FIFE RESEARCHERS studying the resting behaviour of wild sperm whales have found evidence that they literally drift off to sleep and can “cat nap” during rhythmic dives.

The new study, by Dr Patrick Miller of St Andrews University, reveals a novel drifting behaviour, and sperm whales were captured on video as they eerily floated vertically and motionless at or just below the ocean surface.

The research and footage demonstrates that sperm whales in the wild switch off completely for short periods of time, and it is now thought sperm whales may sleep less than any other mammal in the wild.

Dr Miller, of the school of biology’s sea mammal research unit, said the drift diving behaviour had been recorded by tagging devices attached with suction cups to 59 whales around the world.

The whales were recorded consistently performing this behaviour in each location, indicating that it is stereotypical for the entire species.

“Many mammals show species-typical sleeping behaviour, such as dogs circling before lying down, lending support to the idea that sperm whales sleep during these drift dives,” said Dr Miller. “One exciting aspect of this finding is that it suggests the largest toothed whale actually might sleep in a fashion that we recognise as similar to sleep in terrestrial mammals.

“The sperm whale behaviour we describe seems to allow normal-looking quiescent sleep, possibly including REM sleep which has never been clearly observed in any cetacean,” he said.

The related video was co-incidentally captured by a team led by fellow St Andrews researcher Dr Luke Rendell, off the coast of northern Chile.

The video footage shows half a dozen sperm whales, with three of them ‘unusually non-responsive’ to the approaching boat, until it accidentally touched one of them.

The researchers believe the video also seems to show the whales were asleep with both eyes shut.

Previous studies of captive dolphins has suggested cetaceans sleep with one side of the brain at a time.

“Our study presents the first evidence that whales in the wild may sleep with both sides of the brain (bihemispheric),” Dr Miller continued.

“This is supported by the fact that the whales we approached did not react to the boat even though it was visible to both eyes.

“From what we have found, the amount of time sperm whale seem to have for sleep is very little, so it would make more sense for them to sleep both sides at a time.

“Very little is known about sleeping behaviour of whales in the wild as previous research has involved captive cetaceans.

“One of the theories for why cetaceans in captivity sleep with one side of their brain at a time (uni-hemispheric) is so they can keep looking out for their pod-mates or for danger with one open eye.

“Instead, the whales that we approached seemed to be non-responsive with either eye—which is unusual because it is very rare to ever get that close to sperm whales at sea,” he said.

The data also revealed such whales appeared to sleep for just 7.1% of the time which would be less sleep than any mammal and much less than is reported for captive cetaceans.