eduKate Learning Manual
Science | Animal World
Understand → Learn → Test → Transfer → Go Deeper

Weddell Seal

How a Mammal Holds Its Breath for More Than an Hour Without Starving Its Brain

Big Question: How can an air-breathing mammal keep its most oxygen-sensitive organs alive while swimming beneath Antarctic ice with no access to the atmosphere?

Quick Answer

Weddell seals carry unusually large oxygen stores in blood and skeletal muscle. Their blood volume, haematocrit and haemoglobin concentration are high, while swimming muscles contain abundant myoglobin that binds oxygen locally. During a dive, the cardiovascular dive response can slow heart rate and constrict vessels supplying less immediately critical tissues. Blood oxygen is thereby conserved for organs such as the brain and heart, while working muscle can draw more heavily on its own myoglobin store. Metabolic rate can also fall. Most routine dives remain within an aerobic range, but unusually long dives can exceed the aerobic dive limit and accumulate lactate, requiring longer recovery at the surface. Weddell-seal tissues are also unusually tolerant of low oxygen and reperfusion stress.

What You Will Learn

• Where a diving seal stores oxygen.
• Why blood volume matters more than simply having large lungs.
• What haemoglobin and myoglobin do.
• How bradycardia and peripheral vasoconstriction conserve oxygen.
• Why the brain is treated differently from kidneys and some muscles.
• What the aerobic dive limit means.
• Why a 96-minute dive does not mean 96 minutes of ordinary aerobic metabolism.
• How hypoxia and reperfusion tolerance complete the diving system.

Part 1 — The Oxygen Budget Starts Before the Dive

Once a seal submerges, no new oxygen enters from the atmosphere. The body must operate on oxygen already stored.

Those stores are divided among three main compartments:

• lungs — oxygen in respiratory gas;
• blood — oxygen bound mainly to haemoglobin;
• muscle — oxygen bound to myoglobin.

In marine mammals, blood and muscle stores are proportionally much more important than in humans.

Part 2 — A Seal Carries a Large Internal Blood Reservoir

Diving mammals can have much larger blood volumes than terrestrial mammals of similar size. Weddell seals also carry high concentrations of red blood cells and haemoglobin.

Haemoglobin reversibly binds oxygen. More circulating haemoglobin means more oxygen can be carried before submergence.

The spleen can also store concentrated red cells and release them into circulation in many diving mammals, adding flexibility to blood oxygen capacity.

Part 3 — Muscle Has Its Own Oxygen Tank

Skeletal muscle contains myoglobin, an oxygen-binding protein related to haemoglobin.

Weddell-seal swimming muscles contain much more myoglobin than ordinary terrestrial mammal muscles. That lets the muscle carry a local oxygen reserve even when blood flow is reduced during diving.

haemoglobin carries oxygen through blood; myoglobin holds oxygen inside muscle.

Part 4 — Why Not Store Everything in the Lungs?

At depth, increasing pressure compresses gas-filled lungs. In many marine mammals, peripheral airways and alveoli progressively collapse, reducing gas exchange at depth.

This has advantages: it limits nitrogen uptake and therefore reduces decompression risk. But it also means lungs cannot be treated as a continuously available scuba tank.

Large blood and muscle stores are therefore essential.

Part 5 — The Heart Slows

One hallmark of the mammalian dive response is bradycardia: heart rate decreases.

A slower heart generally means lower cardiac output and can reduce the rate at which blood oxygen is delivered and consumed.

But the response is flexible. Heart rate changes with dive depth, duration, exercise and behaviour. A free-ranging seal does not switch to one fixed “dive heart rate.”

Part 6 — Blood Flow Is Redistributed

Peripheral vasoconstriction narrows blood vessels supplying tissues that can tolerate reduced flow for a time.

Perfusion is maintained preferentially to the heart and brain, while organs such as the kidneys can receive much less blood during a prolonged dive.

oxygen is not spent equally across the body.

This creates oxygen triage: protect the organs least tolerant of interruption while allowing more tolerant tissues to operate from local stores or temporarily reduced metabolism.

