Seal facts

• True seal species cannot turn their front flippers to support their body weight. To move on land, they hop on their bellies in an awkward ‘galumph’ or a caterpillar motion.
• Some seal species may be able to distinguish individual stars to navigate.¹
• There is extraordinary size variation between seal species: some seals are the size of a small child, others that are the height of a two-storey house.
• Some seal species can show enormous size differences between sexes. Male elephant seals can weigh as much as ten times the weight of the females.²
• Some seals can sleep for up to twenty minutes underwater at depths of three hundred metres. Others can sleep on the seafloor.³
• Elephant seals are extraordinary divers and can plunge to depths of two kilometres  - that’s deeper than most whale species!⁴
• A seal’s whiskers act as its fingers. They are one of a seal's primary methods of interacting with the world. Some species will ‘sweep’ them across surfaces, while others will push and prod at what intrigues them.
• Seals get most of their freshwater from the food they eat. The fish, squid, and other marine animals which make up their diet, have high water content. This supplies a large portion of their hydration needs.⁵
• Some seals hold their breath while sleeping on land to conserve water.⁶

Seals are among the most successful marine mammal species on Earth. The eighteen living species that make up ‘true seals’, the phocids, have incredible ecological diversity and call waters from the tropics to the poles their home.⁷

Seals are the largest family in the pinniped clade, which includes their relatives walruses and sea lions. Of all the pinnipeds, however, true seals are the most adapted for aquatic life.⁸ They are more streamlined, wrapped up in thick, insulating blubber.⁹ Some species can remain in deep water, holding their breath, for up to two hours.¹⁰

Being more adapted to an aquatic lifestyle can make seals rather ungainly on land - you can tell a true seal species from other pinnipeds (walrus, sea lions etc.) by the way they move about on land. They tend to lie flat on their bellies as their forearms cannot bend to hold up their torsos. They cannot run either, but ‘caterpillar’ inch across the ground or galumph over surfaces. Seals can also be told apart from other pinnipeds by their lack of ears. Where sea lions and fur seals have protruding ears, true seal species only have holes.¹¹

Most true seals swim with their hind legs, hugging their clawed forearms close to their bodies, unlike other pinnipeds, which use hydrodynamic flippers to glide and manoeuvre through the water. Leopard seals are the exception, which have evolved hydrodynamic flippers.¹²

In general, seals prefer cold and temperate waters. Phocid species can be found mostly in polar, subpolar and the waters around North America, northern Europe, South America and Australia. The exceptions are two species of tropical monk seals; one found in Hawaii and another species in the Mediterranean.¹³
  
Around the United Kingdom, grey seals and harbour seals (or common seals) can be seen all over the coast but especially in Teesside, the east coast, Scotland and Northern Ireland.¹⁴

The seal family varies remarkably in size. The length of an average ring seal is 1.5m, whereas the southern elephant seal can reach lengths of 6m.¹⁵

True seals also show the widest size difference within species too. Some species, such as elephant seals, have one of the largest size differences between males and females (known as sexual dimorphism) of any mammals.¹⁶ The male southern elephant seal can be ten times heavier than a female.¹⁷

Studies have recently shown that size difference between male and female mammals is not as common as previously thought.¹⁸ The possible environmental factors behind elephant seals’ enormous size difference continues to interest scientists.¹⁹

Seals can feed on a wide variety of seafood including zooplankton, squids, jellyfish, molluscs and marine fish. Grey seals will even eat other mammals, such as porpoise.²⁰ Other species will occasionally eat seabirds and even other seals.²¹ Some species are more particular, such as the crabeater seal, which despite its name - prefers krill.²²

While most seal species spend their time surrounded by seawater, they still need access to freshwater to drink. To quench their thirst, seals get most of their freshwater out of the food they eat. Harbour seals will get as much as 90 percent of their freshwater from the fish they eat.²³

Seals have some unique adaptations to make sure they don’t waste this water. Because exhaling results in water loss, some species will even sleep holding their breath.²⁴

While seals large eyes might give them an adorable appearance, their size allows them the vital ability to make out details in dark and murky waters.

