The Weddell seal (Leptonychotes weddellii) is the southernmost-breeding mammal on Earth, living year-round on and beneath the fast ice that rings the Antarctic continent. Named after the British navigator James Weddell, who first described the species in the 1820s, these seals spend much of their lives under a ceiling of ice in near-freezing water, routinely diving to several hundred meters and staying submerged for well over an hour. What makes them so compelling to biologists is not just where they live but how their bodies have been reshaped by the demands of that environment, from a cardiovascular system that can throttle itself to a fraction of its resting output to muscles loaded with an oxygen-carrying protein that functions like an onboard scuba tank.
How Weddell Seals Survive Long Dives
When a Weddell seal slips below the ice, its body undergoes a dramatic shift. Heart rate drops from a resting average of roughly 50 beats per minute to about 15, and cardiac output falls from around 40 liters per minute to fewer than 6. Blood flow to the gut and limbs is slashed by more than 90 percent, while flow to the brain stays essentially unchanged. The adrenal glands see a moderate reduction, and even the heart muscle itself receives far less blood. The whole system is reorganized to keep the brain and a few critical organs supplied while everything else goes on standby.1PubMed. Regional blood flow during simulated diving in the conscious Weddell seal
This redistribution is part of the mammalian dive reflex, a set of involuntary responses triggered by submersion. All mammals have a version of it, including humans, but in Weddell seals it is extraordinarily pronounced. The peripheral shutdown means muscles are largely cut off from fresh oxygen delivery for the duration of a dive. That raises an obvious question: how do the muscles keep working?
An Oxygen Reserve Inside the Muscles
The answer centers on myoglobin, a protein packed into muscle tissue that binds oxygen much the way hemoglobin does in the blood. Weddell seal muscles contain exceptionally high concentrations of myoglobin, giving the tissue a dark, almost blackish-red color. During a dive, that stored oxygen is steadily drawn down. Researchers who implanted tiny spectrophotometers on the swimming muscles of free-diving Weddell seals near Ross Island measured a consistent decline in muscle oxygen saturation throughout dives to depths of up to 300 meters. The rate of decline averaged a few percent per minute, and two of the study animals showed occasional partial resaturation late in their dives, suggesting that some oxygen was being transferred back from circulating blood to the muscles even during the dive.2PubMed. Myoglobin saturation in free-diving Weddell seals
One detail that surprised researchers was that the rate of oxygen depletion in the muscle did not track closely with how hard the seal was swimming. In other words, a seal cruising at low power and a seal actively stroking used up their myoglobin-bound oxygen at roughly the same pace. This hints that the oxygen draw is governed more by the seal’s overall metabolic management during a dive than by the immediate mechanical demands of locomotion.
Genetic Signatures of a Diving Lifestyle
The Weddell seal’s extreme physiology leaves fingerprints in its genome. Researchers who assembled and annotated the Weddell seal genome found evidence of evolutionary selection on genes involved in cardiovascular function and lipid metabolism, traits that would directly support a lifestyle of repeated deep dives and a fat-rich diet in freezing water. A separate study looking at small regulatory molecules called microRNAs identified changes in gene regulation connected to hypoxia tolerance, cell-survival pathways that protect against oxygen deprivation, and signaling through nitric oxide, a molecule that helps control blood vessel dilation.3PubMed Central. microRNA profiling in the Weddell seal suggests novel regulatory mechanisms contributing to diving adaptation
These are not single-gene stories. The emerging picture is of a genome that has been tuned across many systems at once: oxygen storage, oxygen conservation, blood flow regulation, and resistance to the tissue damage that oxygen deprivation can cause. It is this last point that interests biomedical researchers. In humans, a sudden loss of blood flow to an organ followed by its restoration causes what is called reperfusion injury, a cascade of damage driven partly by reactive oxygen molecules. Weddell seals do this to themselves with every dive and suffer no apparent harm. Understanding the genetic and molecular basis for that tolerance has implications well beyond marine biology.
Navigating Under a Ceiling of Ice
A Weddell seal diving under fast ice faces a problem that open-ocean seals do not: it has to find its way back to a breathing hole. Miss that hole, and you drown. Studies of how both Weddell and ringed seals find holes under ice have shown that vision is the primary sense involved. Seals used both natural and artificial visual landmarks during their dives, and when researchers blindfolded the animals, diving behavior became highly restricted. The blindfolded seals were reluctant to venture far. They did, however, respond to acoustic signals and move toward them, showing that hearing can serve as a backup when vision is compromised.4Canadian Journal of Zoology. Under-ice movements and the sensory bias of hole finding by ringed and Weddell seals
Under Antarctic fast ice, light conditions vary enormously. Near a breathing hole or a thin spot in the ice, enough light filters through to create a visible beacon from below. In the permanent darkness of winter, or under thick snow-covered ice, the seals rely on learned spatial memory and possibly acoustic cues from other seals or from the ice itself. Weddell seals are among the most vocal of all pinnipeds, producing a rich repertoire of underwater calls. While the primary function of those calls is social, the acoustic environment they create may also serve as an orientation aid.
