How Sperm Whales Use Gas Bubbles to Maintain Their Bizarre Vertical Sleep Posture

The researchers supplemented this field data with a computer simulation that incorporated tissue density, drag, and onboard gas volume to test whether bubble release could explain the observed buoyancy control .

Key findings: bubble releases are pervasive and deliberate

During 61 observed rest periods, bubble releases occurred in approximately 92% of them, with an average of about 8 releases per nap . The pattern varied by depth: whales resting closer to the surface released up to ~11 bubbles per nap, while those ascending from deep foraging dives (over 200 m) released only 3–4 .

Why bubbles? Sperm whales are naturally positively buoyant because of the large spermaceti oil reservoir in their heads. The oil is lighter (less dense) than other body tissues, causing the whales to passively rotate into a head-up posture when not actively swimming . As the whales rest, their lung gas expands at shallower depths, further increasing positive buoyancy. By releasing gas, they reduce this buoyant force, achieving near-neutral buoyancy that keeps them suspended at their desired depth .

Importantly, the whales begin rest dives with less gas volume in their lungs than they do for deep foraging dives, which itself pre-emptively reduces buoyancy . The bubble releases then provide fine-scale adjustments.

Open questions: conscious control and dual functions

The study raises two major unresolved questions:

What triggers each bubble release? The whales may consciously sense their depth and ascent and deliberately release bubbles — or the process might occur automatically during sleep. Patrick Miller suspects sperm whales engage in bi-hemispheric sleep (both brain hemispheres sleeping simultaneously), which would mean they manage buoyancy while fully unconscious — a remarkable feat .

Do the bubbles serve additional functions? The authors note that bubble release could also off-gas excess CO₂ or nitrogen from tissues into the lungs, meaning the behavior may play a role in metabolic gas exchange during rest, not just buoyancy control .

Bottom line

This study provides strong evidence that sperm whales actively regulate their buoyancy by releasing gas bubbles while sleeping vertically, using fine-scale adjustments that prevent them from floating uncontrollably to the surface. The combination of field tagging, acoustic monitoring, and computer modeling confirms that bubble release slows ascent and maintains neutral buoyancy, while leaving open the intriguing questions of whether the whales are conscious during the process and whether the bubbles serve dual respiratory and buoyancy functions.