You may have heard that dolphins sleep with only half of their brains at a time. Like me, you may have assumed that, while the awake hemisphere was capable of preventing the animals from drowning, it wasn’t completely alert. Not so. Apparently, dolphins can respond to echolocation and training cues with full concentration indefinitely. Or at least, for the fifteen straight days demonstrated by this study.
Researchers from marine mammal programs in San Diego, led by Brian Branstetter of the National Marine Mammal Foundation, asked two dolphins (SAY and NAY) to search their water enclosure for simulated echolocation targets. Upon detecting a target, the dolphins could press a response paddle and receive a reward. After training, the dolphins each participated in two 5-day sessions of round-the-clock target simulation. Both of the dolphins maintained a correct response rate of over 75%, but because SAY did a little better (she was correct 95% of the time), she got the honor of participating in a 15-day test. That test was actually supposed to last thirty days, but was cut short by a storm. SAY’s performance remained strong throughout this longer interval.
Credit: Brian Branstetter
During unihemispheric sleep when only half their brains are awake, dolphins are not merely maintaining breathing and other bodily functions. They can respond to their surroundings with full alertness. If you consider that many dolphin populations are at constant risk of attack from sharks, perpetual vigilance sounds like a great idea.