Aug 7 (Reuters) – 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 \u200cupward \u2014 a feat scientists now think is made possible by blowing bubbles.<\/p>\n
Sperm whales are the only whales known to rest vertically in the water, \u200cwith 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 \u200bunclear.<\/p>\n
Now researchers have found that the whales use bubble releases to regulate their buoyancy \u2014 the tendency to float or sink in water \u2014 while resting in a distinctive vertical posture just below the ocean surface.<\/p>\n
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 \u200cof Experimental Biology.<\/p>\n
Scientists believe the animals are asleep, but \u2060prefer the more cautious term because it has not been conclusively demonstrated.<\/p>\n
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 \u2060to detect bubble releases and model their effects on the position in the water of these marine mammals.<\/p>\n
The researchers found that the bubbles reduce a whale's positive buoyancy \u2014 the tendency to float upward in water.<\/p>\n
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, \u200bgases \u200bin their lungs expand because of the lower water pressure nearer the surface. Releasing bubbles \u200bhelps counter that effect, letting the whales remain almost weightless in \u200cthe water and below the surface.<\/p>\n
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."<\/p>\n
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.<\/p>\n
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 \u200cclicking sounds and listening for the returning echoes to map their surroundings.<\/p>\n
"Sperm whales spend around \u200b80% of their time performing very deep dives searching for prey. Because of the intense pressure \u200bduring those dives, their bodies will tend to accumulate undesirable gases like \u200bnitrogen and carbon dioxide that can be slow to transfer from the blood to the lungs. Because they rest at a \u200cvery shallow depth, this is the ideal condition for off-gassing those \u200bundesirable by-products of their deep-dive foraging," Miller \u200bsaid.<\/p>\n
The idea could be tested in future research, Miller said.<\/p>\n
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.<\/p>\n
The researchers also hope to determine whether sperm whales sleep with \u200bone half of the brain at a time, as \u200cdolphins do, or with both cerebral hemispheres asleep. Future studies could use brain-wave sensors or observations of the eyes of the whales while resting \u200bto help answer that question.<\/p>\n