Title - What's worth diving down there?
Inside this section
• A clue: how oxygen varies during diving
• More storage
• Efficient physiology and behavior

A clue: O2 during a dive

Elephant seals can dive further and stay under water much longer than humans. Scientists still don’t have a complete understanding of this remarkable aspect of elephant seal physiology. As previously mentioned, one factor is that animals that dive well have higher oxygen stores than do humans.

Jessica and Paul’s recent research reveals that: 1) elephant seals can tolerate much lower levels of oxygen than humans and other animals, and 2) the rate at which these animals use oxygen can also be altered dramatically in order to provide the most efficient usage possible.

Graduate student Jessica Meir in the field. Jessica Meir in the field. Photographer - Patrick Robinson.

The oxygen data shown in the animation below was collected as part of Jessica Meir's work and it was shown in the animation in the previous section about diving. This time, we're showing oxygen and depth.

The animation shows how much oxygen is left in this animal's veins as blood returns to the heart. The scale to the right shows units of "partial pressure of oxygen." The partial pressure of oxygen represents how much oxygen remains in the blood for the seal to utilize. Larger numbers indicate more.

Observe how the oxygen concentration varies while the animal is diving.

Did you see that:
  • After the seal surfaces, oxygen is replenished as the seal starts to breathe, and the oxygen concentration increases
  • As the animal dives rapidly to depth and the duration of the dive increases, oxygen concentration falls
  • For the range of depths and durations shown here, the deeper and longer duration dives are not using more oxygen than the shallow ones. It is the rate of oxygen depletion that is variable.
  • Approaching the end of the dive, oxygen has fallen to just 3-8 mm of Hg or 5-10% of the original load, well below the blackout tolerance or hypoxemic limit for humans.

More places to store O2

Title - What's worth diving down there?

More O2, a deeper dive - The longer that an animal can hold its breath and stay alive, the longer (and perhaps deeper) it can last on a single dive, all other things being equal.

Comparison of total oxygen stores. Data sources - [43, 44, 45, 46].

Why do animals need oxygen?

Oxygen allows animals to fuel metabolic processes such as respiration that keep cells and the entire body alive. Because animals must respire, oxygen is critical for life. Plants and other photosynthesizers are critical for the survival of animals because they are the only ones capable of producing O2 on Earth - without photosynthetic life, the atmosphere would have no oxygen.

3X the oxygen concentration of humans - Elephant seals carry more than three times the amount of oxygen for every unit of body mass relative to humans. But they don't store and carry it as a gas in their lungs. They store O2 in blood and muscle by attaching it to special protein molecules called globins. Avoiding gas in their lungs prevents some problems such as decompression sickness.

Comparison of stored oxygen distribution. Data sources - [43, 44, 45, 46].

Many people are familiar with hemoglobin, the globin protein that binds and transports oxygen in the blood. As blood circulates, hemoglobin can bind oxygen from the lungs and release it to muscle and other tissues throughout the body. Muscles contain a globin protein called myoglobin. As the O2 moves from hemoglobin to the myoglobin, the muscles can have a ready and dedicated store of oxygen when they need to spring into action. Other critical organs like the brain have also been shown to contain specialized globin proteins recently. Even when O2 is absent in lungs, it can still be stored in blood and muscle.

Beyond storage - Compared with humans and emperor penguins, elephant seals store proportionately more O2 in blood and less in the lungs, but even these impressive oxygen stores are still not enough to account for their incredibly long dives. The world record for a breath hold in a pool is just over 9 minutes for humans. The longest recorded dive on a single breath made by a northern elephant seal is 119 minutes, though most routine dives are closer to 20 minutes.

Making a quantity of oxygen last longer under water

Mammalian diving reflex

When the faces of most mammals are submerged in water, their bodies respond reflexively. A set of responses, called the mammalian diving reflex, operates involuntarily whether an animal is conscious or not. That response can make the difference between life and death for someone who is unconscious in a pool or overboard at sea. Even though humans and elephant seals have the same set of reflexes, it is more pronounced in elephant seals.

Responses to submergence of the face -

  • The heart rate slows to reduce oxygen consumption (bradycardia)
  • Blood vessels and capillaries in extremities are constricted so that oxygen is preferentially available to the brain (peripheral vasoconstriction)

Many physical adaptations and behaviors allow elephant seals to utilize oxygen more efficiently than other air-breathing animals.

They use it more slowly and have a much greater tolerance for running on low levels.

A group of involuntary responses called the mammalian diving reflex that reduce oxygen consumption during a breath-hold dive are more extreme in elephant seals than humans.

Title - What's worth diving down there? How heart rate changes during diving in an elephant seal. Lower heart rates during the dive correspond to reduced oxygen consumption. Graph modified from Andrews and others, 1997 [41].