Sperm whales vent gas bubbles to sleep upright just below the surface

Sperm whales are the only whales known to rest in a vertical position near the ocean surface, a posture that shields them from wave action without the energy cost of a deep dive. A team from the University of St Andrews and Université de Neuchâtel has now identified how they hold that position: they release gas bubbles to fine-tune their buoyancy. The finding appears in the Journal of Experimental Biology.

The whales are naturally buoyant because of the large volume of spermaceti oil in their heads, and as breath-hold divers they face a second problem — gas in their lungs expands as they drift upward, pushing them toward the surface. Venting bubbles counteracts both effects, letting the animals settle at near-neutral buoyancy. Researchers gathered the evidence using suction-cup tags on whales off Norway that recorded audio and 3-D movement; clear bubble sounds plus a simulation built from tissue density, drag, and body-gas volume confirmed the mechanism. The data also showed resting whales begin these dives with less lung gas than they carry on deep foraging dives.

What makes the behavior notable is that these are precise, continuous adjustments made while the animal is apparently asleep — control that has no parallel in land mammals. The authors also speculate the bubbles may be tied to off-gassing of excess CO2 or nitrogen from tissue into the lungs, hinting at a possible role in metabolic gas exchange.