Part 7 — The Brain Still Needs Oxygen

Mammalian brains are usually extremely sensitive to interrupted oxygen delivery.

Weddell seals preserve cerebral perfusion during diving and have molecular features associated with strong control of brain blood vessels. Diving pinniped brains also show high antioxidant defences and adaptations that help tolerate severe hypoxaemia.

In classic physiological studies, Weddell seals tolerated extremely low arterial and cerebral venous oxygen tensions during prolonged breath holds before obvious neurological impairment appeared.

Part 8 — Muscle Can Become More Independent

If blood flow to muscle decreases, myoglobin-bound oxygen can support local aerobic metabolism.

Studies of Weddell seals indicate that muscle perfusion patterns can vary, and not every dive follows the same simple “blood off, muscle isolated” model. Some muscle blood flow may persist depending on dive conditions.

The important principle is flexible matching of oxygen stores to workload.

Part 9 — What Is the Aerobic Dive Limit?

The aerobic dive limit, or ADL, is the dive duration beyond which post-dive blood lactate begins to rise substantially, signalling greater reliance on anaerobic metabolism.

For many adult Weddell seals in a recent large study, calculated aerobic dive limits were around 19 minutes.

Most dives stay near or below aerobic thresholds because aerobic diving is efficient: the seal can surface, exchange gases and dive again without a long lactate-clearing recovery.

Part 10 — Then How Can a Dive Last 96 Minutes?

An extreme dive can exceed the normal aerobic budget.

During such a dive, oxygen stores become deeply depleted and some tissues increasingly use anaerobic glycolysis. Lactate accumulates. On surfacing, the seal must spend longer recovering, restoring oxygen stores and clearing metabolic by-products.

So a 96-minute dive is impressive precisely because it is not ordinary. Modern records describe it as several times the estimated aerobic dive limit.

maximum capability ≠ routine operating mode.

Part 11 — Why Extreme Dives Are Strategically Timed

Long dives have costs. If they require long recovery, they reduce the fraction of time available for repeated foraging.

A 2024 study tracking 59 adult Weddell seals over thousands of seal-days found that the most extreme dives were not distributed randomly through the day. Seals appeared to schedule demanding dives in ways that balanced physiological cost with changing prey availability and Antarctic light conditions.

Physiology and behaviour therefore meet: the animal does not merely possess a maximum dive time; it chooses when pushing closer to that limit is worthwhile.

Part 12 — Coming Back Up Is Also a Physiological Problem

When normal blood flow returns to previously constricted tissues, oxygen returns rapidly. This can create reactive oxygen species and reperfusion stress.

Diving mammals possess strong antioxidant systems and cellular mechanisms that reduce injury from repeated cycles of low oxygen and reoxygenation.

A successful dive therefore includes descent, oxygen management, ascent and recovery.

Someone Put Instruments on Seals Under Antarctic Ice

Weddell seals became foundational animals in diving physiology because researchers could work around stable breathing holes in Antarctic sea ice.

Gerald Kooyman and later researchers measured dive depth, duration, heart rate, blood gases, lactate, blood volume and oxygen stores. Modern biologging tags now record months of natural behaviour rather than isolated experimental dives.

watch the dive → measure oxygen stores → record heart and blood → define aerobic limit → tag wild animals → connect physiology to behaviour.

How Do We Know?

• Time–depth recorders measure natural dive duration and depth.
• Blood-volume measurements quantify the circulating oxygen reservoir.
• Haemoglobin and haematocrit assays estimate blood oxygen capacity.
• Muscle biopsies measure myoglobin concentration.
• Heart-rate recorders reveal bradycardia and recovery.
• Blood gas measurements reveal severe hypoxaemia.
• Post-dive lactate helps define the aerobic dive limit.
• Molecular studies test hypoxia and antioxidant adaptations in brain and vessels.

Observation vs Inference

• Observation: heart rate falls during many dives.
• Observation: kidney perfusion can decrease while cerebral perfusion is preserved.
• Observation: blood and muscle contain large oxygen stores.
• Observation: lactate rises after sufficiently long dives.
• Inference: selective circulation and local muscle stores stretch the usable oxygen budget.
• Behavioural inference: seals trade dive reward against recovery cost.

