Sperm whales regulate their buoyancy to rest vertically just beneath the ocean surface by deliberately releasing bubbles from their blowholes, according to a recent study published in the Journal of Experimental Biology. The findings explain how the world’s largest toothed whales manage massive, oil-filled heads without floating away during naps.
Anyone who has tried to keep a cork underwater knows it takes effort. Yet sperm whales manage to rest suspended beneath the ocean surface despite having massive heads filled with oil, wax, and air that would otherwise send them drifting upward. Scientists now understand that this capability relies on a deliberate bubble-release mechanism.
Sperm whales are the only whales known to rest vertically in the water with their heads pointing upward. Researchers believe this posture protects them from surface wave movements without expending the energy required to dive deeper. However, the precise physical mechanism that keeps them submerged had remained unclear until recently.
Tracking Marine Mammals Off the Coast of Norway
To investigate how these marine mammals remain submerged while resting, researchers from the University of St. Andrews in Scotland and the University of Neuchâtel in Switzerland attached small suction-cup tags to 42 sperm whales in the Loften Islands off the coast of Norway. The devices recorded sound and three-dimensional movement data, allowing scientists to measure depth, orientation, and bubble release.
The findings, published in the Journal of Experimental Biology, revealed that the whales doze underwater for 10 to 15 minutes at a time. During these resting periods, the animals utilize bubble releases to counteract their natural positive buoyancy.
Varying Buoyancy Across Different Resting Depths
Sperm whales rest in three distinct ways. They can put their heads down and dive slightly before their buoyant heads turn upward, they can sink gently tail-first about eight meters down, or they can rest as they ascend from deep dives exceeding 200 meters. These different behaviors require varying amounts of oxygen and different management of buoyancy.
Deep-diving whales accumulate significant oxygen needs, and the intense pressure of deep water compresses the air in their lungs, making them less buoyant. According to the research, whales drifting up from deep foraging dives blew bubbles only three or four times during their restful ascent. By contrast, whales dozing closer to the surface blew considerably more bubbles, releasing them around 11 times per power nap.
“However, when the whales rest closer to the surface, the air in their lungs will slowly cause them to rise to the surface. Blowing bubbles releases some of the air, making them less buoyant in the process.”
Professor Patrick Miller, University of St. Andrews
Metabolic Off-Gassing and Deliberate Bubble Release
Sperm whales spend roughly 80 percent of their time performing deep dives to hunt prey such as giant squid, navigating dark waters using echolocation by producing loud clicking sounds and listening for returning echoes. That intense pressure causes their bodies to accumulate undesirable gases like nitrogen and carbon dioxide.
Because the whales rest at very shallow depths, the behavior creates an ideal condition for off-gassing those metabolic by-products from their tissues into the lungs. Furthermore, researchers noted that the bubble releases appear entirely intentional. After tracking the animals with sound-recording tags for more than two decades, researchers from the University of St. Andrews, Scotland, and the University of Neuchâtel, Switzerland, confirmed a distinct behavioral pattern.
“we only hear bubbles being released in this specific resting behavioral context, so incidental release of gases is something sperm whales just don’t seem to do.”
Professor Patrick Miller, University of St. Andrews
Unresolved Questions About Marine Mammal Sleep
Despite these findings, several questions remain regarding the cognitive state of resting sperm whales. Scientists refer to the behavior as resting rather than sleeping because definitive sleep confirmation requires measuring brain activity, a feat not yet accomplished in sperm whales.
Researchers do not yet know whether sperm whales sleep with one cerebral hemisphere at a time, similarly to dolphins, or if they experience full bi-hemispheric sleep. Future studies may incorporate brain-wave sensors or detailed eye observations to determine their exact neurological state.
“Blowing bubbles to make sure they don’t come too near the surface requires the whales to sense their environment and realize that they are coming too close to the surface. Whether they turn out to be partially awake or fully asleep, sperm whales have definitely come up with a creative solution to ensure a restful slumber beneath the waves.”
Noémie Freymond, University of Neuchâtel
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