Sperm Whales Blow Bubbles to Maintain Buoyancy While Sleeping Vertically

Sperm whales are known for their dives into the deep ocean. But when it is time to rest, the largest toothed whale on the planet adopts one of the strangest postures in the animal kingdom. They hover vertically in tight groups just beneath the waves, looking like a row of floating toy soldiers with their heads pointed toward the sky.

How Sperm Whales Maintain Vertical Posture During Rest

That bizarre, head-up resting pose was first documented by naturalists in 2008. The species is the only type of whale known to rest in this vertical alignment, an orientation scientists believe shields the animals from surface wave action while sparing them the energy required to descend back into the deep ocean.

Maintaining that position, however, presents a stubborn physics puzzle. Sperm whales are naturally buoyant because of the vast quantities of spermaceti oil stored inside their massive heads, and they also carry air inside their lungs as breath-hold divers. As an animal rests and begins to drift upward even slightly, the water pressure drops, causing the gas in its lungs to expand. Without a countermeasure, that expanding gas would force the whale to rocket straight to the surface.

Suction-Cup Tags and Computer Simulations Reveal Bubble Mechanics

To solve the mystery, researchers from the University of St Andrews and Université de Neuchâtel deployed small recording tags held by suction cups on sperm whales off the Norwegian coast. The devices recorded three-dimensional body movements, depth data, and the acoustic signature of underwater exhalations.

The study, published in the University of St Andrews, used data to build a computer simulation incorporating tissue density, water resistance, and internal gas volumes to test whether those exhalations accounted for the stability.

Sperm Whales Release Gas Bubbles to Maintain Buoyancy While Sleeping

“This study confirms that Sperm Whales exquisitely control their buoyancy by releasing gases to remain submerged with near-neutral buoyancy just below the sea surface. It is particularly fascinating they are able to make these fine-scale adjustments while they are thought to be asleep, a challenge totally alien to terrestrial mammals like us humans.”

Professor Patrick Miller, Sea Mammal Research Unit at the University of St Andrews

Shallow Naps Versus Deep Ascents Change How Whales Vent Gas

At greater depths, the surrounding water pressure already compresses the air inside the whales’ bodies, reducing its natural buoyant pull. Consequently, those deep-water resting whales do not need to modulate their buoyancy as frequently.

The simulation confirmed that release of bubbles functioned to reduce the whales’ positive buoyancy, enabling them to remain submerged while resting.

Unresolved Questions About Sleep States and Metabolic Off-Gassing

While the buoyancy explanation accounts for the presence and timing of the bubbles, scientists do not yet know whether the whales execute these adjustments consciously or while fully unconscious. Like dolphins, cetaceans are known to practice unihemispheric sleep, resting one half of the brain at a time. However, the stillness of sperm whales during these vertical rests leads researchers to suspect they may achieve full bi-hemispheric sleep.

Additionally, researchers emphasize that buoyancy control may not be the only purpose behind the exhalations. The vented air could serve a secondary metabolic function.

“The authors suspect that releasing bubbles could be related to the off-gassing of excess CO2 or N2 from tissues into the lungs, so this behaviour could also have implications for metabolic gas exchange.”

Study authors, via University of St Andrews