New research suggests sperm whales deliberately release air to fine-tune their buoyancy, helping them remain suspended vertically beneath the surface during their unusual resting periods.
Resting sperm whales blowing bubbles
Sperm whales are famous for their extraordinary deep dives, but scientists have now uncovered another notable adaptation: they appear to use bubbles as a form of buoyancy control while resting.
Researchers from the University of St Andrews and Université de Neuchâtel found that resting sperm whales (Physeter macrocephalus) release gas as they drift vertically below the surface. By reducing the volume of gas within their bodies, the whales can counter their tendency to become increasingly buoyant as they rise towards shallower water.
Sleeping vertically
Sperm whales display an unusual resting behaviour in which they hang almost vertically in the water column, sometimes resembling enormous pillars suspended beneath the sea.
The behaviour is thought to allow the animals to sleep or rest away from surface wave action without expending the energy required to descend into deeper water. However, maintaining that position presents a physical problem.
As a breath-holding whale rises, decreasing water pressure allows gas in its lungs to expand according to Boyle’s law. That makes the animal increasingly buoyant and would eventually cause it to float towards the surface.
The researchers found that releasing bubbles appears to solve the problem.
Tagged whales reveal the mechanism
The study analysed data from 42 sperm whales fitted with suction-cup recording tags off northern Norway between 2005 and 2019. The tags recorded depth, movement and underwater sound. Seventeen whales were observed resting, with individual resting dives lasting between six and 31 minutes.
For detailed modelling, the researchers examined ten whales displaying a characteristic resting pattern in which they initially descended head-down before rotating into a head-up position.
All ten released bubbles during these resting dives.
Computer simulations showed that the whales’ observed movements could be reproduced when their internal gas volume was progressively reduced, supporting the hypothesis that bubble release allows them to regulate buoyancy. Models assuming a constant gas volume produced substantially poorer matches to the actual dive profiles.
Preparing for a rest
There was another intriguing finding. The whales appeared to begin resting dives carrying less gas than during deep foraging dives.
That suggests buoyancy control may begin even before the resting phase: the animals appear to adjust how much gas they carry according to what they are about to do.
Researchers cannot yet say exactly how consciously the whales control the process. Nor can they confirm from behavioural observations alone that the animals are actually asleep; measuring brain activity would be required to establish that conclusively.
The authors also note another possibility. Releasing bubbles could help eliminate excess carbon dioxide or nitrogen accumulated during diving, meaning the behaviour may serve both buoyancy regulation and physiological gas exchange.
What appears deceptively simple—a sleeping whale blowing bubbles—may therefore be another finely tuned adaptation to life as one of the ocean's most accomplished breath-hold divers.
