A seal can slip through water with remarkable ease, then reach the beach and move as though its legs have been lost in the post. A sea otter wears a fur coat that needs constant cleaning. A walrus searches the seabed with sensitive whiskers and sucks clams from their shells. These animals are mammals, just as we are, but cold seawater presents their bodies with some rather different problems. Comparing them reveals how bodies keep heat, save oxygen, and find food—and why an oil spill or disappearing sea ice can damage much more than the scenery.
In the News
Sea otters get more prey and reduce tooth damage using tools — Reuters, 16 May 2024. Researchers studying southern sea otters found that using objects to open prey helped protect their teeth. Tools also made harder food accessible. The findings connect behaviour with feeding ability and survival.
Bird flu is decimating seal colonies. Scientists don’t know how to stop it — Associated Press, 22 March 2024. The report described severe outbreaks affecting seals and sea lions, including thousands of deaths in South America. Researchers were investigating how infection spread. The losses show how disease can affect predators and the ecosystems they inhabit.
Europe just had warmest March on record — Reuters, 08 April 2025. The report also described the lowest Arctic sea ice extent recorded for March in the 47-year satellite record. Sea ice is part of the habitat used by walruses and some seals. Its location and seasonal availability affect where animals can rest and feed.
What do you think?
1] Why do you feel colder in wet clothes than in dry clothes?
2] Why does a thick blanket help even though it produces no heat?
3] Why might a small animal lose heat faster than a large one?
4] How does moving quickly change your breathing?
5] Why is it difficult to hold your breath while running?
6] What problems would you face if lunch were always underwater?
7] Why is a smooth shape useful when moving through water?
8] Why might equipment that works well in water work badly on land?
9] How could you find something in a dark room without seeing it?
10] What could the ripples behind a moving object tell you?
11] Why do people use tools to open difficult containers?
12] What could happen if you had to open every food packet with your teeth?
13] Why does cleaning equipment sometimes help it work properly?
14] How could dirt change the usefulness of a coat?
15] Why might a parent need more food while caring for a baby?
16] How could a safe resting place help an animal find food?
17] What risks appear when a large crowd suddenly runs towards one exit?
18] How could removing one kind of eater change a whole garden?
19] Why might two people disagree about bringing an animal back to an area?
20] What observations would help you judge whether a protection plan works?
Did You Know?
1] A sea otter has loose skin beneath its forelegs that can hold food collected underwater. Its shopping bag is part of its armpit.
2] Both male and female walruses have tusks. Each tusk is an enlarged upper canine tooth.
3] Harbour seal pups can swim soon after birth. Swimming lessons begin before anyone has arranged a timetable.
4] Sea otter pups are initially so buoyant that diving is difficult. Their air-filled coats help them float while their mothers search for food.
Vocabulary
- hind flippers — 뒷지느러미
- ear openings — 귓구멍
- drag — 물이나 공기가 움직임을 방해하는 저항
- blood vessels — 혈관
- body core — 몸의 중심부
- fat reserves — 몸에 저장된 지방
- scarce — 부족한
- vital organs — 생명 유지에 필수적인 기관
- sensory equipment — 감각을 감지하는 신체 구조
- visibility — 눈으로 볼 수 있는 정도
- haul out — 물 밖으로 나와 쉬다
- stranded — 해안에 갇혀 돌아가지 못하는
- abandoned — 버려진
- grooming — 털을 손질하고 깨끗하게 하는 행동
- trapped air — 갇혀 있는 공기
- heat loss — 열 손실
- body mass — 몸의 질량
- reproduction — 번식
- prey — 잡아먹는 먹이 동물
- tooth damage — 치아 손상
- buoyant — 물에 잘 뜨는
- drifting — 물살에 떠밀려 이동하는 것
- tusks — 상아처럼 길게 자란 송곳니
- seabed — 바다 밑바닥
- suction — 압력 차이로 빨아들이는 작용
- calves — 어린 바다코끼리들
- feeding grounds — 먹이를 찾는 지역
- trade-offs — 한 가지 이점을 얻는 대신 다른 이점을 포기하는 관계
- entanglement — 그물이나 줄에 몸이 얽히는 것
- shellfish — 조개와 게 같은 껍데기 있는 수산동물
Glossary
Adaptation — An inherited feature can help an animal live in its environment. It may involve body structure or how the body works. Such features develop in populations over many generations. 적응 형질.
