Dolphins keep an eye open while sleeping

Dolphins have a clever trick that doesn’t jump into the air to catch fish: They can overcome lack of sleep and remain constantly alert for days by resting one half of their brain while the other half remains conscious.

Because they have to periodically go up to get some fresh air and keep an eye on potential predators, dolphins can’t curl up and zone at night like land mammals. So they have to stay somewhat conscious and sleep with the proverbial eye open.

Sam Ridgway of the U.S. Navy’s Marine Mammal Program wondered if this constant vigilance was blunting their senses, as lack of sleep does in humans (as anyone who has shot a sleepless night knows).

To study the effects of this sleep pattern on dolphins, Ridgway and colleagues trained two dolphins to respond to a 1.5-second beep emitted randomly against a 0.5-second beep background. (The sounds were faint enough not to interfere with the dolphins in their daytime swims around their pool, but the random tone still caught the dolphins’ attention.)

Even after listening to the tone for five days in a row, the dolphins continued to respond to the beep as vividly as they did at the beginning.

Then, two of the researchers, Allen Goldblatt and Don Carder, designed a visual stimulus test to see if dolphins were just as vigilant with their eyes. They also continued to see if the dolphins responded to the sound signals.

Dolphins have binocular vision (their eyes sit on either side of their heads), so the researchers trained one of the dolphins (named Say) to recognize two shapes, either three horizontal red bars or a vertical green bar. They first trained Say with his right eye.

Scientists thought that since half of the dolphin’s brain would be asleep during the test, Say would only recognize the shapes with the eye connected to the conscious half of his brain. But she had a surprise in store for them: She trained her left eye on the shapes, even though that eye hadn’t seen the shapes before.

Ridgway a dit que cela doit signifier que les informations sont transférées entre les deux hémisphères du cerveau.

The dolphins turned out to be just as sharp with their eyes as with their ears: After 120 hours, they still saw the shapes.

The researchers checked the dolphins’ blood for physical signs of sleep deprivation, but found none.