Sperm Whales Blow Bubbles to Stay Upright While Sleeping, New Study Reveals

Key findings

• Bubble release was pervasive: During 61 rest periods, bubble releases occurred in approximately 92% of them, with an average of about 8 releases per nap. Whales resting closer to the surface released up to ~11 bubbles per nap, while those ascending from deep dives (over 200 m) released only 3–4 .
• Mechanism confirmed: The simulation showed that releasing gas reduces the whales' positive buoyancy, counteracting the expanding lung gas that would otherwise cause them to float upward. This enables near-neutral buoyancy while sleeping vertically just below the surface .
• Why vertical? Sperm whales are naturally positively buoyant because of the large spermaceti oil reservoir in their heads, which causes them to rotate into a head-up posture when not actively swimming .
• Starting gas volume differs: The whales begin rest dives with less gas volume in their lungs than they do for deep foraging dives, which itself reduces buoyancy at the start .

Open questions

• What triggers each release? It is unknown whether the whales consciously sense their depth and ascent and deliberately release bubbles, or if this occurs automatically during sleep. If they are sleeping with both brain hemispheres (bi-hemispheric sleep, which Miller suspects ), they would be managing buoyancy while fully unconscious — a remarkable ability .
• Additional functions? The authors note that bubble release could also serve to 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.