Sperm whales regulate their buoyancy during vertical sleep by intermittently releasing gas bubbles through their blowholes, according to a study published in the Journal of Experimental Biology. Researchers using suction-cup tags on 42 animals in Norway found that near-surface resting dives prompted significantly more bubble releases than deep-water ascents.
Marine biologists have long puzzled over how the world’s largest toothed whales manage to rest without drifting away. Unlike most marine mammals that sleep horizontally or at the surface, sperm whales hang vertically in the water column like columns of toy soldiers, remaining completely motionless for short power naps that typically last 10 to 15 minutes. This vertical posture protects the animals from turbulent surface wave action while conserving the energy required to reach deeper waters.
Yet this resting strategy presents a stubborn physics problem. A sperm whale’s head takes up roughly a third of its total body mass and contains large amounts of spermaceti oil, wax, and trapped air—all of which are naturally buoyant. Because sperm whales are breath-hold divers that store air before descending, their bodies act like corks, tending to pull them upward. When a resting whale slowly drifts toward the surface, the surrounding water pressure drops, causing the gas in its lungs to expand and increasing its upward lift.
Tracking Sleeping Giants Off the Norwegian Coast
To solve the mystery of how sperm whales stay submerged during these vertical rests, an international team of researchers from the University of St Andrews and the University of Neuchâtel deployed advanced recording technology. Researchers collected data by placing small tags using suction cups on Sperm Whales off the Norwegian coast, specifically around the Lofoten Islands in northern Norway. These suction-cup devices recorded the animals’ three-dimensional movements, orientation, depth, and nearby acoustic data.
Depth, Pressure, and the Mechanics of Bubble Release
The study revealed that the frequency of bubble release is directly tied to the depth at which the whales choose to rest. Whales sleeping closer to the surface released bubbles about 11 times per power nap, whereas animals resting after ascending from deep dives of more than 200 meters produced only three or four bubble events.
“Deep diving means the whales need a lot of oxygen, but the pressure of the water compresses the air in their lungs causing the whales to be less buoyant. This means they need to blow fewer bubbles to stop themselves from floating upward. However, when the whales rest closer to the surface, the air in their lungs will slowly cause them to rise to the surface. Blowing bubbles releases some of the air, making them less buoyant in the process.”
Prof. Patrick Miller, University of St. Andrews, via Advocateanddemocrat
To test whether these exhalations accounted for their stability, the scientific team constructed a computer simulation based on 10 whales that began their resting dives head-down. The model incorporated body density, water resistance, and onboard gas volume.
Unresolved Biological Questions About Cetacean Sleep
While the mechanical role of the bubbles is clear, researchers emphasize that questions remain regarding the whales’ state of consciousness during these resting bouts. Because marine mammals cannot breathe automatically like land animals, scientists debate whether sperm whales experience unihemispheric sleep—shutting down one half of the brain at a time like dolphins—or enter a fully unconscious state.
“No one knows whether sperm whales sleep as we do, or if they sleep like dolphins and keep one half of their brain awake while the other snoozes. Blowing bubbles to make sure they don’t come too near the surface requires the whales to sense their environment and realize that they are coming too close to the surface.”
Noémie Freymond, University of Neuchâtel, via Advocateanddemocrat
Furthermore, investigators note that buoyancy regulation may not be the sole function of the exhalations. The study suggested that releasing bubbles could also relate to the offgassing of excess carbon dioxide or nitrogen accumulated in tissues during prolonged hunts, pointing toward broader implications for metabolic gas exchange.