Insulation — Insulation slows the movement of heat. Fur containing trapped air and thick fat can provide it. Insulation helps conserve heat but does not create heat. 단열.
Blubber — Blubber is a thick layer of tissue rich in fat beneath the skin of many marine mammals. It helps reduce heat loss. It also stores energy that the body can use. 피부 아래의 두꺼운 지방층.
Haemoglobin — Haemoglobin is a protein in red blood cells. It binds oxygen and carries it around the body. Diving mammals use oxygen held in blood while they are underwater. 혈액에서 산소를 운반하는 단백질.
Myoglobin — Myoglobin is a protein in muscle cells. It holds oxygen that muscles can use. High amounts help diving mammals maintain muscle activity without taking a new breath. 근육에서 산소를 저장하는 단백질.
Metabolism — Metabolism includes the chemical reactions that keep a body alive. These reactions release usable energy from food and build needed materials. Some energy becomes heat, helping a mammal stay warm. 생명을 유지하는 몸속 화학 작용인 물질대사.
Pinniped — A pinniped is a mammal in the group containing seals, sea lions, and walruses. Its limbs are adapted as flippers. A sea otter is a marine mammal but is not a pinniped. 물범·바다사자·바다코끼리를 포함하는 기각류.
Natural selection — Individuals in a population have inherited differences. Some differences improve survival and reproduction in particular conditions. Those features can become more common over many generations. 자연선택.
Food web — A food web connects the feeding relationships among organisms in an ecosystem. One species may eat several kinds of food and have several predators. Changing one population can therefore affect others. 먹이그물.
Trophic cascade — A trophic cascade is a chain of ecological effects across feeding levels. A predator can change the abundance or behaviour of its prey, which then affects other organisms. Otters reducing urchin grazing on kelp provide one example. 먹이 관계를 따라 이어지는 연쇄적인 생태계 변화.
Seals: Excellent Swimmers, Awkward Pedestrians
A harbour seal is a true seal: it has small ear openings, short front flippers, and powerful hind flippers. In water, its smooth body reduces drag, the resistance that slows movement. It pushes forward mainly by moving its rear body and hind flippers from side to side. On land, those hind flippers cannot rotate underneath it for walking. The result is a rolling, wriggling journey across the beach. Sea lions have visible ear flaps and can walk on four flippers. The difference is anatomical, not a failure of ambition. A seal has not forgotten how to walk; its body has evolved for another job.
1] How do a harbour seal’s body shape and hind flippers help it swim?
2] Which two features help you distinguish a true seal from a sea lion?
Cold water carries heat away from a body much faster than still air does. A seal therefore needs insulation, something that slows heat loss. Beneath its skin lies blubber, a thick layer of tissue rich in fat. Blubber also stores energy, which can support the animal when food is scarce. Blood vessels near the skin can narrow, reducing the amount of warm blood exposed to the cold. This helps protect the warm body core, where vital organs work. A seal’s round appearance is therefore useful equipment. Looking like an overfilled cushion has advantages when the cushion must hunt fish in winter.
1] What two useful functions does blubber perform?
2] How can narrowing blood vessels near the skin reduce heat loss?
A seal cannot breathe underwater. Instead, it carries oxygen into a dive and carefully manages the supply. Haemoglobin, a protein in blood, carries oxygen; myoglobin stores oxygen in muscles. Humans have both proteins too, but diving seals have much greater oxygen stores for their body size. During a dive, the heart can slow and blood flow to some tissues can decrease. Oxygen remains available to vital organs, especially the brain and heart. These changes extend underwater feeding time, but do not make oxygen unlimited. Diving ability also varies greatly between species. A harbour seal and an elephant seal are not interchangeable submarines.
1] Where do haemoglobin and myoglobin hold oxygen?
2] Why does controlling blood flow help a seal remain underwater?
A harbour seal’s whiskers are sensory equipment. Sensitive nerves at their bases detect tiny movements in water, including trails left by swimming fish. This helps the seal hunt when visibility is poor. Its eyes also work well in dim light. After hunting, it may haul out: leave the water to rest on a beach, rock, or ice. Harbour seal mothers nurse their pups with rich milk, helping them grow and build fat reserves. A resting seal is therefore not necessarily stranded, and a lone pup is not automatically abandoned. Getting close can interrupt rest or keep a mother from returning.
1] How can whiskers help a seal find fish it cannot clearly see?
2] Why should people avoid approaching a resting seal or a lone pup?