When submerged, their pupils quickly dilate to a large size.²⁵ Their retinas include a high number of rod cells. These cells are especially sensitive to light, making them effective for low-light vision.²⁶

On land, seals’ pupils reduce to a tiny hole, to reduce glare. A protective mucus, which keeps eyes wet and clean of pathogens, is the reason why seals can have a teary-eyed appearance.²⁷

A sense of touch is also vital to seal’s exploring a water environment. In many species whiskers act as fingers and, according to biologists, are one of the mammal’s primary methods of sensing the world around them. Most species use their whiskers in different ways: some species ‘sweep’ them across surfaces while others will prod and push to investigate their environment.²⁸

Some seals can use their whiskers for hunting. When a fish moves through the water, it leaves a subtle trail of disturbance in its wake. Scientists have found that some species of seal can sense these vibrations with their face whiskers, and like underwater detectives, follow the trail to catch up with their meal.²⁹ By some accounts, they can do this up to thirty seconds after the prey has fled.³⁰

On the surface, seals have a few other tricks to navigate. While there is a growing body of evidence that some seal species use visual landmarks to map their surroundings, other studies have also found that harbour seals could be using the stars in the night sky to orient themselves.³¹

With their big eyes and whiskered snouts, seals can look remarkably similar to domestic dogs. In the Netherlands, the resemblance is the reason for their Dutch moniker of “zeehond”, or sea dog.

Scientists have found that, while seals are distantly related to dogs and share common ancestors, it’s likely their evolutionary paths split 50 million years ago.³²

Unsurprisingly, seals’ closer living relatives are the other pinniped species (walruses, fur seals, and sea lions).³³ When it comes to living land animals, seals are thought to be more closely related to bears and the mustelids (a group of animals that includes weasels, otters, raccoons and skunks) than dogs.³⁴

Seals evolved from a terrestrial, carnivorous mammal millions of years age.³⁵ These ancestors likely started to hunt in freshwater environments and gradually adapted to marine environments - eventually adopting a fully aquatic lifestyle. Over time, they developed streamlined bodies, flippers, and the ability to hold their breath for extended periods.

The exact ancestry of seals is still contentious, but the remains of ancient relatives help teach scientists how seals may have entered the marine world. The most widely accepted theory is that pinnipeds (seals, sea lions, walruses etc.) descend from a single ancestor that was either otter or bear-like animal 27 million years ago.³⁶ One possible predecessor, Puijila darwini (24-21 mya.) look like otters. They represent a “missing link” between terrestrial mammals and seals. While they did not have flippers, they did have enlarged or webbed feet, a step towards a fully aquatic lifestyle.³⁷ Another possible relative, Kolponomos, was more bear-like, and could crush marine shells in its powerful teeth.³⁸

Why would they have chosen water over a life on land? Some scientists believe that during the late Oligocene (~28-23 mya.), changes in ocean circulation caused abundant upwelling of nutrients in concentrated areas. This resulted in an abundance of food.³⁹ Seal ancestors likely evolved to take advantage of these abundant feeding grounds.⁴⁰

By 20 million years ago, the fully-flippered Enaliarctos, was swimming in the North Pacific. While it did not swallow its prey whole, like modern seals, it did have a streamlined body, a small tail and flipper-like limbs.⁴¹

The first phocids (seals) were initially shallow water divers, who evolved better diving abilities over time.⁴² Around 18 million years ago, true seals split into two groups, the northern seals (such as harbour and grey seals) and the southern seals (such as elephant seals, leopard seals, and monk seals).⁴³ These groups make up the modern seal species we see today. 

There’s extreme variety in the size of seal species. The largest of them all, the male southern elephant seal, can reach 6m long and can weigh in at 4000kg, that’s larger than a White Rhino!⁴⁴  If these giants were terrestrial animals, they would be the world’s second largest animal (only elephants are bigger). Elephant seals are the largest true carnivore (belonging to the order Carnivora) alive today.

Like all animals, the difference in size of seal species is a product of evolutionary adaptations to their environment. Here are a few examples:

Having a larger body helps to conserve heat in cold conditions due to a lower surface are to volume ratio. It’s a pattern seen across animals; species found in colder regions tend to be larger than their counterparts in warm regions. 
Being bigger also allows you to store more oxygen and dive longer, so gaining size could be an advantage for seals that forage in deep waters. Finally, some seals have sexual selection behaviour where being big is a huge advantage.

