New research from the University of St Andrews and Université de Neuchâtel has uncovered the mechanism which allows sperm whales to sleep in a unique vertical position, just under the ocean surface.

Sperm whales are the only whale known to rest vertically. It’s thought to enable them to sleep effectively by buffering them from surface wave action, while avoiding the energy needed to dive to deeper depths. However, until now, it’s been unclear how they maintained their position just below the sea surface.

In a paper published in the Journal of Experimental Biology, researchers discovered that while resting, sperm whales release gas bubbles to help them regulate their buoyancy and remain below the sea surface. This regulation is crucial as sperm whales are positively buoyant naturally due to the large amounts of spermaceti oil in their heads. Additionally, sperm whales are breath-hold divers; if they slowly drift up while resting, the gas in their lungs will slowly expand. Releasing bubbles enables them to counteract the expanding diving gas and achieve neutral buoyancy.

Researchers were able to collect data by placing small tags using suction cups on sperm whales off the Norwegian coast. The tags record sound and three-dimensional animal movements. Clear bubble sounds were recorded by the tags, and animal movement data was used to create a simulation based on tissue density, drag through the water, and gas volumes in their body.

Results from the simulation confirmed that release of bubbles functioned to reduce the whales’ positive buoyancy, enabling them to remain submerged while resting. Additionally, the results indicate that sperm whales start resting dives with less diving gas volume than for deep foraging dives.

Professor Patrick Miller from the Sea Mammal Research Unit at the University of St Andrews said: “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.”

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 behavior could also have implications for metabolic gas exchange.