A group of five captive dolphins in France have been recorded making whale-like noises late at night — despite the fact that they have only heard whale sounds as recordings during their daytime dolphin shows.

If the sounds are confirmed to be mimicking whales, it would be the first example of dolphins "saving up" a sound to practice later. And since the whale sounds are only uttered at night, it's possible the whale sounds are a dolphin version of sleep-talking.

The dolphins, all of whom were born in captivity, have never had the opportunity to hear a whale sing except on the soundtrack to their daily shows at the French aquatic park Planète Sauvage. Amid music, bird cries and other marine sounds, that 21-minute soundtrack features a couple of minutes of whale song.

The dolphins have never been heard mimicking that whale song during or after shows, but when researchers recorded all of the dolphin vocalizations for nine days and eight nights between November 2008 and May 2009, they heard 25 instances of dolphin sounds never heard before. Though rare — about 1 percent of all the dolphin noises recorded — the sounds sounded strikingly like whale calls. These strange sounds occurred only at night during dolphin "rest periods," mostly between midnight and 3 a.m.

To make sure they weren't hearing things, the researchers played slowed-down and regular-speed audio of the calls to 20 volunteers, along with regular dolphin whistles, slowed-down dolphin whistles, and real whale songs. They found that the volunteers correctly identified dolphin whistles as dolphin whistles and whale song as whale song 88 percent to 99 percent of the time. But 72 percent of the time, the listeners misconstrued the dolphin's whale-like whistles as real whale song. [Listen to dolphin's sleep-talk]

Dolphins, like birds, are known copy-cats, but their mimicry has always been confined to the time right after hearing an odd sound, at least as far as anyone knew. The Planète Sauvage dolphins, however, only make the whale sounds at night, most likely when they're sleeping or at least resting. This suggests that they could be rehearsing their daily shows in their minds at night, the researchers reported online in the journal Frontiers in Comparative Psychology Dec. 29, 2011. It's possible the dolphins are even asleep as they make the whale-like noises, meaning they are essentially sleep-talking in 'whale.'

The recordings are "the first report of mimicries of sounds heard during special events produced by dolphins in a resting/sleeping context," the researchers wrote in the journal. "This finding opens very large perspectives for future investigations on dolphin learning processes and 'mental representations.'"

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