The Weddell seal (Leptonychotes weddellii) is a marine mammal. These large pinnipeds are the southernmost breeding mammals on Earth, inhabiting the icy waters surrounding the Antarctic continent. Their robust, streamlined bodies, typically dark gray to brown with lighter patches, can reach lengths of 2.5 to 3.5 meters and weigh between 400 and 600 kilograms, with females often being slightly larger than males. Their round faces, large eyes, and sensitive whiskers give them a distinctive appearance.
Life in the Antarctic Ice
Weddell seals primarily inhabit the “fast ice” of Antarctica, sea ice attached to the coastline. This stable ice platform provides an environment for them to rest, molt, and give birth to their pups. While they prefer fast ice, some individuals may utilize drifting pack ice during winter or move between the two.
Their existence is closely tied to access to water, and they frequently gather around natural cracks and holes in the ice. These seals are adapted for hunting beneath the ice, where light levels can be extremely low. Their diet primarily consists of fish, particularly the Antarctic silverfish and Antarctic toothfish, but also includes cephalopods like squid and octopus, and some crustaceans. Weddell seals are efficient underwater hunters, capable of surprising prey. They have even been observed blowing bubbles into ice crevices to flush out hiding fish, making them easier to catch.
Remarkable Adaptations
Weddell seals possess specialized adaptations that enable their survival in the harsh Antarctic environment. A prominent adaptation is their ability to maintain breathing holes in thick sea ice, which can be several meters deep. They use their unique dentition, specifically their forward-jutting canine and incisor teeth, to rasp and grind away new ice that forms over existing cracks or to create new holes. This continuous effort, however, leads to significant tooth wear over their lifespan, which can eventually hinder their ability to feed and maintain these access points, sometimes contributing to mortality in older seals.
Their diving capabilities allow them to forage at considerable depths and remain submerged for extended periods. Weddell seals can dive to depths exceeding 600 meters (over 2,000 feet) and hold their breath for up to 80 minutes, though typical dives are often shallower and shorter, around 15-20 minutes to depths of 200-400 meters. These feats are supported by physiological mechanisms, including a large blood volume—up to 21% of their body mass—and a high concentration of oxygen-storing proteins like hemoglobin in their blood and myoglobin in their muscles. During dives, their heart rate significantly decreases, a phenomenon known as bradycardia, and blood flow is redirected from extremities to essential organs like the brain and heart, conserving oxygen. A thick layer of blubber, which can be over 2 inches thick, provides insulation against the frigid Antarctic waters, allowing them to maintain their core body temperature.
Behavior and Conservation
Weddell seals are solitary outside of the breeding season, but they congregate in loose groups around breathing holes and cracks in the ice. They are known for their complex underwater vocalizations, which include a variety of sounds such as trills, whistles, chirps, and low-frequency buzzes, some extending into the ultrasonic range up to 50 kHz. These sounds are believed to play a role in social interactions, territorial defense, and communication, especially during the breeding season.
Breeding occurs on the fast ice, with females giving birth to a single pup between September and November, depending on the latitude. Mothers provide maternal care, nursing their pups for approximately 45 to 50 days. During this period, pups rapidly gain weight, doubling their 25-30 kg birth weight in about 10 days, thanks to the rich milk provided by their mothers. Mating takes place underwater towards the end of the nursing period, with males defending underwater territories around breathing holes to gain access to females. Females exhibit delayed implantation, meaning the fertilized egg does not implant in the uterus for a month or two after mating.
Weddell seals face predation primarily from Orcas (killer whales) and leopard seals, particularly when at sea or on pack ice, with younger seals being more vulnerable. Despite these predators, their preference for fast ice offers some protection. The species was discovered and named in the 1820s by British sealing captain James Weddell during his expeditions to the Southern Ocean.
Currently, the Weddell seal population is considered stable, with estimates ranging from 200,000 to 1,000,000 individuals, and is classified as “Least Concern” by the IUCN. They are protected under the Antarctic Treaty and the Convention for the Conservation of Antarctic Seals (CCAS). Research continues to monitor potential impacts of climate change on their sea ice habitat, which could affect their long-term survival.