Anyone who has tried to keep a cork underwater knows it takes effort. Yet sperm whales can rest suspended beneath the ocean surface despite having massive heads filled with oil, wax and air that would seem to send them drifting upward — a feat scientists now think is made possible by blowing bubbles.
Sperm whales are the only whales known to rest vertically in the water, with their heads pointing upward. Scientists believe this position helps protect them from the movement of surface waves without requiring the energy needed to dive deeper, but exactly how they remain submerged had remained unclear.
Now researchers have found that the whales use bubble releases to regulate their buoyancy — the tendency to float or sink in water — while resting in a distinctive vertical posture just below the ocean surface.
Researchers refer to the behaviour as "resting" rather than "sleeping" because sleep is typically confirmed by measuring brain activity, something that has not yet been possible in sperm whales, according to Patrick Miller, a marine mammal researcher at the University of St Andrews in Scotland and senior author of the study published in the Journal of Experimental Biology.
Scientists believe the animals are asleep, but prefer the more cautious term because it has not been conclusively demonstrated.
To investigate how they remain submerged while resting, the researchers attached small suction-cup tags to sperm whales off the coast of Norway. The devices recorded sound and three-dimensional movement data, allowing scientists to detect bubble releases and model their effects on the position in the water of these marine mammals.
The researchers found that the bubbles reduce a whale's positive buoyancy — the tendency to float upward in water.
Sperm whales naturally tend to float because of the large amounts of spermaceti oil, a waxy substance in their heads. As resting whales slowly drift upward, gases in their lungs expand because of the lower water pressure nearer the surface. Releasing bubbles helps counter that effect, letting the whales remain almost weightless in the water and below the surface.
Miller said that the bubble releases appear to be deliberate. After more than two decades of tracking sperm whales with sound-recording tags, "we only hear bubbles being released in this specific resting behavioural context, so incidental release of gases is something sperm whales just don't seem to do."
Scientists also suspect the behaviour could help the whales remove excess gases such as carbon dioxide and nitrogen from tissues into the lungs, potentially playing a role in their metabolic gas exchange.
Sperm whales are the world's largest toothed whales. They can dive to extreme depths to catch prey like giant squid, navigating in dark waters using echolocation by making loud clicking sounds and listening for the returning echoes to map their surroundings.
"Sperm whales spend around 80% of their time performing very deep dives searching for prey. Because of the intense pressure during those dives, their bodies will tend to accumulate undesirable gases like nitrogen and carbon dioxide that can be slow to transfer from the blood to the lungs. Because they rest at a very shallow depth, this is the ideal condition for off-gassing those undesirable by-products of their deep-dive foraging," Miller said.
The idea could be tested in future research, Miller said.
The new study leaves some questions unanswered. For instance, it remains to be demonstrated whether the whales consciously release the bubbles and what cues tell them when an adjustment is needed.
The researchers also hope to determine whether sperm whales sleep with one half of the brain at a time, as dolphins do, or with both cerebral hemispheres asleep. Future studies could use brain-wave sensors or observations of the eyes of the whales while resting to help answer that question.
For now, the findings indicate that, like humans, sperm whales may be willing to do whatever it takes to get a good rest.
