Diving

Comparing limits with humans

To get a feel for extreme diving by elephant seals, with regard to dive depth and duration, consider the limits of humans. Who are the world's best air-breathing divers? Compare your thoughts with observation.

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Sources of data cited with Dive Stats table below.

        

The distinctive dive pattern

Tracking shows that northern elephant seals make vertical commutes many times a day. They regularly dive to depths of 400-800 m for 20-30 minutes at a time. Their maximum dive depth and duration are even more impressive (see Dive Stats table below). They spend most of their time at depth and just a few minutes at the surface between dives. These animals dive in this pattern along their distant migration path as they forage for food.

See a dive pattern for a juvenile elephant seal that was tracked by Jessica Meir. Although the exact path isn't known, this animal crossed Monterey Bay from south to north.

Why dive so deep?

Access to more ocean - With or without scuba gear, human divers barely scrape the surface of the ocean. Wide areas of the world oceans are 3,000-6,000 meters deep and exceptionally deep areas occur in narrow trenches more than 11,000 meters deep.

Although unmanned remote vehicles have come a long way, human visits to the depths of the ocean are still technically challenging and as rare as visiting outer space. New tools are just beginning to reveal the wealth of life in deeper water. We're seeing what remains after fishing in the dark for over half a century.

What is so interesting down there? During the day, around the world's open oceans, many prey species congregate in one or more layers at midwater depths near 100-500 meters. These daytime aggregations can be so thick that sonar waves bounce off these layers, as if they were solid like the ground. This daytime feature is called the deep-scattering layer or DSL. At night, the DSL disappears as these animals spread out and move closer to the surface to hunt for surface-dwellers under the cloak of darkness. This daily mass movement of life, called the diel vertical migration is the largest multispecies migration on the planet and it happens every sunset and sundown as the deep scattering layers break up and reform.

Most air-breathing animals can't get to the depths where all this tasty seafood hides during the day. By being able to dive deep, elephant seals are able to leave behind competitors who might outcompete them back at the surface and to avoid shallow-roaming large predators that want to eat them.

Elephant seals are specially outfitted to hunt in this environment where it is deep, cold and dark. Blubber, big eyes with keen vision that can sense flashes of bioluminescence in darkness, and many other adaptations make their lifestyle possible.

The world's best air-breathing divers

One of the world's largest living mammals, the sperm whale is the deep-diving champion. The sperm whale dives down to nearly 2 km (one and a quarter miles) and stays under for over an hour on a regular basis. That puts it near food that includes colossal squid and the Patagonian tooth fish (Dissostichus eleginoides, also known as Chilean sea bass). At that dive range, sperm whales can access about a third of the world's ocean volume.

The much smaller beaked whale is a close second. Not much is known about these animals because they are rarely seen except when they've beached themselves.

Of the "smaller" mammals, elephant seals are the deepest divers.

Of the animals that are not mammals, a bird - the penguin, and a reptile - the sea turtle are also expert divers.

Comparison of dive statistics for air-breathing animals.

Species	Maximum depth (meters)	Maximum duration (minutes)	Total oxygen stores (ml/kg>	Hemoglobin (g/dl)	Myoglobin (g/100g)	Blood volume (ml/kg)
Humans Homo sapiens	214	91	28 (20	14.5	0.3-0.4	70
Leatherback turtle Dermochelys coriacea	1230	67.3	27	15.6	4.9	77
Bottlenose dolphin Tursiops truncatus	390	8	36	14	3.3	71
Emperor penguin Aptenodytes forsteri	564	23.1	53	18	6.4	100
Northern elephant seal Mirounga angustirostris	1,581	119	97	25	6.5	250
Beaked whale Ziphius cavirostris	1,888	85	unknown	unknown	4.3	unknown
Sperm Whale Physeter macrocephalus	2,250	138	77	22	5.0	200

        Click to see reference list.     Updated: September 23, 2009.

Problems faced by human divers

Problems fall into three main categories -

• running out of oxygen - limits to the amount of oxygen our bodies carry
• the minimum oxygen levels at which our bodies remain functional
• negative behavior of gases

Animals like the elephant seal, whales, turtles, penguins, and dolphins breathe air into gas-filled lungs, just like us. They can only dive as far as a breath of air allows. So why can some animals dive much further before suffering ill consequences? Physiologists want to know how they do it and ecologists want to know why they do it.

