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Last updated on April 18, 2014 at 9:07 EDT

Dolphins Able To Sleep While Awake

May 2, 2009

Dolphins have an uncanny ability to maintain a constant 24-hour vigilance without sacrificing rest or mental acumen.

Because of their need to always be aware of their surroundings and protect themselves from potential predators, dolphins are able to let one side of their brain sleep while the other remains perfectly conscious and acutely aware for days at a time.

Sam Ridgway from the US Navy Marine Mammal Program and his colleagues from San Diego and Tel Aviv set out to test two dolphins over a 5-day period to see if their auditory or visual awareness would dull after successive days without respite.

To test their auditory skill without sleep, they sounded a 1.5-second beep randomly against a background of 0.5-second beeps. Ridgeway explains that the beeping was low enough to allow them to swim around the enclosure without being disturbed, but the randomness of the 1.5-second beeping would evoke a response and get them moving every single time. This affirmed that their auditory vigilance was not dulled at all even after 5 days.

Next, two of the researchers, Allen Goldblatt and Don Carder, administered the visual stimulus test while they were listening to the beeps to see if they maintained an equal level of vigilance with their eyes.

Dolphins have very limited vision due to their eyes being located on opposite sides of their heads, known as binocular vision. With this in mind, they trained one of the dolphins they named “ËœSay’ to recognize either 3 horizontal red bars or one vertical green bar with each eye independently.

They assumed that with half of the brain unconscious during the test, the dolphin would only be cognizant of the shapes seen through the eye connected to the conscious half of the brain.

Surprisingly, when the team began training Say’s left eye, they discovered they she was already able to recognize the shapes even though the left eye and the side of the brain that was asleep before had never seen them.

Ridgeway concluded that the dolphin’s inter-hemispheric commissures connecting the two halves of the brain could be responsible for transferring the visual information.

This test proved to be an incredible challenge for the team. Not only did they have to train the dolphins to recognize shapes, they also had to monitor and reward them continually over a 120-hour period of time while testing their response to both the auditory and visual stimulation.

The research team was astounded to report that after 5 days of listening to randomized 1.5-second beeps against the steady .5-second beep background and being constantly tested for the recognition of 2 shapes, the dolphins still responded with phenomenal accuracy.

Blood tests were also performed to further test the dolphins’ blood for physiological signs of sleep deprivation, but they only served to confirm that there were none. With 5 days of unbroken vigilance, the only ones showing signs of fatigue were likely the researchers.

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