Newborn dolphins and killer whales do not sleep for a whole month after birth, new research has revealed, and neither do their mothers, who stay awake to keep a close eye on their offspring.
The feat of wakefulness is remarkable given that rats die if forcibly denied sleep. And in humans, as any new parent will tell you, sleep deprivation is an exquisite form of torture.
The surprising sleeping patterns of captive killer whales - Orcinus orca - and bottlenose dolphins - Tursiops truncates - in the early months of life were observed by a team led by Jerome Siegel of the University of California at Los Angeles, US.
Unlike all animals previously studied, which maximise rest and sleep after birth to optimise healthy growth and development, the cetaceans actively avoided shut-eye. "The idea that sleep is essential for development of the brain and body is certainly challenged," says Siegel.
Coming up for air
The patterns observed contrast with that seen in adult cetaceans, which normally "sleep" for 5 to 8 hours a day - either floating at the surface or lying on the bottom before rising periodically for air.
But the newborn whales and dolphins were continually active, surfacing for air every 3 to 30 seconds. They also kept at least one eye open to track their mothers, who seemed to set the frenetic pace by always coursing ahead of their offspring.
Siegel and his colleagues found that, over months, mothers and offspring gradually increased the amount of rest until it approached that of normal adults. And measurements of the stress hormone cortisol showed that levels were normal, so the animals were not apparently stressed by their insomnia.
The researchers suggest that for cetaceans, the ability to keep on the go after birth has several advantages. It makes it harder for predators to catch them because "in the water, there's no safe place to curl up", Siegel notes.
It also keeps their body temperature up while their layer of insulating blubber builds up. The mystery, he says, is how the cetaceans seem to avoid the penalties of sleep deprivation seen in all other mammals.
Eyes open, wide awake?
"It's an extraordinary finding," says Jim Horne, director of the Sleep Research Centre at the University of Loughborough, UK. "Normally, newly born mammals and their mothers stay asleep for as long as they can after birth."
Horne says that if it had been practically possible, measurement of brain activity would have provided better confirmation that the animals were awake than simply checking if they had at least one eye open.
"You can't be entirely sure that they're actively awake all the time, not going into a drowsy, trance-like state," he says. "But they are certainly showing extensive periods of wakefulness."
Horne says that humans sometimes fall asleep with their eyes open, so it is conceivable that nature allows the dolphins and whales to do the same.
Journal reference: Nature (vol 435, p1177)