Limited oxygen stores

A body is only so big and it has a limited capacity to carry oxygen. In the few minutes you can remain conscious on a breath of air, you can only swim so far with the body you have. The best deep divers like the elephant seal and sperm whale are big animals. They carry a lot of oxygen and importantly, more of it per unit of body mass than a human. And they have the right kind of body to go further on the same tank of gas, possessing efficient fins instead of hands for swimming. Adaptations involve tradeoffs - hands have more dexterity than fins, but hands don't propel us very efficiently through water. No single body type being perfect for all situations allows for the great diversity of life on Earth.

Poor hypoxemic tolerance

Hypoxemic tolerance is the lowest level of oxygen that an animal can tolerate.

Humans lose consciousness well before all the oxygen in their body is used up. Loss of consciousness while underwater leads to drowning and death. Elephant seals can function until their oxygen stores are nearly empty, allowing them to stay down longer on the same amount of oxygen.

When you try to hold your breath, that irresistible urge to take a breath is triggered by too high levels of carbon dioxide (CO₂) gas rather than too low levels of oxygen (O₂) gas. Sensitivity to elevated levels of CO₂ before running out of O₂ provides a safety cushion. It tells us that taking a breath is our highest priority.

CO₂ is produced as a byproduct of respiration as your body metabolizes O₂. During a breath hold dive, CO₂ accumulates in your system unless you exhale.

Sometimes expert divers want to delay the signal from CO₂ so they can stay under longer. By hyperventilating before diving, they lower their pre-dive CO₂ load relative to O₂. But sometimes they run out of oxygen before the CO₂ trigger occurs and experience shallow water blackout that leads to drowning and death.

Decompression sickness

Decompression sickness is also known as the bends because it can causes damage and pain at joints. It is also called caisson disease because it causes illness in people who work in pressurized structures called caissons.

We tend to think of crushing pressures in the deep sea. We know that the downward pressure exerted by even one meter thickness of overlying water is more than that exerted by the entire columnn of air above sea level. Yet we also see delicate animals living at all depths in the sea without being crushed. Since water hardly compresses at all compared tp highly compressible gases such as air, animals that are filled with water do not experience the same problems faced by animals that are partly filled with air.

At high pressure, air compresses and changes from a gas into a liquid and it tends to dissolve into liquids such as water or blood. The reverse happens when pressure is reduced, liquefied gas turns back into a gas and expands significantly. Gases that were dissolved in water or blood come back out, sometimes in inopportune places.

When someone dives and the surrounding pressure increases, the lungs compress as air compresses. Gases also dissolve into the body. When the pressure is reduced, the process reverses, gases in the lung expand and dissolved gas comes out of the body. When humans try to surface too fast, gas bubbles can form in blood vessels, joints and other unfortunate places rather than in the lungs. Gas bubbles in those places create immediate and delayed problems that can cause permanent damage as well as death. Thus divers learn the importance of a slow accent and that if needed to take a break just below the surface.

Not just diving, but any activity that involves a rapid drop in pressure can cause similar problems. Flying in an unpressurized aircraft and stepping into outer space are two such situations.

Elephant seals use several strategies to avoid decompression sickness. They don't carry compressible gas to the deep in the first place, so it's not there to reappear as they surface. They lack some of sinuses that house air in a human head. They expel air before diving by collapsing lungs encased in flexible rather than rigid ribs.

Nitrogen narcosis

Natural air is a mixture of nitrogen, oxygen, argon and carbon dioxide. Nitrogen is the most abundant and it is inert. Inert means that it doesn't react much with other chemicals. Both nitrogen and oxygen circulate through our bodies, but nitrogen keeps circulating without getting used up. Because oxygen is removed from circulation as it gets stored and metabolized, O₂ doesn't return into the blood stream. Over time, that makes the relative concentration of nitrogen high.

When the concentration of nitrogen becomes too rich, at high pressure at depth, humans experience nitrogen narcosis or rapture of the deep. Effects resemble alcohol intoxication and include disorientation, very poor judgment, and euphoria. Different mixes of dive gases offer advantages, but also carry different risks. Any effect that impairs judgment is extremely hazardous while diving.

All of these problems are complex and not entirely understood. Diving provides a rare opportunity to experience an alien world and lifestyle, but it can be unforgiving when you're a human rather than an elephant seal.