London, Feb 22 : Researchers studying sperm whales have determined that the marine species could be the least sleep-dependent mammals known in the animal kingdom.
According to a report in Nature News, the researchers include a team led by Luke Rendell at the University of St Andrew's, UK, that were monitoring calls and behaviour in sperm whales off the northern Chile coast, when they observed an unusual sleeping pattern in the animals.
The team observed a pod of whales hanging vertically in the water, their noses poking out of the surface. At least two of the whales were facing the boat, but not a single animal responded.
When the researchers tried to move their boat away from the whales, they nudged one of the animals on the way out.
"We had no idea how they would react; each of the animals probably weighed up to twice as much as our boat, and could have sunk us," said Rendell.
But, after an initial jolt of activity the whales timidly moved away, and within fifteen minutes, were bobbing peacefully at the surface again.
When Patrick Miller, also at the University of St Andrew's, saw the video footage from the encounter, the odd observation began to make sense.
Miller and his colleagues had earlier attached data-logging suction cups to 59 sperm whales to monitor the animals' depth and behaviours as they travelled around the globe.
They had found that the whales spent about 7% of their time drifting inactive in shallow water. What they were doing or why they were doing it had been a mystery, but seeing footage from Rendell's experience determined that the whales were sleeping.
Whales and dolphins have only ever been seen allowing one brain hemisphere to rest at a time, keeping one eye open. This is presumably because they need to do important things that require physical activity, such as coming to the surface to breathe or avoid predators. They never fully let their guard down.
But now Miller, Rendell and their colleagues report that sperm whales seem to sleep fully while drifting, either at the surface, or at 10 metres depth. Their naps seem to last for ten to fifteen minutes, during which time they do not breathe or move.
According to the report, if the whales are fully asleep while in the drifting mode, then they sleep very little: just 7% of the time. That contrasts sharply with smaller beluga and grey whales, which sleep for 32% and 41% of the time, respectively.
Such a meagre amount of sleep designates the sperm whales as the least sleep-dependent mammals known.