In elephant seals, males fight for control over a harem of females. That means that being bigger than other males means you a far more likely to pass on your genetic lineage, creating a massive selection pressure for large males. It’s one of the biggest reasons that elephant seals get so large.⁴⁵ Male elephant seals can be ten times larger than females – a trait called “sexual dimorphism”. This difference is one of the largest in the animal kingdom.⁴⁶

But being big isn’t always good – it means you need a lot more food to survive, reproduce more slowly, and you aren’t as agile as smaller animals. Seals like the ringed seal have evolved smaller bodies to be more energy efficient and highly agile – able to evade predators and catch prey in complex habitats such as rocky shores and ice floes. 

Seals are so varied in their species, size and characteristics that it can be difficult for scientists to say overall if they represent a danger to humans. In general, seals tend to be curious around humans but can be unpredictable when threatened. When swimming, they usually keep distance from humans – although bolder individuals might approach with curiosity.⁴⁷

Seals have shared coastal areas with humans for millennia, and archaeological remains have shown that they were an important part of subsistence of aboriginal groups 1,000 years ago.⁴⁸ What they think of us, of course, remains a mystery.

For fishermen who make a living out of their catch, seals have been seen as a nuisance. The mammals can eat the fish from fishing gear, which causes considerable economic losses to small-scale fishermen. How to fairly and safely manage such human-wildlife conflicts is an ongoing debate in fishing regions of the world.⁴⁹

Perhaps unsurprisingly, seals can be harmed by human activity. Human disturbance can have a large impact on haul-out behaviour of seals resting or pupping on land. Disturbances such as loud cars, or boats can cause seals to “flush” en masse into the seal.⁵⁰ These “flushing events” can cost seals a lot in terms of their energy and can disrupt vital nursing sites.⁵¹

The current advice is to calmly observe a seal and not to approach or touch them. Crucially, pups that are mistaken for abandoned, are in fact usually waiting for their mothers to return. Though fascinating to observe, true seal species need some respectful distance for their safety and our own.⁵²

Seals, just like us, need oxygen to survive, which they gather by breathing air at the surface. What is special about phocid species is how long they can go without inhaling. Smaller seal species, such as harbour seals, can go twenty minutes without taking a breath, while larger seals such as hooded seals can go over an hour.⁵³ What allows them to do it?

One of their adaptations is the way they store oxygen in their bodies. While humans store a large portion of oxygen in the lungs, seals siphon the oxygen to different parts of the body, becoming their own internal SCUBA tanks.

Seals do this using two specialized molecules: haemoglobin, a molecule that stores oxygen in the blood, and myoglobin, which stores oxygen in muscle cells. The blood of some species can store four times as much oxygen as a human, while the muscle tissue can store six times more than a human.⁵⁴ Most seal species also have a large spleen compared to other mammals, which is another important storage area for the oxygenated blood.⁵⁵

Another ability is seals’ extraordinary control over their blood flow.  When diving, seals can reduce their heart rate, as well as divert blood flow to focus on essential areas: the muscles, the brain and the heart. Simultaneously, blood-flow can be reduced for the non-essential diving organs such as the kidneys, stomach and body extremities, such as skin and blubber.⁵⁶ A recent study of harbour seals found that individuals will begin to redistribute their blood flow around fifteen seconds before a dive, which may suggest a level of cognitive control.⁵⁷

All seals are proficient divers, but elephant seals are in a league of their own.
The largest males can be submerged for up to two hours and some can dive more than two kilometres deep – that’s equivalent to more than two Burj Khalifas stacked on top of one another.⁵⁸
 
Astonishingly, they only need as much as two minutes of recovery between dives.⁵⁹ This ability allows them to spend 90 percent of their time submerged. That’s more time than most whales!⁶⁰
 
Part of their prowess comes down to an extraordinary amount of blood. Even if a human was the same size, an elephant seal would have three times the amount of blood volume. Their blood also contains a higher amount of haemoglobin, the molecule that transports oxygen around the body.⁶¹
 
When an elephant seal takes a dive, it first empties its lungs to decrease buoyancy. While this seems counterintuitive, the animals only rely on their lungs for four percent of stored oxygen. It’s muscle tissue account for 28 percent and the oxygenated blood in its body for a whopping 68 percent.⁶²
 
As with other seals, the elephant seal cuts blood flow to some of its organs and lowers its heart rate. In extremes, diving heart rates have been recorded as low as two beats per minute, a pace that would be considered a heart attack in most other species.⁶³ As it moves, the seal “glides” gently as opposed to swimming, in order to keep its energy and oxygen expenditure low.⁶⁴
 
