Scientists tagging 42 sperm whales off Norway discovered that the world’s largest toothed mammals sleep vertically near the ocean surface, carefully releasing gas bubbles to regulate their buoyancy and counteract lung expansion.
When it comes to marine life, some behaviors remain beyond everyday human imagination. Researchers from the University of St Andrews and the Université de Neuchâtel have provided a rare look into the secretive resting habits of sperm whales, revealing a buoyancy-control mechanism used while the massive marine mammals sleep vertically just beneath the ocean surface. The findings are published in the Journal of Experimental Biology.
University of St Andrews and Université de Neuchâtel
How Vertical Naps Work in the Water Column
Unlike most marine mammals, sperm whales sleep in a strange upright position, bobbing almost motionless with their heads pointing toward the surface and their bodies trailing downward like a row of toy soldiers. These resting bouts usually last between 10 and 15 minutes. By hanging several meters deep, the whales stay close enough to the surface to breathe easily upon waking, while positioning themselves far enough down to avoid getting knocked around by surface waves or drifting into deeper waters.
Because sperm whales are air-breathing mammals that must think about taking every single breath, deep sleep as humans experience it is impossible because they would drown. Instead, scientists believe they likely switch off one half of their brain at a time—known as unihemispheric sleep—which also allows them to keep watch for potential threats.
Spermaceti Oil and Lung Gas Expansion
The Buoyancy Problem and Off-Gassing
Staying suspended in an upright posture presents a distinct physical challenge. Sperm whales possess massive heads filled with a waxy substance called spermaceti oil, and they are naturally positively buoyant. As breath-hold divers, the gases inside their lungs compress during deep dives but expand as they ascend toward the surface to rest.
If a resting whale slowly drifts upward, the expanding lung gas would cause it to float uncontrollably to the surface. To combat this, the animals engage in carefully releasing bubbles of gas while they snooze, effectively acting as off-gassing to stay submerged just below the waves.
As sperm whales are breath-hold divers, if they slowly drift up while resting, the gas in their lung will slowly expand,
write the authors in the study. Releasing bubbles would enable them to counteract the expanding diving gas and achieve neutral buoyancy.
Suction-Cup Tags off the Coast of Norway
Tagging the Slumbering Giants
To uncover this behavior, researchers deployed small suction-cup tags on sperm whales off the coast of Norway. These benign tags recorded detailed movement data alongside the acoustic sound of bubbles forming during rest periods. The team tagged 42 sperm whales with special recorders that could record depth and capture the sound of bubbles while the whales were snoozing and ran a simulation to find out whether bubble releases can help sperm whales regulate their net buoyancy,
they write.
The data revealed a clear pattern based on where the resting dives began. Sperm whales catching 40 winks closer to the surface blow more bubbles than those sleeping on the way back from deep dives of 200m or more: around 11 bubble releases compared to three or four, respectively. In deeper waters, the air in the whales’ lungs is compressed more severely, and so less buoyant. In cases where resting dives started at depths exceeding 100m, the number of bubble releases decreased,
they say in the paper, This suggests that these individuals did not need to modulate buoyancy via bubble release to rest long enough because of the greater ascent distances.
Professor Patrick Miller
Professor Patrick Miller from the Sea Mammal Research Unit at the University of St Andrews highlighted the precision involved in these underwater naps, stating: Sperm whales have an incredible ability to control their buoyancy by releasing gases when they are resting.
Beyond buoyancy control, scientists note that there may also be other reasons sperm whales blow bubbles during their naps. While our results indicate that presence and timing of bubble releases suggest they are used to maintain neutral buoyancy, it does not necessarily follow that this represents the only function of bubble releases during resting,
they say in the paper, noting it could be related to the process of taking in oxygen and releasing excess carbon dioxide. While researchers have yet to capture brain activity via an electroencephalogram (EEG) to formally confirm slumbering sperm whales are asleep—leading them to describe the animals as ‘resting’ rather than sleeping—the findings demonstrate that these animals maintain remarkably fine positioning control even while resting.
