Schematic image showing sperm whales using bubbles while resting. Image purely for illustrative purposes.
In one of the ocean’s strangest sleep routines, sperm whales can hang almost motionless in the water like giant living pillars. Now, a new study suggests that the giants maintain this improbable pose by using a surprising trick: bubbles.
Data from whales tagged off northern Norway indicate that with every exhalation, whales eliminate some of the gas inside their bodies. Thus, they reduce their buoyancy and prevent an unwanted rise to the surface. The finding reveals a stunning new dimension to what was already one of the animal kingdom’s most mysterious forms of sleep.
A Most Mysterious Sleep
Resting sperm whales look surreal. They hang almost motionless, usually head-up, beneath the reach of breaking waves. After roughly 10 to 20 minutes, they rise to breathe.
How or why they do this exactly is still unclear. Scientists formally described this behavior only in 2008 after fitting whales around the world with suction-cup tags. The instruments recorded depth and movement as the animals entered passive “drift-dives.” These resting bouts seem to only occupy 7.1% of the total recording time.
In addition to the biology questions, the behavior posed a stubborn physics problem.
Air makes an animal float. So do many lipid-rich tissues, including those packed into the sperm whale’s enormous head. Sperm whales experience particularly strong upward buoyancy, especially in the top 100 metres of the ocean. So, how do resting whales remain submerged and not float towards the surface?
Noémie Freymond of the Université de Neuchâtel and Alec Burslem and Patrick Miller of the University of St Andrews decided to investigate. They examined recordings from suction-cup tags deployed on 42 sperm whales around Norway’s Lofoten Islands. The tags measured depth and three-dimensional body movement while recording nearby sounds. Seventeen of the whales rested during the deployments, providing the researchers with synchronized records of posture, motion and the distinctive sounds of underwater exhalations.
The records revealed three broad resting patterns:
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- some whales descended head-first before their buoyant bodies rotated upward;
- others sank backward, tail-first, by about eight meters;
- a third group rested while drifting upward after dives deeper than 200 meters.
But across all those different maneuvers, bubbles repeatedly appeared at moments when the whales needed to curb their ascent.
The Physics of a Sleeping Giant
The key element is pressure.
As you go deeper underwater, water pressure starts to squeeze gas. Compressed gas occupies less space and provides less lift. As the animal rises, the pressure eases, the gas expands and its upward pull grows stronger.
That effect becomes especially important near the surface, where relatively small changes in depth produce large proportional changes in pressure. A whale drifting upward can therefore face a feedback loop: rising expands its gas, and the expanding gas makes it rise faster.
To avoid getting caught in that loop, whales perform one simple trick: they exhale.
A sped-up recording of Sperm Whale off-gassing bubbles to maintain vertical positioning. Credits: University of St. Andrews.
Whales resting close to the surface released bubbles about 11 times during a typical rest. Whales relaxing during an ascent from more than 200 metres released bubbles only three or four times. At greater depth, pressure had already compressed the gas inside their bodies, reducing its contribution to buoyancy. This seemed to support the idea.
To add further weight, the researchers then built a computer model of the forces acting on the whales. It incorporated tissue density, water resistance and the changing volume of gas as pressure rose or fell. The model showed that bubble releases could reduce positive buoyancy enough to explain how the whales maintained their positions. It also indicated that whales began resting dives with less gas than they carried into deep foraging dives, placing them closer to neutral buoyancy from the start.
“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,” explains Professor Patrick Miller from the Sea Mammal Research Unit at the University of St Andrews.
Sleep or Something Like It
Sperm whales resting. University of St. Andrews.
The next question is how do whales control this so well when they’re sleeping?
For starters, the word sleep here requires a bit of attention. It’s tempting to say whales are sleeping, but we don’t actually know what they’re doing. The animals become motionless and unusually unresponsive, behavior consistent with sleep, but movement tags cannot reveal whether both halves of the brain shut down together. No one has recorded the brain activity of free-ranging sperm whales during these rests. So, their version of “sleep” might be different than ours.
Studies of dolphins and other toothed whales in captivity have found what researchers call unihemispheric sleep: one side of the brain rests while the other remains more alert. That arrangement may help an animal continue swimming, monitor its surroundings and return to the surface to breathe. Researchers still do not know whether vertically resting sperm whales use the same system, but it is a plausible hypothesis.
The bubbles add another layer to that mystery.
To adjust its position, a whale must somehow detect that it has become too buoyant or has risen too far. It must then release an appropriate amount of gas. The action could reflect conscious monitoring by a partly awake brain, or an automatic response controlled without full awareness.
For now, scientists have uncovered one part of the physics questions; but the biological mystery is still unanswered.
The study was published in the Journal of Experimental Biology.