Most of us start to tire after about half a day without any sleep. Staying awake for five in a row would be extremely difficult and even if you could manage it, you’d be a physical and mental wreck by the end. But not all animals suffer from the same problem. A dolphin can stay awake and alert for at least 5 days straight, chaining together all-nighters without any noticeable health problems or loss of mental agility.
The two halves of a dolphin’s brain can sleep in shifts, “shutting down” one at a time so that the animal is always half-awake. They can truly sleep with one eye open, an essential skill for an animal that has to be constantly watching for predators and timing its breaths.
Previously, Sam Ridgway from the US Navy Marine Mammal Program found that bottlenose dolphins could respond to a distinct noise for 5 straight days without any dip in accuracy. He trained a female called Say to recognise occasional 1.5-second beeps amidst a background of shorter 0.5-second ones. When she heard the longer tone, she pressed a lever for food. Say was slower to respond at night, but overall, her reaction times didn’t slow over the five day run.
It was an impressive performance, but one that didn’t require much in the way of thought. This time, Ridgway wanted to see if the dolphin’s mental skills would take a hit after five days of continual vigilance. To do that, he trained Say, and another male called Nay, to make different noises for two different visuals – a whistle for a single, vertical, green bar of light, or a burst of sonar pulses for three, horizontal, red bars. Incidentally, dolphins are colour-blind; the colours were for the scientists’ benefit.