Published online 21 February 2008 | Nature | doi:10.1038/news.2008.613

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Sperm whales found fast asleep at sea.

Luke Rendell, University of St Andrews. Drifting off: these sperm whales didn't seem to notice the boat, till it bumped them. Watch the video.

An accidental encounter with a pod of sleeping sperm whales has opened researchers’ eyes to some unknown sleep behaviours of these giant sea creatures. Counter to previous assumptions, and unlike smaller cetaceans, the whales seem to enter a period of full sleep. But they also sleep for a very limited time per day, hinting that they could be the least sleep-dependent mammals known.

A team led by Luke Rendell at the University of St Andrew’s, UK, were monitoring calls and behaviour in sperm whales (Physeter macrocephalus) off the northern Chile coast when they accidentally drifted into the middle of 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.

“It was actually pretty scary. The boat had drifted into the group with its engine off [while] I was below decks making acoustic recordings,” says Rendell. “Once I saw the situation I decided the best thing to do was to try and sail our way out of the group rather than turn the engine on and have them all react.”

The researchers was almost successful, but unfortunately they nudged one of the whales 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. If they had decided to take action collectively — sperm whales do engage in communal defence [against] killer whales — then we could have been in real trouble,” Rendell says. Fortunately for everyone on board, after an initial jolt of activity the whales timidly moved away, and within fifteen minutes were bobbing peacefully at the surface again.

Drifters

When the video footage from the encounter was seen by Patrick Miller, also at the University of St Andrew’s, 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 clarified things: 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 these observations have been strictly limited to captive environments, where brain waves can be monitored easily, and have not been conducted in the larger whales.

But now Miller, Rendell and their colleagues report in Current Biology that sperm whales seem to sleep fully while drifting, either at the surface, or at 10 metres depth1. Their naps seem to last for ten to fifteen minutes, during which time they do not breathe or move. Rendell and his crew couldn’t tell whether the sperm whales in this encounter had one eye open or not, as the whales' eyes were underwater. But the whales' total unresponsiveness leads the researchers to suspect that both eyes were closed.

Power naps

If the whales are fully asleep while in the drifting mode documented by Miller 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. (The current record-holder is the giraffe, which sleeps for 8% of the time.)

But it is also possible that sperm whales engage in two types of sleep: full sleep while drifting at the surface, and half sleep, which has not yet been documented. “This finding raises the possibility that in the wild, cetaceans have flexibility in the type and depth of sleep that they enter, which is intriguing because this is behaviour that we have seen in birds too,” says Niels Rattenborg, a specialist in avian sleep at the Max Planck Institute for Ornithology in Seewiessen, Germany.

The limitation of the study is that it is observational. To confirm the finding, brain-wave activity needs to be measured in sperm whales in the wild. But the technology to do that is far off.

“In the immediate future, we need to get a diver in the water with a camera observing the whales as they sleep, to see if they sleep with one eye open,” says Rattenborg. But such a study could be very dangerous. Although the whales are not aggressive towards humans, a slap from a startled dorsal fin would be devastating. “I’m certainly not going to be the one getting in the water with them,” says Rattenborg.

Watch a video of the whales (credit Luke Rendell, University of St Andrews).