Dolphins vigilant round-the-clock, even while asleep

WASHINGTON - Dolphins have a clever way of remaining vigilant round-the-clock, by sending half their brain to sleep, while keeping the other half awake.

Sam Ridgway from the US Navy Marine Mammal Program and colleagues from San Diego and Tel Aviv wondered whether the dolphins’ unrelenting vigilance tired them or weakened their senses.

Ridgway set about testing two dolphins’ acoustic and visual vigilance over a five day period to find out how well they functioned after days without a break.

Ridgway and his colleagues trained two dolphins to respond to a 1.5 second beep sounded randomly against a background of 0.5 second beeps every 30 seconds.

Ridgway explained that the sounds were low enough for the dolphins to barely notice them as they swam through their enclosure, but the animals sprung into action every time they heard the 1.5 second tone, even after listening to the sounds for five days without a break.

Next colleagues Allen Goldblatt and Don Carder designed a visual stimulus to test the dolphins’ vigilance while they continued listening to the repetitive beeps.

Knowing that the dolphins’ binocular vision is limited because their eyes are situated on opposite sides of their heads, Kamolnick trained one of the dolphins, Say, to recognize two shapes (either three horizontal red bars or one vertical green bar) with her right eye before training her to recognize the same shapes with the left eye.

Goldblatt and Carder reasoned that if half of her brain were asleep during testing, the dolphin would only see the shapes through the eye connected to the conscious half of the brain. But the team were in for a surprise when they began training Say’s left eye.

Say already recognized the shapes, even without her left eye seeing them previously. Ridgway explained that the visual information must be transferred between the two hemispheres connected by inter-hemispheric links.

Amazingly, even after five days of listening to the 1.5-second beeps against the 0.5 second beep background, the dolphins were still responding as accurately as they had done at the beginning of the experiment, said a US Navy release.

These results were published in the Friday issue of the Journal of Experimental Biology.


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