"After being awake for many hours or days, humans and other animals are forced to stop all activity and sleep," Branstetter added. "Dolphins do not have this restriction, and if they did, they would probably drown or become easy prey."
Branstetter and his colleagues worked with two bottlenose dolphins, a male named Nay and a female named Say, from a special floating platform that was outfitted with eight underwater sound receiver/emitters. As the dolphins checked out the devices with their echolocation, the devices would echo back the sounds, creating 'phantom targets'. When they heard the echo, the dolphins would press a paddle. If they correctly identified the phantoms, a tone would sound and they would receive a fish as a reward. If they pressed the paddle when there was no echo, there was no tone and no reward.
Image courtesy Brian BranstetterNay and Say were tested in three different sessions, each lasting 5 days straight —which would have caused sleep-deprivation symptoms in other mammals — but they maintained a 99% success rate throughout the tests (Say performed slightly better than Nay).
The researchers ran a third session, specifically with Say due to her enthusiasm for the test, with the intent to run it for 30 days straight. A storm cut the experiment short after only 15 days, however Say maintained her exceptional performance the entire time.
"The demands of ocean life on air breathing dolphins have led to incredible capabilities, one of which is the ability to continuously, perhaps indefinitely, maintain vigilant behaviour through echolocation.'' said Branstetter.
This form of sleep is called 'unihemispheric slow-wave sleep'. One side of the brain goes into deep sleep, with the corresponding eye closed, while the other side remains active and the eye corresponding to that side stays open. Several other species have been shown to do this, including two other species of dolphin, porpoises, beluga whales and pilot whales, northern fur seals, two sea lion species, the amazonian manatee and several species of bird, including the mallard duck and the common chicken.
In the discussion section of their paper, Branstetter and his colleagues noted that their hypothesis that dolphins can maintain indefinite activity through unihemispheric sleep needs further study for conclusive evidence, but they also said:
"From an anthropomorphic viewpoint, the ability of the dolphin to continuously monitor its environment for days without interruption seems extreme. However, the biological, sensory and cognitive ecology of these animals is relatively unique and demanding. If dolphins sleep like terrestrial animals, they might drown. If dolphins fail to maintain vigilance, they become susceptible to predation. As a result, the apparent "extreme" capabilities these animals possess are likely to be quite normal, unspectacular, and necessary for survival from the dolphin's perspective."
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