The sperm whale sleeps vertically, motionless, some twenty metres below the surface. That position raises a difficulty that is easily underestimated: its head makes up close to a third of its body and consists of oil, wax and air, three materials that float. Such an animal ought to rise like a cork. A Swiss and British team shows that it counters this by releasing bubbles, and quantifies the mechanism for the first time.
Vertical sleep, described in 2008
The behaviour was characterised in 2008 in Current Biology by Patrick Miller and his colleagues. The sperm whale rests by drifting below the surface in a stereotyped vertical position, without swimming, in naps of ten to fifteen minutes, generally at around twenty-five metres depth.
Two advantages are attributed to this posture. Staying below the surface keeps the animal out of the agitation of the waves. And remaining shallow spares it the transit cost of a deep dive, an expensive business for an animal that is not seeking to feed. Our page on sleep in cetaceans sets this peculiarity alongside the other strategies found in the group.
One observation remained unexplained: during these phases, sperm whales release bubbles. That is the subject of the new study, by Noémie Freymond, of the University of Neuchâtel, with Alec Burslem and Patrick Miller, of the University of St Andrews.
The physical problem: Boyle’s law
A breath-hold diver carries a volume of gas in its lungs and airways. That gas obeys an elementary law: at constant temperature, the product of pressure and volume stays constant. Compress it and it takes up less room; release the pressure and it expands.
Underwater, pressure rises by about one atmosphere every ten metres. An instability follows from this: the higher the animal rises, the more its gas expands, the more water it displaces, and the more it floats. The process feeds on itself, since floating more means rising more.
One technical point governs everything that follows: the expansion does not depend on the distance covered but on the ratio of the pressures. Ten metres of ascent therefore have quite different effects depending on the starting depth.
| Situation | Ascent | Gas expansion |
|---|---|---|
| Resting during the ascent from a deep dive | from 200 m to 190 m | × 1.05 |
| Nap at the usual depth | from 25 m to 15 m | × 1.40 |
| Drift towards the surface | from 15 m to 5 m | × 1.67 |
| Tail-first sinking, return to the surface | from 8 m to 0 m | × 1.80 |
By expelling gas, the animal reduces the volume it displaces and offsets the expansion so as to stay neutral. Ballast regulation, in effect.
Protocol
Watching a whale blow bubbles twenty-five metres down is in practice impossible. The researchers therefore worked from on-board recordings. Tags attached by suction cups to 42 sperm whales off the Lofoten Islands, in northern Norway, picked up three pieces of information at once: the sound of the bubbles expelled through the blowhole, the animal’s depth, and the orientation of its head, upwards or downwards.
The tags then detach and are recovered at the surface for analysis. Of the 42 individuals, the authors kept the ten that rested mainly during head-first dives, the configuration in which the mechanism is easiest to read.
From these recordings they built a simulation incorporating tissue density, the drag coefficient and the volume of gas carried, then set it against the real depth profiles. The reasoning runs as follows: if the predicted trajectory matches the observed trajectory only once bubble release is taken into account, then that release does indeed fulfil a regulatory function.
Results
All ten individuals released bubbles during their resting dives, without exception, and the simulation confirms that this release allows them to stay submerged instead of drifting towards the surface.
The most telling demonstration lies in the comparison between resting modes, because it follows the prediction of the table above exactly. A sperm whale dozing near the surface blows bubbles about eleven times per nap. A sperm whale resting as it comes up from a dive of more than two hundred metres blows only three or four. At depth the gas is already compressed and the drift slow; near the surface, the expansion runs away and forces repeated purges.
A second result concerns the starting point, and it points to a finer form of regulation still. Sperm whales begin their resting dives with a smaller volume of gas than they take down for a deep feeding dive. The adjustment is therefore not made only en route, through the bubbles, but also beforehand, by metering the initial inhalation.
Two questions the study leaves open
Do the bubbles serve some other purpose as well? The authors write explicitly that they cannot rule out this release being linked to the removal of excess carbon dioxide or nitrogen from the tissues into the lungs. What is at stake goes beyond buoyancy: a diver making repeated breath-holds accumulates dissolved nitrogen, with the risk of decompression sickness that this implies. The two functions are not mutually exclusive, and the question of nitrogen management in this species remains open, as our page on how the sperm whale dives without decompression sickness sets out.
Is the animal really asleep during this time? This is the most unsettling question, and it is raised by the journal itself. Blowing a bubble at the right moment so as not to drift implies perceiving one’s own depth, and therefore keeping watch on one’s surroundings. Yet it is not known whether the sperm whale sleeps as we do, or in the manner of dolphins, one cerebral hemisphere at a time, the other staying awake. The mechanism described here could be the work of a half-awake brain, or of an automatism. Nothing yet settles the question.
Two limits of scope deserve a final mention. The analysis rests on ten animals from a single population, the one that frequents the Lofoten Islands. And the core of the demonstration is a simulation: it establishes that bubble release is enough to account for the observed trajectories, which amounts to strong consistency, but it does not directly measure the volume of gas expelled.
What this work brings to light
Sleep in a marine animal is not a switching-off. It requires holding a position in a fluid whose pressure varies with every metre, without spending the muscular energy that the animal is precisely trying to save. The sperm whale’s solution is remarkably frugal: rather than swimming to hold station, it adjusts its own density, as a submarine would trim its ballast.
The result adds to what is known of diving physiology in this species, a champion of depth records. It also illustrates what on-board tags make possible: reconstructing, from a sound and a depth curve, a behaviour that no one has ever been able to observe directly.
Reference: Freymond N., Burslem A. and Miller P. J. O., “Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus)”, Journal of Experimental Biology, 229(14), jeb251920, 15 July 2026. The data are deposited in open access on Open Science Framework. Summary: Kay J., “Blowing bubbles helps sperm whales sleep soundly beneath the waves”, Inside JEB, 229(14), jeb252978. Resting behaviour originally described by Miller P. J. O., Aoki K., Rendell L. and Amano M., Current Biology, 18(1), R21-R23, 2008. The fieldwork was funded by several naval research programmes, American, Dutch, British and French, under which these tagging campaigns were carried out.
Did you find this article useful?
Add L'info des Cétacés to your preferred sources on Google. Our stories will be suggested to you more often, and the choice can be undone as easily as it is done.
Follow us on GoogleThis preference is saved in your Google account and can be changed at any time. It is neither a certification nor a recommendation from Google.
Leave a review