Breathing holes do not maintain themselves. Weddell seals keep them open by abrading the ice with their teeth, grinding away new growth with their upper canines and incisors. This behavior takes a visible toll: older seals often have severely worn or broken teeth, and dental condition is one of the factors thought to limit lifespan in the species. A seal that can no longer maintain its breathing holes is effectively locked out of the water, and in an environment where all food comes from below the ice, that is a death sentence.
What Weddell Seals Eat
The image of the Weddell seal as a specialist predator of large Antarctic fish turns out to be incomplete. Camera-equipped studies of lactating females in Erebus Bay recorded over 800 prey encounters and found that the most commonly targeted prey were crustaceans, making up about 46 percent of encounters. Antarctic silverfish accounted for roughly 19 percent. Only two encounters out of 846 involved juvenile Antarctic toothfish, a species that has attracted attention partly because of the commercial toothfish fishery operating in the Southern Ocean.5Polar Biology. Prey targeted by lactating Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica
There was substantial individual variation. Some seals appeared to specialize on certain prey types while others were more generalist. The differences also varied by location, suggesting that local prey availability shapes foraging decisions. This kind of individual dietary flexibility is probably important for a predator living in an ecosystem where prey distribution changes with season, ice coverage, and oceanographic conditions.
Raising Pups on the Ice
Weddell seals give birth on the fast ice in the austral spring, typically in sheltered spots near tidal cracks. Mothers nurse their pups for about six weeks, during which the pup roughly triples its birth weight. The milk is extraordinarily rich in fat, as is typical of the phocid (or “true”) seals, which pursue a strategy of intensive, short-duration nursing. But fat is not the only important component. Analysis of Weddell seal milk has shown that fasting mothers devote a meaningful amount of glucose to synthesizing milk sugars, estimated at around 39 grams per day, an amount comparable to what the mother’s own brain requires.6PubMed. Milk composition in the Weddell seal Leptonychotes weddellii: evidence for a functional role of milk carbohydrates in pinnipeds
Because the mother is fasting during this period, all of that glucose has to come from internal sources, primarily the breakdown of body protein through gluconeogenesis. That is a substantial metabolic cost on top of the already enormous energy investment of producing high-fat milk. The payoff is that the sugar in the milk helps the pup efficiently deposit both fat and protein, accelerating growth and allowing earlier weaning. For an animal breeding on sea ice that could break up unpredictably, getting the pup to independence quickly is a survival advantage.
Staying Warm on the Ice Surface
Weddell seals are well-insulated for life in cold water, but they spend a considerable amount of time hauled out on the ice surface, where wind and radiation create a different set of thermal challenges. Thermal modeling of hauled-out seals found that radiation accounted for the majority of heat loss, followed by conduction through the ice and convection from wind. Leaner seals, with thinner blubber, lost proportionally more heat through conduction.7PubMed Central. Thermoregulatory costs in molting Antarctic Weddell seals: impacts of physiological and environmental conditions
These thermal costs spike during the annual molt, when seals shed and replace their fur. Molting requires increased blood flow to the skin, which means the blubber layer’s insulating effect is partly bypassed. Research on molting Weddell seals confirmed that thermoregulatory costs rise during this period and climb higher in colder, windier conditions. Seals tend to haul out in more sheltered locations during the molt, and they reduce their time in the water. The molt is energetically expensive enough that it represents a distinct vulnerable period in the annual cycle, particularly for animals already in lean condition after the breeding season.
Population Dynamics and the Importance of Ice
The Weddell seal population in McMurdo Sound has been the subject of one of the longest-running demographic studies of any marine mammal, with tagging and annual surveys stretching back to the 1960s. Mark-recapture analyses have shown that juvenile survival is influenced by sex, birth cohort, and year, and that sea-ice extent plays a role in immigration patterns. When fast ice diminishes, seals from surrounding areas are either displaced into or gain easier access to the core study area in Erebus Bay.8Canadian Journal of Zoology. Age-specific survival, abundance, and immigration rates of a Weddell seal (Leptonychotes weddellii) population in McMurdo Sound, Antarctica
The species’ dependence on stable fast ice for breeding and haul-out sites makes it sensitive to changes in the ice regime. The location of pupping colonies is determined largely by physical factors like glacial movement and tidal action rather than biological considerations such as exposure or snow drift. This means the seals are essentially at the mercy of ice dynamics for the suitability of their breeding habitat.
