Dolphins: While We Sleep, They Echolocate the Night Away

Bottlenose dolphin. Photo by Peter Asprey, Scotland. Creative Commons.

I got a touch of dolphin envy when I first saw a new study in the journal PLoS One that shows these marine mammals can stay partly awake for at least 15 days, keeping track of their environment. (I’m useless and cranky if I get less than 7 hours of shut-eye each night.)

A group of researchers led by Brian Branstetter, of the National Marine Mammal Foundation, conducted the experiment on two bottlenose dolphins: a 30 year-old female (SAY) and a 26-year-old male (NAY) in a portable floating pen in the San Diego Bay.

Each dolphin was trained to monitor the pen for electronically simulated (“phantom”) targets in eight different locations and press a response paddle when they detected them.

Like bats, dolphins emit sounds (in their case, clicks) and listen to them bounce off objects through a process called echolocation. But bats typically snooze away the day. Dolphins stay more vigilant, 24/7. At night, one hemisphere of their brain remains alert while the cerebral cortex and thalamus on the other side goes to sleep, the article explains. (If only I could achieve unihemispheric sleep, I could get so much more accomplished.)

The experiment took place over three, five-day sessions. As the PLoS one article states, “The dolphins would have no knowledge of where or when a phantom target would occur and would have to continuously echolocate on the eight stations to detect a phantom target.” Trainers reinforced correct responses with fish.

SAY was 96 to 99 percent accurate in detecting echoes from the research project’s “phantom echo generator.” NAY was 75 to 86 percent accurate. Because of her “superior performance,” SAY was selected for another 15-day test. She continued to echolocate successfully during that time.

So why are dolphins peppier than the rest of us?

Previous research shows that unihemispheric sleep likely evolved in dolphins because of the need to remain close to the surface for breathing. It’s possible, the PLoS One article states, that it also serves to keep them alert to predators. (Sharks are a frequent threat.)

“From an anthropomorphic viewpoint, the ability of the dolphin to continuously monitor its environment for days without interruption seems extreme,” the article states. “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.”

I think it’s time to set aside my jealousy. At least I can sleep well, knowing there are no sharks about.

Related articles

• How Dolphins Stay Awake for Two Weeks (news.discovery.com)
• Dolphins can sleep one-half of their brain at a time say researchers (slashgear.com)
• Dolphins stay awake for 15 days (telegraph.co.uk)
• Dolphins Can Sleep With Only Half Their Brain at a Time (sciencespacerobots.com)