Elephant seals are so good at diving that scientists are still trying to figure out how they do it. Their ability to routinely survive extreme hypoxemia (low levels of oxygen in the blood) and recover from deep dives so fast are still among the great mysteries of the deep.⁶⁵

For humans, holding your breath while sleeping (called sleep apnoea) is not good for you. This is not the case for seals, who will hold their breath while sleeping for several minutes - even when on land! This ability means that not only are they able to sleep underwater, but also conserve their vital freshwater stores, which are normally exhaled as water vapour during breathing.⁶⁶
 
For species that spend many months at sea, sleeping is a risky business. Unlike many marine mammals, true seals cannot put one half of their brain to sleep at a time. They need to shut off their entire brain, like humans do.⁶⁷ With predators stalking the waters, they need special techniques to keep them safe. Recently, scientists uncovered extraordinary habits of one sea-going species. Elephant seals, which are at risk of killer whales and white sharks, have been observed sinking to around one hundred metres on their rest dives - below the preferred danger zone of their predators.⁶⁸
 
It is at these depths they do something a little eerie. They turn their bellies upwards, close their eyes and fall into deep sleep. They drift in this state, belly up and eyes closed, sinking a downward corkscrew spiral, to greater depths and greater depths (around 300 metres).⁶⁹  These extreme power naps last around twenty minutes a time and are repeated to make up a total of just two hours of sleep per day.⁷⁰

Seals face considerable pressures, and several species are considered endangered according to the International Union for the Conservation of Nature.
 
The Hawaiian monk seal has been in decline for decades and now numbers roughly 1600 individuals.⁷¹ Caspian seals, which live in the brackish water of the Caspian Sea are considered endangered, with a 2005 survey estimating 111,000 individuals remaining.⁷² Experts believe it has declined by ninety percent in the last century.⁷³
 
Among the most at risk of extinction is the Mediterranean monk seal, which was declared ‘critically endangered’ in 2015. The species had its numbers vastly reduced by hunting during the Roman Empire, again in the Middle Ages and has faced numerous human pressures since.⁷⁴

The species, which was once described flocking on Mediterranean beaches, have now colonised sea caves, in what is considered a reaction to the high human presence in the region. The Mediterranean monk seal is now one of the rarest marine mammals in the world, with an elusive population of around 600 individuals.⁷⁵
 
The Caribbean monk seal has not had confirmed sightings in the past 50 years and is considered extinct.⁷⁶
 
Some seal species are proof of potential for recovery. During the nineteenth century, many seal species near to human populated areas were intensely hunted for fur and oil. Gray seals were hunted so extensively in North America, they were rarely seen in the region for most of the 20th century, while northern elephant seals were hunted to near-extinction.⁷⁷  With the demise of the sealing industry as well as the protection of haul-out sites, both populations have rebounded dramatically.⁷⁸

Featured image © Seiji Seiji | Unsplash

Fun fact image © Mana5280 | Unsplash

Quick facts

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2. “Pinniped | Mammal Suborder.” n.d. Encyclopedia Britannica. https://www.britannica.com/animal/pinniped.

3. Ross, Heather. 2021. “What Do Seals Eat?” A-Z Animals. October 27, 2021. https://a-z-animals.com/animals/seals/what-do-seals-eat/; Kienle, Sarah S., and Annalisa Berta. 2015. “The Better to Eat You With: The Comparative Feeding Morphology of Phocid Seals (Pinnipedia, Phocidae).” Journal of Anatomy 228 (3): 396–413. https://doi.org/10.1111/joa.12410.

4. Cooke, Corinne. 2019. “What Animals Eat Seals?” Sciencing. November 22, 2019. https://www.sciencing.com/animals-eat-seals-6775626/.

5. Seals: 2021. “Seals: Diet, Habitat, Behaviour, and Conservation | IFAW.” IFAW. 2021. https://www.ifaw.org/uk/animals/seals#faqs; “Common Seal | Devon Wildlife Trust.” 2025. Devonwildlifetrust.org. 2025. https://www.devonwildlifetrust.org/wildlife-explorer/marine/marine-mamm….