Common Misconceptions and Repairs

Misconception	Better model
Weddell seals have gigantic lungs that hold 96 minutes of air.	Blood and muscle oxygen stores are major components, and extreme dives exceed routine aerobic budgets.
The heart nearly stops and all tissues receive no blood.	Bradycardia and vasoconstriction are graded and tissue-specific; brain and heart perfusion are prioritised.
Every long dive is anaerobic from the start.	Dives begin using stored oxygen; anaerobic contribution rises as aerobic stores become depleted.
96 minutes is a normal Weddell-seal dive.	It is an extreme recorded duration; routine dives are much shorter.
Myoglobin is haemoglobin inside muscle.	They are related oxygen-binding proteins with different structures and locations.
Surfacing instantly resets the animal.	Extreme dives require recovery, gas exchange and metabolic restoration.

Checkpoint Questions

1. Where does a Weddell seal store oxygen?
2. What does haemoglobin do?
3. What does myoglobin do?
4. Why is bradycardia useful?
5. What is peripheral vasoconstriction?
6. Why is brain perfusion prioritised?
7. What is the aerobic dive limit?
8. Why does lactate rise after some extreme dives?
9. Why are maximum dives not a good description of routine behaviour?
10. Why does recovery count as part of diving physiology?

Apply It — Two Seals With the Same Oxygen Store

Seal A and Seal B begin with the same total oxygen store. Seal A maintains a high heart rate and perfuses most tissues normally. Seal B slows its heart and reduces blood flow to less critical organs.

Which animal should preserve blood oxygen for the brain longer? What cost might Seal B pay in the under-perfused tissues?

Answer Key

Open after attempting the question

Seal B should preserve central blood oxygen longer because oxygen delivery to peripheral tissues is reduced. Those tissues may rely more on local oxygen stores, reduce metabolism or eventually use anaerobic pathways, producing metabolites that must be managed after the dive.

Can You Explain WHY?

• Why is a large blood volume useful underwater?
• Why does muscle myoglobin reduce competition with the brain for blood oxygen?
• Why can slowing circulation extend a dive but not make oxygen unnecessary?
• Why do the longest dives create a time cost after surfacing?

World Connection

Weddell seals live around Antarctica, often beneath sea ice where access to breathing holes is limited. Their physiology is inseparable from that environment: oxygen management matters because the next breath may be hundreds of metres and many minutes away.

Primary Science Bridge

• Mammals breathe air with lungs.
• The heart pumps blood.
• Blood transports oxygen.
• Muscles need energy for movement.
• Organisms can adjust body functions to environmental demands.

Secondary / JC Resolution

Simple idea	Higher-resolution science
Seal stores oxygen	Blood volume, haemoglobin, haematocrit and myoglobin
Heart slows	Autonomic dive response and reduced cardiac output
Blood goes to brain	Regional vascular resistance and oxygen prioritisation
Long dive exceeds oxygen	Aerobic dive limit, glycolysis and lactate
Seal recovers	Reoxygenation, lactate clearance and oxidative-stress defence

Deep Science Window — Oxygen Is a Budget and a Routing Problem

Two animals can start with the same total oxygen and achieve different dive times depending on how fast they spend it and which tissues receive it.

Diving performance therefore emerges from storage × demand × routing × tolerance, not storage alone.

Evidence Boundaries

• 96-minute maximum ≠ routine dive.
• Calculated ADL ≠ identical hard cutoff for every individual dive.
• Bradycardia ≠ fixed heart rate.
• Peripheral vasoconstriction ≠ complete shutdown of every muscle and organ.
• Large oxygen stores ≠ no anaerobic metabolism on extreme dives.
• Weddell-seal physiology ≠ every marine mammal uses identical proportions and strategies.

Research Sources and Further Reading

• Communications Biology — Breath-hold capacities and dive rhythmicity in Weddell seals
• Full open-access version of the 2024 Weddell-seal study
• Peripheral vasoconstriction and brain perfusion in Weddell seals
• Antioxidant defence in the brain of deep-diving pinnipeds
• Classic Science study — maximum diving capacities of the Weddell seal