Dolphins stay awake for 15 days by sleeping with one half of brain Each of the eight targets consisted of a device that picked up dolphin sound pulses and sent back "phantom" echoes. When a dolphin detected an echo from an activated target, it responded by pressing a paddle. Correct detection triggered a tone, signalling success, and the dolphin was rewarded with a fish. False alarms led to no tone and no reward. Over three sessions of five continuous days both dolphins did well, with success rates of up to 99%, but Say outperformed her male partner. The scientists then went on to test Say further by repeating the same experiment over a period of 30 days. In the event, a storm cut the trial short after 15 days had elapsed. However, during this time Say's performance hardly deteriorated at all. The findings were published yesterday in the online journal Public Library of Science ONE. Lead researcher Dr Brian Branstetter, from the National Marine Mammal Foundation in San Diego, said: "These majestic beasts are true unwavering sentinels of the sea. 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. The difference in performance between Nay and Say was probably down to personality, said the scientists. Say appeared much more motivated and keen on the task, often producing "victory squeals" when she correctly responded to a target. Dolphins use their sonar-like ability to navigate, find prey, detect predators, and co-ordinate group behaviour. Brain wave measurements have confirmed that the creatures are capable of "unihemispheric sleep" - sleeping with just one side of the brain. When dolphins sleep in this way, they often keep one eye open. Many dolphin populations are exposed to almost constant risk of shark attack, and the animals commonly carry bite marks, said the researchers. Continually listening to their echo signals allowed them to counter the shark threat, even in murky waters. The scientists wrote: "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.