Recent work in the northern Weddell Sea has examined how seal populations respond to variation in sea-ice seasons. During a period from the late 1990s to mid-2000s when warming paused temporarily and the ice season extended, Weddell seals appeared to benefit. Ice-tolerant seal species that can also use open water did not show the same positive response. This divergence underscores that the Weddell seal is genuinely ice-obligate, not merely ice-associated, and that its fortunes are tied more directly to ice persistence than those of its neighbors.9PubMed Central. Temporary Absence of Warming in the Northern Weddell Sea Validates Expected Responses of Antarctic Seals to Sea Ice Change
Weddell Seals as Oceanographic Instruments
Because Weddell seals reliably travel to places that are nearly impossible for human researchers to reach in winter, scientists have enlisted them as data collectors. In the austral winter of 2007, a Weddell seal fitted with a miniaturized instrument that measures water conductivity, temperature, and depth traveled across the central southern Weddell Sea continental shelf. The seal returned 750 water-column profiles, 250 of which extended all the way to the sea floor.10Geophysical Research Letters. Wintertime ocean conditions over the southern Weddell Sea continental shelf, Antarctica
This kind of data is enormously valuable because it comes from a season and region where ship-based sampling is essentially impossible. Pack ice blocks vessels, and the perpetual darkness and extreme cold make other observation platforms impractical. Seal-borne instruments have since been used across multiple Antarctic seal species to map water masses, track currents beneath the ice, and detect changes in ocean temperature and salinity that are relevant to understanding ice-shelf stability and global ocean circulation. The Weddell seal, with its deep dives and long winter residency under the ice, is a particularly good platform for this work.
A Distinctive Gut Microbiome
Even the bacteria living inside Weddell seals reflect the species’ particular biology. Comparison of the gut microbiomes of Weddell seals and southern elephant seals in Antarctica found that the two species harbor distinctly different bacterial communities, with only about 7 percent of microbial types shared between them. Weddell seals carried certain bacterial families that were entirely absent from elephant seals. Interestingly, no sex-based difference in gut bacteria was detected in Weddell seals, while in the much more sexually dimorphic elephant seals the effect of sex was strong. That pattern suggests that extreme differences in body size between males and females can reshape the internal microbial ecosystem, an effect that does not arise in the more modestly dimorphic Weddell seal.
Research into the gut microbiome of Antarctic seals is still in early stages, but the distinctness of each species’ microbial community raises questions about how diet, physiology, and environment interact to shape what lives inside these animals. Weddell seals’ prey base, cold-water physiology, and long fasting periods during breeding and molting all create conditions that may select for particular microbial inhabitants.
The Tooth Problem
One of the more unusual constraints on Weddell seal biology is dental wear. These seals use their forward-projecting canines and incisors to ream out breathing holes in the ice, scraping and grinding at the edges to keep them open. The behavior is essential, since without access to open water for breathing the seals cannot survive. But ice is abrasive, and over years the teeth wear down significantly. Older individuals frequently show broken canines, worn incisors, and exposed tooth pulp, which can lead to infection.
Dental condition correlates with age and body condition, and severe dental deterioration is thought to be one of the key factors capping lifespan in this species. A Weddell seal with healthy teeth can live into its late twenties or beyond, but tooth loss and breakage can make it impossible to maintain holes, effectively ending the animal’s ability to access food and breathing opportunities. In other seal species that do not maintain ice holes, dental wear follows a different and less life-limiting trajectory. For the Weddell seal, teeth are not just for eating; they are infrastructure.
Weddell Seals and the Toothfish Fishery
The Antarctic toothfish (Dissostichus mawsoni), marketed globally as “Chilean sea bass,” supports a significant commercial fishery in the Southern Ocean. Because Weddell seals were long thought to be major predators of toothfish, concerns arose about competition between the fishery and the seals. The camera-based foraging studies discussed earlier suggest that, at least for lactating females in McMurdo Sound, toothfish encounters are extremely rare. Only two out of 846 prey encounters involved juvenile toothfish.11Polar Biology. Prey targeted by lactating Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica
That does not entirely settle the question, because the study captured one demographic segment in one location. Adult male Weddell seals, which dive deeper on average, might encounter larger toothfish more often. And seals in other parts of Antarctica may rely on different prey assemblages. Still, the finding shifts the narrative away from the idea that Weddell seals and the toothfish fishery are in direct competition and toward a more nuanced picture in which crustaceans and smaller fish dominate the seal’s actual diet. For fishery managers, that distinction matters when setting catch limits and assessing ecosystem impacts.