How do sperm whales dive without decompression sickness?

A professional diver working in saturation can hold out for a few hours at three hundred metres down, after several days of acclimatisation and with as many decompression stops on the way back up. Without that equipment, he would be doomed within minutes: nitrogen narcosis, decompression sickness, loss of consciousness. The sperm whale, for its part, descends past a thousand metres every day on a single breath taken at the surface. Some exceptional dives reach more than three thousand metres: a pressure of three hundred atmospheres, the equivalent of three hundred kilos bearing on every square centimetre of its body. How does it do it?

Short answer: unlike the human diver, who breathes air under pressure throughout the descent and therefore builds up nitrogen in the blood, the sperm whale makes a breath-hold dive on a single volume of air taken at the surface. Under pressure, its lungs collapse from a hundred metres down, pushing the air back into rigid airways where no gas exchange can take place any longer. With no nitrogen uptake, there is no decompression sickness. Added to this are a massive concentration of muscle myoglobin that stores oxygen, a diving bradycardia that slows the metabolism, and a vasoconstriction that directs blood towards the vital organs.

Why humans suffer and sperm whales do not

To understand what separates the two, one has first to understand what happens to a diver. A scuba diver breathes compressed air delivered from a cylinder. As the diver descends, the ambient pressure rises, and with it the amount of nitrogen dissolved in the blood (Henry’s law). At three metres, twice as much nitrogen is being breathed in; at twenty metres, three times as much. That nitrogen accumulates slowly in the tissues, above all in fatty tissue. When the diver comes up too fast, the nitrogen cannot be expelled through the lungs quickly enough and forms microbubbles in the vessels. This is decompression sickness: joint pain, spinal cord lesions, gas embolism. Without decompression stops, it is fatal.

The sperm whale, by contrast, does not breathe underwater. It dives on a single breath. That initial volume of air is compressed progressively as the pressure rises. At a hundred metres, lung volume is reduced to about ten per cent of its value at the surface. At a thousand metres, it is less than one per cent. The remaining air is pushed out of the pulmonary alveoli (where gas exchange takes place) and into the upper airways (trachea, larynx), which are structurally rigid and impermeable to the passage of gas. If the air is no longer in contact with the blood, no nitrogen can be absorbed, and so nothing can form bubbles on the way back up.

A massive oxygen store in the muscles

If the sperm whale does not breathe for an hour and a half, how do its tissues stay oxygenated? The answer lies in a protein called myoglobin. Myoglobin is the muscular equivalent of haemoglobin in the blood: it binds oxygen and stores it locally. All mammals have some, but diving cetaceans concentrate astronomical quantities of it. One study (Mirceta et al., 2013) measured a muscle myoglobin content in the sperm whale about ten times that of a human muscle.

This extreme concentration is so high that it poses a chemical problem: myoglobin molecules, which carry a positive charge, ought in theory to aggregate and precipitate within the muscle. Evolution, however, has selected in all diving mammals (cetaceans, seals, penguins) a particular surface charge that prevents this aggregation. The marker is reliable enough that Mirceta and colleagues were able to reconstruct the dive depth of fossil mammals purely by analysing the charge on their myoglobin.

The physiological slowdown of the dive

When the sperm whale begins its dive, its body switches into economy mode. This is the diving response, a mechanism shared by all diving mammals and even in part by humans: plunging the face into cold water is enough to trigger it slightly.

The heart rate falls sharply: from sixty beats per minute to about ten. This bradycardia reduces the heart’s own oxygen consumption and slows the distribution of blood around the body. At the same time the peripheral arteries contract, which is peripheral vasoconstriction. Blood is redirected towards the vital organs: brain, heart, lungs. The muscles, meanwhile, go on working on their own myoglobin reserves, becoming temporarily almost anaerobic. They build up lactate at concentrations that would be intolerable in a human, but which cetaceans clear without harm once back at the surface.

Finally, cetaceans tolerate high levels of carbon dioxide in the blood without triggering the breathing reflex, whereas it is precisely the build-up of CO₂, and not the lack of oxygen, that forces a human back up to breathe.

The architectural role of the rete mirabile

To this is added a remarkable vascular structure: the rete mirabile (“wonderful net”), a dense network of small interwoven arteries and veins. In diving cetaceans this network is particularly well developed at the base of the brain and in the thorax. It serves to damp the abrupt swings in blood pressure that go with rapid changes of depth, to regulate temperature by countercurrent exchange, and to store a volume of oxygenated blood that can be drawn on during the dive. It is the biological equivalent of a hydraulic capacitor. See our dedicated page.

A persistent puzzle: scars in the bones

The picture of a sperm whale wholly immune to the strains of diving needs qualifying. In 2004, Michael Moore and Greg Early published a disquieting study in Science: on old sperm whale skeletons held in museum collections, they observed characteristic lesions in the femoral heads and in certain vertebrae, pitting, erosion, microfractures. These lesions resemble those found in human divers affected by chronic decompression injury (dysbaric osteonecrosis).

The conclusion: the sperm whale is not entirely out of reach of it. It tolerates strains that would be lethal in other mammals, but that tolerance is not absolute. Subclinical incidents appear to occur over the animal’s lifetime. The current hypothesis is that particularly intense sequences of dives, or disturbance forcing an abrupt ascent, can cause bubbles to form. It is precisely this mechanism that is invoked today to explain the mass strandings of beaked whales exposed to military sonar: see our dedicated feature.

Sources

Kooyman, G.L. & Ponganis, P.J. (1998). “The physiological basis of diving to depth: birds and mammals”. Annual Review of Physiology, 60, 19-32.
Moore, M.J. & Early, G.A. (2004). “Cumulative sperm whale bone damage and the bends”. Science, 306(5705), 2215.
Mirceta, S. et al. (2013). “Evolution of mammalian diving capacity traced by myoglobin net surface charge”. Science, 340(6138), 1234192.
Ponganis, P.J. (2015). Diving Physiology of Marine Mammals and Seabirds. Cambridge University Press.
Fahlman, A. et al. (2020). “Lung function in cetaceans”. Respiratory Physiology & Neurobiology, 275.