6.  Miyazaki, Nobuyuki. 2017. “Baikal Seal.” Elsevier EBooks, December, 57–58. https://doi.org/10.1016/b978-0-12-804327-1.00055-8; Britannica Kids. “Elephant Seal,” n.d. https://kids.britannica.com/students/article/elephant-seal/438007

7. Government of Canada, Fisheries and Oceans Canada. 2016. “Ringed Seal.” November 25, 2016. https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/ringedseal-p…; Red List of South African Species. “Elephant Seal,” n.d. https://speciesstatus.sanbi.org/taxa/detail/2137/

8. Britannica. 2019. “Seal | Mammal.” In Encyclopaedia Britannica. https://www.britannica.com/animal/seal-mammal; Gisonna, Kelly, and John P Rafferty. 2024. “Baikal Seal | Diet, Habitat, Lifespan, & Facts.” Encyclopedia Britannica. November 18, 2024. https://www.britannica.com/animal/Baikal-seal;

9. “Mediterranean Monk Seal : First Edition | IUCN Library System.” 2025. Iucn.org. May 21, 2025. https://portals.iucn.org/library/node/52224; UCN Red List of Threatened Species. “The IUCN Red List of Threatened Species,” n.d. https://www.iucnredlist.org/ja/search?query=Hawaiian%20monk%20seal&sear…

Fact file:

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4. Adachi, Taiki, Akinori Takahashi, Daniel P Costa, Patrick W Robinson, Luis A Hückstädt, Sarah H Peterson, Rachel R Holser, Roxanne S Beltran, Theresa R Keates, and Yasuhiko Naito. 2021. “Forced into an Ecological Corner: Round-The-Clock Deep Foraging on Small Prey by Elephant Seals.” Science Advances 7 (20). https://doi.org/10.1126/sciadv.abg3628; McIntyre, T., P. J. N. de Bruyn, I. J. Ansorge, M. N. Bester, H. Bornemann, J. Plötz, and C. A. Tosh. 2010. “A Lifetime at Depth: Vertical Distribution of Southern Elephant Seals in the Water Column.” Polar Biology 33 (8): 1037–48. https://doi.org/10.1007/s00300-010-0782-3.

5. Renouf, Deane, Elizabeth Noseworthy, and Mary C. Scott. 1990. “DAILY FRESH WATER CONSUMPTION by CAPTIVE HARP SEALS (PHOCA GROENLANDICA).” Marine Mammal Science 6 (3): 253–57. https://doi.org/10.1111/j.1748-7692.1990.tb00251.x.

6. Blackwell, Susanna B., and B. J. Le Boeuf. 1993. “Developmental Aspects of Sleep Apnoea in Northern Elephant Seals,Mirounga Angustirostris.” Journal of Zoology 231 (3): 437–47. https://doi.org/10.1111/j.1469-7998.1993.tb01930.x; Lyamin, Oleg I., and Jerome M. Siegel. 2019. “Sleep in Aquatic Mammals.” Handbook of Sleep Research 30: 375–93. https://doi.org/10.1016/b978-0-12-813743-7.00025-6.

7. Adachi, Taiki, Akinori Takahashi, Daniel P Costa, Patrick W Robinson, Luis A Hückstädt, Sarah H Peterson, Rachel R Holser, Roxanne S Beltran, Theresa R Keates, and Yasuhiko Naito. 2021. “Forced into an Ecological Corner: Round-The-Clock Deep Foraging on Small Prey by Elephant Seals.” Science Advances 7 (20). https://doi.org/10.1126/sciadv.abg3628

8. Thompson, L. A., and T. A. Romano. 2019. “Effects of Health Status on Pressure-Induced Changes in Phocid Immune Function and Implications for Dive Ability.” Journal of Comparative Physiology B 189 (5): 637–57. https://doi.org/10.1007/s00360-019-01228-6.

9. Lyamin, Oleg I., and Jerome M. Siegel. 2019. “Sleep in Aquatic Mammals.” Handbook of Sleep Research 30: 375–93. https://doi.org/10.1016/b978-0-12-813743-7.00025-6.

10. Meir, Jessica U., Patrick W. Robinson, L. Ignacio Vilchis, Gerald L. Kooyman, Daniel P. Costa, and Paul J. Ponganis. 2013. “Blood Oxygen Depletion Is Independent of Dive Function in a Deep Diving Vertebrate, the Northern Elephant Seal.” Edited by Patrick J. O. Miller. PLoS ONE 8 (12): e83248. https://doi.org/10.1371/journal.pone.0083248.

11. Hammill, Mike O. 2018. “Earless Seals.” Encyclopedia of Marine Mammals, 284–89. https://doi.org/10.1016/b978-0-12-804327-1.00112-6.

