Sperm whales have long posed a puzzle for marine scientists: the only whale species known to rest vertically just beneath the ocean surface. This posture appears to help them sleep while limiting the jolting effects of surface waves, without paying the energetic cost of descending to deeper waters. But how do they stay in place, close enough to breathe yet stable enough to avoid drifting upward?
A new study by researchers from the University of St Andrews and Université de Neuchâtel reveals the mechanism that makes this near-surface “standing sleep” possible. The team reports that resting sperm whales actively release gas bubbles to regulate buoyancy, keeping them close to neutral buoyancy under the water’s surface.
The key challenge is physical: sperm whales are naturally positively buoyant. Their large heads contain spermaceti oil, which lowers overall density and tends to push them upward. If they simply relaxed without control, they would slowly rise while resting—until their physiology and the surrounding environment forced them to change behavior.
The answer involves diving gas dynamics. Because sperm whales are breath-hold divers, the gas already in their lungs expands gradually as external pressure decreases with ascent. That expansion would normally increase buoyancy and encourage the whales to rise faster. By releasing bubbles during rest, the animals counteract this shift, effectively reducing their net buoyancy.
To test the idea, the researchers used suction-cup tags deployed on sperm whales off the Norwegian coast. These devices recorded both three-dimensional movement and sound. The tags captured “clear bubble sounds,” providing direct evidence that bubble release occurs while whales are resting rather than only during active dives.
Using the movement and acoustic data, the team built a simulation that incorporated tissue density, drag forces, and changes in internal gas volume. The model reproduced the observed requirement: bubble release reduces positive buoyancy enough for whales to remain submerged with near-neutral stability.
The findings also suggest a behavioral implication. Resting dives appear to begin with less diving gas volume than deeper foraging dives, indicating that whales may fine-tune their internal conditions before switching into sleep mode. The authors further speculate that bubble release may connect to off-gassing of excess CO2 or N2 from tissues into the lungs, potentially influencing metabolic gas exchange as well.
Subject of Research: Animals (marine mammals; sperm whales)
Article Title: Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus)
Web References: http://dx.doi.org/10.1242/jeb.251920
References: Journal of Experimental Biology, doi:10.1242/jeb.251920
Image Credits: University of St Andrews
Keywords: sperm whales; buoyancy regulation; marine ecology; breath-hold diving; gas bubbles; neutral buoyancy; tagging; underwater sleep; off-gassing; animal physiology
