A sperm whale’s head takes up about a third of its total body mass and is packed with oil, wax, and air. Since these components are buoyant, scientists have wondered how the whales stay underwater without behaving like corks. The new study suggests a surprising solution: during sleep, sperm whales regulate their buoyancy by actively modulating the gas volume they carry—specifically by blowing bubbles from the blowhole.

To test this idea, the team used advanced suction-attached recording devices on 42 sperm whales around the Lofoten Islands in northern Norway. The instruments captured bubble sounds, recorded the timing and frequency of bubble release, and tracked key context variables such as whale depth and head orientation (up versus down). The recorders were later recovered after the animals surfaced.

The results show that resting depth strongly predicts bubble activity. Whales sleeping closer to the surface released bubbles about 11 times per power nap, while whales resting after deep dives of more than 200 meters produced only around three to four bubble events during their ascent-related rest. In other words: the whales blow more bubbles when they are “buoyancy-prone” to rise too quickly.

The technical logic behind this pattern is tied to pressure and oxygen supply. At greater depths, the surrounding pressure compresses air in the whale’s lungs and reduces buoyancy, meaning the animals need fewer bubble releases to prevent upward drift. Closer to the surface, air expands gradually and pushes the whale upward, so blowing bubbles releases some onboard gas and reduces lift, helping the whale stay in the right depth window.

The study also raises a biological question: is bubble blowing a purely physiological response driven by sensing buoyancy, or does it reflect a conscious strategy during sleep? Because researchers don’t yet know whether sperm whales sleep with both hemispheres fully inactive or in a partially awake state, the behavioral control behind bubble release remains uncertain.

Still, the pattern is clear—and it reframes what “sleeping underwater” means for one of the ocean’s most iconic giants. Rather than floating passively, sperm whales appear to fine-tune their internal gas balance in real time, trading small, periodic bubble releases for sustained, stable rest below the surface.

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Subject of Research: Animals
Article Title: Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus Linnaeus)
News Publication Date: 23-Jul-2026
Web References: https://doi.org/10.1242/jeb.251920
References: Freymond, N., Burslem, A. and Miller, P. J. O. (2026). Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus Linnaeus). J. Exp. Biol. 229, jeb.251920. doi:10.1242/jeb.251920.
Image Credits: Stéphane Granzotto www.stephanegranzotto.com
Keywords: sperm whale, buoyancy regulation, sleeping behaviour, bubble release, onboard gas volume, underwater resting, marine mammals, Journal of Experimental Biology

A new study by researchers from the University of St Andrews and Université de Neuchâtel reveals the mechanism that makes this near-surface “standing sleep” possible. The team reports that resting sperm whales actively release gas bubbles to regulate buoyancy, keeping them close to neutral buoyancy under the water’s surface.

The key challenge is physical: sperm whales are naturally positively buoyant. Their large heads contain spermaceti oil, which lowers overall density and tends to push them upward. If they simply relaxed without control, they would slowly rise while resting—until their physiology and the surrounding environment forced them to change behavior.

The answer involves diving gas dynamics. Because sperm whales are breath-hold divers, the gas already in their lungs expands gradually as external pressure decreases with ascent. That expansion would normally increase buoyancy and encourage the whales to rise faster. By releasing bubbles during rest, the animals counteract this shift, effectively reducing their net buoyancy.

To test the idea, the researchers used suction-cup tags deployed on sperm whales off the Norwegian coast. These devices recorded both three-dimensional movement and sound. The tags captured “clear bubble sounds,” providing direct evidence that bubble release occurs while whales are resting rather than only during active dives.

Using the movement and acoustic data, the team built a simulation that incorporated tissue density, drag forces, and changes in internal gas volume. The model reproduced the observed requirement: bubble release reduces positive buoyancy enough for whales to remain submerged with near-neutral stability.

The findings also suggest a behavioral implication. Resting dives appear to begin with less diving gas volume than deeper foraging dives, indicating that whales may fine-tune their internal conditions before switching into sleep mode. The authors further speculate that bubble release may connect to off-gassing of excess CO2 or N2 from tissues into the lungs, potentially influencing metabolic gas exchange as well.

Subject of Research: Animals (marine mammals; sperm whales)
Article Title: Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus)
Web References: http://dx.doi.org/10.1242/jeb.251920
References: Journal of Experimental Biology, doi:10.1242/jeb.251920
Image Credits: University of St Andrews

Keywords: sperm whales; buoyancy regulation; marine ecology; breath-hold diving; gas bubbles; neutral buoyancy; tagging; underwater sleep; off-gassing; animal physiology