12. Hocking, David P., Felix G. Marx, Shibo Wang, David Burton, Mark Thompson, Travis Park, Ben Burville, et al. 2021. “Convergent Evolution of Forelimb-Propelled Swimming in Seals.” Current Biology 31 (11): 2404-2409.e2. https://doi.org/10.1016/j.cub.2021.03.019.

13. Hückstädt, Luis A. , and Ryan R. Reisinger . 2022. Review of Habitat Utilization and Behavior of Phocid Seals in Relation to Oceanography. In Ethology and Behavioral Ecology of Phocids, edited by Daniel P. Costa and Elizabeth A. McHuron, 127–78. Cham: Springer Cham. https://link.springer.com/chapter/10.1007/978-3-030-88923-4_5.

14. Thompson, Dave, Callan D. Duck, Chris D. Morris, and Debbie J. F. Russell. 2019. “The Status of Harbour Seals (Phoca Vitulina) in the UK.” Aquatic Conservation: Marine and Freshwater Ecosystems 29 (S1): 40–60. https://doi.org/10.1002/aqc.3110; Russell, Debbie J. F., Chris D. Morris, Callan D. Duck, Dave Thompson, and Lex Hiby. 2019. “Monitoring Long‐Term Changes in UK Grey Seal Pup Production.” Aquatic Conservation: Marine and Freshwater Ecosystems 29 (S1): 24–39. https://doi.org/10.1002/aqc.3100.

15. Kovacs, Kit M, John J Citta, Tanya M Brown, Runé Dietz, Steve Ferguson, Lois A Harwood, Magali Houde, et al. 2021. “Variation in Body Size of Ringed Seals (Pusa Hispida Hispida) across the Circumpolar Arctic: Evidence of Morphs, Ecotypes or Simply Extreme Plasticity?” Polar Research 40 (September). https://doi.org/10.33265/polar.v40.5753; “OBIS-SEAMAP Species Profile - Mirounga Leonina.” 2024. Duke.edu. 2024. https://seamap.env.duke.edu/species/180671/html; Páez-Rosas, Diego, Marjorie Riofrío-Lazo, Jorge Ortega, Juan de Dios Morales, Raúl Carvajal, and Juan José Alava. 2018. “Southern Elephant Seal Vagrants in Ecuador: A Symptom of La Niña Events?” Marine Biodiversity Records 11 (1). https://doi.org/10.1186/s41200-018-0149-y.

16. Tarnawski, B.A., G.H. Cassini, and D.A. Flores. 2014. “Skull Allometry and Sexual Dimorphism in the Ontogeny of the Southern Elephant Seal (MiroungaLeonina).” Canadian Journal of Zoology 92 (1): 19–31. https://doi.org/10.1139/cjz-2013-0106; Galimberti, Filippo, Simona Sanvito, Chiara Braschi, and Luigi Boitani. 2007. “The Cost of Success: Reproductive Effort in Male Southern Elephant Seals (Mirounga Leonina).” Behavioral Ecology and Sociobiology 62 (2): 159–71. https://doi.org/10.1007/s00265-007-0450-y.

17. British Antarctic Survey. “Parental Investment in Southern Elephant Seals (Mirounga leonina).” Last modified August 2, 2022. https://www.bas.ac.uk/data/our-data/publication/parental-investment-in-….

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19.Volzke, Sophia, Jaimie B. Cleeland, Mark A. Hindell, Stuart P. Corney, Simon J. Wotherspoon, and Clive R. McMahon. 2023. “Extreme Polygyny Results in Intersex Differences in Age-Dependent Survival of a Highly Dimorphic Marine Mammal.” Royal Society Open Science 10 (3). https://doi.org/10.1098/rsos.221635.

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22. Hückstädt, Luis A., Andrea Piñones, Daniel M. Palacios, Birgitte I. McDonald, Michael S. Dinniman, Eileen E. Hofmann, Jennifer M. Burns, Daniel E. Crocker, and Daniel P. Costa. 2020. “Projected Shifts in the Foraging Habitat of Crabeater Seals along the Antarctic Peninsula.” Nature Climate Change 10 (5): 472–77. https://doi.org/10.1038/s41558-020-0745-9.

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24. Blackwell, Susanna B., and B. J. Le Boeuf. 1993. “Developmental Aspects of Sleep Apnoea in Northern Elephant Seals,Mirounga Angustirostris.” Journal of Zoology 231 (3): 437–47. https://doi.org/10.1111/j.1469-7998.1993.tb01930.x

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