Sea Otters: A Fur Coat That Requires Constant Maintenance
Sea otters live along parts of the North Pacific coast. Unlike seals and walruses, they lack a thick insulating layer of blubber. Their dense fur traps air close to the skin, forming a barrier against cold water. Grooming removes dirt, separates hairs, and helps maintain that air layer. An otter rubbing and rolling at the surface may therefore be maintaining essential insulation rather than enjoying a spa appointment. Oil can damage this system by coating the fur and preventing it from trapping air properly. Once cold water reaches the skin, the otter loses heat rapidly, even though its coat still looks thick.
1] How does trapped air help keep a sea otter warm?
2] Why can oil on the fur be dangerous even when the fur remains in place?
Insulation alone does not solve a sea otter’s heating problem. Its metabolism, the chemical activity that keeps its body working, produces heat as it uses food. Adult sea otters commonly eat roughly 20–30% of their body mass each day, although needs vary with size, activity, and reproduction. Their meals include sea urchins, crabs, clams, and other animals. Feeding takes time and energy, so repeated disturbance by boats or people can be costly. An otter forced to swim away loses resting time and must spend extra energy. Its appetite is less a charming personal weakness than the operating cost of being warm in cold water.
1] Why do sea otters need a large daily food supply?
2] How can repeated disturbance make their energy balance harder to maintain?
Some sea otters use rocks as tools to break hard shells. An otter may float on its back with a rock on its chest, then strike a shell against it. Loose skin beneath a foreleg can hold food gathered during a dive. In a study published in 2024, researchers followed 196 southern sea otters and found that tool use was associated with less tooth damage. Tools also helped animals eat prey that would be difficult to open by biting alone. Not every otter uses tools equally often. The behaviour shows how learning and physical ability can work together; teeth need not perform every kitchen task.
1] How can a rock help an otter obtain food while protecting its teeth?
2] Why is it inaccurate to say that every sea otter uses tools in the same way?
A sea otter mother usually raises one pup at a time. She carries it on her chest, feeds it milk, and carefully grooms its fur. A young pup’s fluffy coat traps so much air that it cannot initially dive effectively. The mother must leave it floating while she searches underwater for food. Adults also rest at the surface, sometimes in groups called rafts, and may use kelp to reduce drifting. Some hold paws, but this is not a rule for every sleeping otter. Rest matters because swimming and keeping warm cost energy. A floating otter can be working on the serious business of doing very little.
1] Why is grooming especially important for a young sea otter pup?
2] How can kelp help an otter rest at the surface?
Walruses: Finding Clams with a Moustache
A walrus is a pinniped, belonging to the same broad group as seals and sea lions. It lives in the Arctic and nearby northern seas. Its large body and blubber slow heat loss, while flippers move it through water and support movement on land. Both males and females have tusks, which are greatly enlarged upper canine teeth. Tusks help with displays and competition and can help an animal pull itself onto ice. They are not its main tools for digging up dinner. The face may suggest a heavily armed predator, but many walrus meals consist of small animals hiding quietly in seabed mud.
1] What are walrus tusks, and do both sexes have them?
2] Why does a large body help a walrus conserve heat?
Finding a clam beneath cloudy water is a problem of touch as much as sight. A walrus sweeps its sensitive whiskers across the seabed to locate food. It can uncover buried prey with movements of its mouth and jets of water. To remove clam meat, it uses its lips and powerful tongue to create suction: lower pressure inside the mouth draws the soft tissue inward. It often leaves the shell behind. This is a specialised feeding method, not ordinary chewing with impressive teeth. The walrus carries its food detector and suction equipment on the same face. The moustache has a demanding job.
1] What role do whiskers play when a walrus searches the seabed?
2] How does suction help a walrus eat a clam?
Walruses must come out of the water regularly to rest. For Pacific walrus females and calves, floating sea ice provides a resting platform near feeding areas in the Bering and Chukchi Seas. The location matters: ice above a shallow seabed allows access to buried prey, while ice above very deep water may not. Mothers nurse their calves and maintain close contact with them. Resting on ice between feeding trips can reduce the travel needed to find food. Calling sea ice merely frozen water misses its biological role. For a walrus family, it can be a bedroom located conveniently above the supermarket.
1] Why is ice over a shallow seabed more useful for feeding than ice over very deep water?
2] How can a nearby ice platform reduce the energy needed to obtain food?
