Carnivora > Family Phocidae > Genus Leptonychotes > Species Leptonychotes weddellii

Scientific synonyms

Phoca weddellii (original combination, Lesson 1826)

Recent studies

Recent research using satellite tags and biologgers has revealed detailed diving physiology and foraging strategies, contributing significantly to understanding of extreme mammalian diving adaptations.

Genome and DNA

Diploid chromosome number is 2n=34, consistent with other phocid seals; genomic studies have contributed to understanding pinniped evolutionary relationships.

Compared with similar species

Compared to the leopard seal, Weddell seals have a much smaller head and more docile temperament, feeding primarily on fish rather than other seals or penguins. They are less aggressive and more sedentary than crabeater or Ross seals.

Weddell Seals can hold their breath for over 80 minutes and dive deeper than 700 meters. They are the southernmost breeding mammal on Earth, with colonies found within sight of the South Pole region's coastal areas.

Where and how to see it

Best observed near McMurdo Sound and other Antarctic research station areas during the austral spring and summer (October-February) when they haul out on fast ice to breed and molt.

Risks to humans

No notable risk to humans; they are generally docile and approachable, though caution is advised as with any wild marine mammal.

Human uses

Historically hunted in limited numbers by early Antarctic expeditions for food and dog food, but there is no significant commercial use today; they are primarily valued for scientific research.