Sperm whales sleep vertically and blow bubbles. Scientists discovered why

Sperm whales can doze almost motionless while positioned vertically just below the ocean surface. Scientists noticed that during this type of rest the animals release air bubbles. The latest research suggests that this is not random behavior, but a way of controlling buoyancy and maintaining a stable position in the water.

How sperm whales sleep and rest

Sperm whales can rest in the water in several ways: by submerging tail-down to a depth of about 8 m, diving head-down, or slowly descending to depths of up to 200 m and relaxing during the ascent.

The vertical sleeping posture was documented for the first time relatively recently. In 2008, French and British researchers observed a group of sperm whales off the coast of Chile and noticed something that had rarely been described before: the animals remained motionless in the water, almost perfectly vertical, with their heads pointing upward. They did not react to the presence of boats and stayed in this position for several minutes.

Previously, scientists believed that such large marine mammals slept mainly while moving slowly or at greater depths. Vertical “hovering” turned out to be a behavior characteristic of short naps lasting usually 10 to 15 minutes. It allows the whales to maintain a stable position just below the water surface while still enabling a quick return to breathing.

Studying underwater sleeping habits

Scientists from the University of St Andrews set out to investigate why the sperm whale’s massive head, which contains a large spermaceti organ filled with a waxy substance, does not cause the animal to lose stability while resting.

The team conducted research around the Lofoten archipelago off the coast of Norway. Using specialized devices that recorded movement, dive depth, and sounds associated with gas release, they observed 42 resting sperm whales. They analyzed both vertical drifting just below the surface and head-down dives. The sensors recorded dive depth and duration, as well as the animals’ movements and body orientation.

How releasing air helps sperm whales rest

The researchers found that the closer the resting whales were to the surface, the more air bubbles they released into the water. The phenomenon can be explained by Boyle’s law, according to which gas volume decreases as pressure increases and expands as pressure decreases.

When a sperm whale dives, water pressure increases and the gas in its lungs is compressed. As a result, the whale’s buoyancy decreases. During ascent, the drop in pressure causes the gases to expand, increasing buoyancy. The partial release of gas from the respiratory system may help sperm whales maintain an appropriate depth without the need for constant active swimming. As a result, the animals do not have to rest in the deep sea and reduce the energy costs associated with returning to the surface.

The researchers also propose that the release of air bubbles may be linked to sperm whale metabolism and physiological processes that occur during diving. Carbon dioxide is produced as a byproduct of metabolism, while nitrogen can accumulate in tissues under increased pressure.

The scientists emphasize that depth regulation during rest is an active process in sperm whales. It does not necessarily imply conscious control by the animals, but it is an intriguing result of evolutionary adaptation that helps reduce energy expenditure during sleep.

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main photo: Wayne Hoggard/NOAA

source:

Freymond, N., Burslem, A. and Miller, P. J. O. (2026). Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus). J. Exp. Biol. 229, jeb251920. https://doi.org/10.1242/jeb.251920