From small boats off Norway’s Lofoten Islands, researchers used poles to attach recording tags to the backs of sperm whales. Suction cups held the devices in place as the whales slipped below the surface.

As the animals stopped swimming and rotated into their upright resting positions, microphones on the tags recorded bursts of bubbles. Those releases appeared to keep the whales from slowly floating upward as they rested.

Sperm whales are the only whales known to rest vertically, with their heads pointed upward and their bodies hanging below. Researchers think this position shields them from surface waves without requiring them to expend energy to reach deeper water. Until now, however, they did not know how the whales maintained that position.

New research from the University of St Andrews and Université de Neuchâtel, published in the Journal of Experimental Biology, found that the whales release gas to control their buoyancy.

“It is particularly fascinating they are able to make these fine-scale adjustments while they are thought to be asleep, a challenge totally alien to terrestrial mammals like us humans,” said Patrick Miller, a professor with the Sea Mammal Research Unit at the University of St Andrews, in a press release.

Read More: Sperm Whale Clicks Aren’t Random — They Follow Human‑Like Sound Rules

How Sperm Whales Sleep Vertically Below the Ocean Surface

The tagged whales rested in three ways. Some moved directly into a head-up position near the surface, while others drifted upward from deep dives. The most common way involved diving head-down and then rotating until the head pointed upward.

Those head-down dives reached an average depth of 90 feet (27.5 meters) before the whales began resting. Individual resting dives lasted from 6 to 31 minutes, with an average of 19 minutes.

Low-density organs near the front of a sperm whale’s body, including its lungs and spermaceti organ, help rotate its head-up, while the gas it carries makes it more buoyant. As the whale rises, water pressure falls, and the gas expands, increasing the force pushing it toward the surface.

Releasing bubbles reduces the gas volume and brings the whale closer to neutral buoyancy, allowing it to remain submerged without swimming.

Resting Sperm Whales Carried Less Gas

The study included recordings from 42 male sperm whales tagged across eight summers between 2005 and 2019. Seventeen rested while the tags were attached, and researchers heard bubbles during 56 of 61 rest periods, or nearly 92 percent.

The whales released bubbles an average of about eight times per resting dive, while one head-down/head-up dive included 36 separate releases.

To test whether those bubbles kept the whales below the surface, the team built a computer model based on 10 whales that began their resting dives head-down. The model calculated how each whale would rise or sink based on its body density, the resistance of the surrounding water, and the amount of gas it carried.

The model closely matched almost 90 percent of the recorded dive sections. When the researchers kept the amount of gas constant, the simulated whales rose much faster than the real ones. The estimated amount of gas fell by an average of 5.2 percent from one section to the next, supporting the idea that releasing bubbles slowed their ascent.

The whales also began resting dives with about 8 milliliters of gas per kilogram of body mass, compared with 26.4 milliliters during dives to search for food. For a 30,000-kilogram whale, the resting amount would equal about 234 liters. Video footage also showed whales appearing to exhale before some resting dives and then descend head-down without taking another breath.

Sperm Whale Bubbles May Also Release Excess Gas

Researchers still do not know what prompts each bubble release or whether the whales make these adjustments deliberately. The bubbles may also clear excess carbon dioxide or nitrogen from their bodies, meaning buoyancy control may not be their only function.

Sleep is usually a moment of surrender, but for a sperm whale hanging upright beneath the waves, it still requires control.

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in the Journal of Experimental Biology:Buoyancy regulation through modulation of onboard gas volume by resting sperm whales (Physeter macrocephalus)