Pacific walruses also rest on land, so losing sea ice does not mean that every animal immediately drowns. The difficulty is where suitable beaches lie relative to food and how many animals gather there. Large coastal groups can contain thousands of walruses. Sudden disturbance can cause a rush towards the water, and calves may be crushed by larger animals. Longer journeys from shore to feeding grounds can also increase energy costs. Local communities help protect resting groups by reducing disturbance from people, boats, and aircraft. These measures address immediate risks, although they cannot replace all the feeding access once provided by offshore ice.
1] What two problems can arise when many walruses must rest on coastal beaches?
2] Why does protecting a resting beach help without solving every problem caused by ice loss?
Comparing Their Lives: Evolution, Food Webs, and a Warmer Ocean
Seals, sea otters, and walruses are mammals: they breathe air, maintain warm bodies, and feed their young with milk. Their ancestors lived on land, but different branches developed different ways to survive at sea. Seals and walruses belong to the pinnipeds; sea otters belong to the weasel family. Bones and DNA provide evidence of these relationships. Natural selection favours inherited features that help animals survive and reproduce in particular conditions. It does not plan improvements or supply whatever an animal wants. Fur, blubber, flippers, and feeding tools involve trade-offs. There is no perfect marine mammal, just several workable arrangements with rather different maintenance bills.
1] Which two animals in this lesson belong to the pinnipeds?
2] Why does evolution produce different workable adaptations rather than one perfect animal?
Sea otters can change a habitat through what they eat. Sea urchins graze on kelp, large seaweeds that form underwater forests. Where otters reduce urchin grazing, more kelp may survive. This chain of effects is called a trophic cascade. Kelp forests provide food and shelter for many other organisms, so an otter’s meal can affect animals it never meets. However, the result depends on local conditions, including temperature, nutrients, and other predators. Otters cannot guarantee a healthy forest everywhere. Researchers compare places and follow changes over time to test these effects. An appealing animal is not evidence; observations are considerably more useful.
1] How can an otter eating sea urchins help kelp survive?
2] Why must scientists investigate local conditions before predicting an otter’s effect on a forest?
Human activities affect these animals through specific biological weaknesses. Fishing nets can trap a diving seal and prevent it from reaching air. Oil can ruin a sea otter’s fur insulation. Loss of suitable sea ice can change a walrus’s access to food and safe resting places. Disease can also spread through wildlife populations, sometimes across species. Effective protection therefore needs more than a general wish to help. It requires evidence about the animal, the hazard, and the place. Reducing entanglement, preventing spills, and protecting resting areas solve different problems. A clean beach is useful, but it cannot untangle a net offshore.
1] Why can entanglement in a fishing net kill an animal adapted to diving?
2] Why should protection measures match each animal’s particular biological needs?
Recovery can bring benefits and disagreements. More sea otters may support kelp forests, but they also eat shellfish valued by fishers. Walrus protection must consider the food needs and knowledge of Indigenous coastal communities. Scientists use observations, tracking devices, and biological samples to understand movement, survival, and reproduction. Such evidence helps identify vulnerable feeding or resting areas and assess whether protection works. Future outcomes remain uncertain because oceans, prey, and human activities change together. Decisions should state those uncertainties and compare real costs. Understanding these animals helps people choose useful actions, rather than buying a hopeful slogan and expecting it to patrol the coast.
1] Why might some fishers be concerned about increasing sea otter numbers?
2] How can scientific evidence and local knowledge improve protection decisions?
Homework
Writing tasks
1] Explain in approximately one page why wet clothes make a person cold and how a sea otter’s fur reduces heat loss. Include trapped air and the effects of oil.
2] Explain in approximately one page how a seal’s blubber works. Address insulation, stored energy, and why blubber is different from a coat.
3] Explain in approximately one page why running makes breath-holding difficult and how seals manage oxygen during dives. Include blood, muscles, and the heart.
4] Explain in approximately one page why a body suited to swimming may move awkwardly on land. Compare a true seal with a sea lion.
5] Explain in approximately one page how an animal can find food when it cannot see clearly. Compare seal whiskers detecting water movement with walrus whiskers examining the seabed.
6] Argue in approximately one page whether using a tool can be as useful as having stronger teeth. Use the sea otter study and explain the limits of your comparison.
7] Explain in approximately one page why a sea otter mother needs time to feed and rest. Address grooming, milk production, and disturbance by people.
8] Explain in approximately one page how the location of a resting place changes access to food. Use Pacific walruses and distinguish shallow feeding areas from deep water.
9] Explain in approximately one page how changing one kind of eater can change a habitat. Describe the otter–urchin–kelp relationship and at least one condition that can affect it.
10] Argue in approximately one page how a coastal community should judge a marine mammal protection plan. Include biological evidence, fishing livelihoods, local knowledge, and uncertainty.
Debate topics
1] Should authorities support sea otter reintroduction where local shellfish fishers object?
Side A — Yes, because: 1) otters can help restore ecological relationships; 2) healthier habitats can benefit other wildlife; 3) lost populations may deserve recovery.
Side B — No, because: 1) shellfish catches and incomes may fall; 2) ecological benefits vary between locations; 3) communities should help design and assess any proposal.
2] Should some wildlife resting areas be closed to visitors during sensitive seasons?
Side A — Yes, because: 1) disturbance can waste energy; 2) crowded walruses can injure calves when frightened; 3) breeding animals may need uninterrupted access to resting places.
Side B — No, because: 1) responsible viewing can support local incomes; 2) education can improve public understanding; 3) distance rules and limited access may sometimes protect animals without full closure.
3] Should limited conservation funding prioritise preventing immediate local hazards over long-term habitat research?
Side A — Yes, because: 1) reducing entanglement can prevent deaths now; 2) protecting resting sites addresses known risks; 3) spill prevention protects existing animals and habitats.
Side B — No, because: 1) research can reveal hazards that are currently missed; 2) changing ice and prey can alter future needs; 3) poorly targeted action may waste funds without improving survival.
Answer the following questions in full sentences. If you don’t know the right answer, add a (?) mark, and later we can look at it together.
1] What makes seals, sea otters, and walruses mammals?
2] How do a true seal’s hind flippers help it swim?
3] Why can a true seal not walk on its flippers like a sea lion?
4] What are the two main functions of blubber?
5] How can blood vessels near the skin help conserve body heat?
6] What different roles do haemoglobin and myoglobin play?
7] How does the diving response help a seal manage oxygen?
8] How can seal whiskers help locate prey in cloudy water?
9] How does grooming maintain a sea otter’s insulation?
10] Why do sea otters need to eat a large amount of food each day?
11] How can rock tools help sea otters protect their teeth?
12] Why are young sea otter pups initially poor divers?
13] What are walrus tusks, and what functions can they serve?
14] How does a walrus find and remove food from clam shells?
15] Why is sea ice above shallow feeding areas useful to Pacific walruses?
16] What risks can calves face in large coastal walrus groups?
17] What evidence helps scientists identify evolutionary relationships among mammals?
18] How can sea otter predation cause a trophic cascade in a kelp forest?
19] How do fishing nets and oil create different biological dangers for marine mammals?
20] Why should conservation decisions include local livelihoods, biological evidence, and uncertainty?
Teaching illustrations and similes
1] Streamlined bodies — Moving a smooth object through water is easier than pushing a broad, flat object. Limit: a seal also needs active muscles and flipper movements; shape alone does not make it swim.
2] Blubber — Think of built-in insulation around a warm room. Limit: blubber is living tissue and stores usable energy, whereas a building’s insulation does not feed the building.
3] Blood flow and warmth — Reducing warm blood near the skin resembles reducing hot water flowing through an exposed pipe. Limit: the body must still supply tissues with enough blood to function.
4] Oxygen storage — Blood and muscle provide oxygen reserves, rather like supplies carried for a journey. Limit: oxygen is held by proteins, not stored in little tanks, and the reserves remain limited.
5] Whisker sensing — Imagine noticing a person moving nearby by feeling the air disturbance with sensitive fingertips. Limit: seal whiskers sense water movement, and a human hand does not have the same sensitivity.
6] Sea otter fur — Trapped air resembles the air inside a padded winter jacket. Limit: an otter maintains the air layer through grooming, and oil can damage the system.
7] Metabolism and food — A warm-running machine needs a continuing fuel supply. Limit: living metabolism also repairs and builds the body; food serves more purposes than fuel alone.
8] Walrus feeding — Compare drawing soft food through an opening by lowering pressure in a syringe. Limit: walruses create suction with living mouth structures and must first locate and uncover the prey.
9] Sea ice and feeding access — A resting platform above food resembles accommodation beside a workplace. Limit: ice moves and melts, and access depends on water depth and prey distribution.
10] Trophic cascades — Removing heavy grazers from a garden can allow plants to recover. Limit: kelp forests also respond to temperature, nutrients, and other organisms, so one predator cannot control everything.
