Do Dolphins speak during their sleep?

Péos, Mininos, Cécil, Teha, and Amtan are performing dolphins at the Planète Sauvage dolphinarium in Port-Saint-Père, France. Every day, as music and sounds of the sea play in the background, they show off their swimming, jumping, and ball-catching skills for an adoring audience and squawk and whistle just like dolphins should. But at night, they make strange noises that researchers believe are imitations of humpback whale songs included in the performance soundtrack. If so, the identification of this unexpected repertoire would mark the first time that dolphins have been heard to rehearse new sounds hours after hearing them rather than right away, providing insights into how they store and process memories.

Researchers discovered the dolphins' midnight melodies by accident. Ethologist Martine Hausberger of the University of Rennes 1 in France and her colleagues had hung underwater microphones in the tank because little is known about what dolphins sound like at night. One night, they suddenly heard 25 new sounds (see below) that the dolphins had never made before, although they weren't sure which of the five animals was talking. Because dolphins are known for mimicry, the researchers examined their complex daytime environment to determine where the noises might be coming from. They finally zeroed in on the new soundtrack that Planète Sauvage was playing during performances, which included music, sea gulls' calls, the dolphins' own whistles, and humpback whale calls.

When the researchers used a computer program to compare auditory recordings of the whale calls with the mysterious nighttime noises, it showed that the two sounds were very similar. And because the dolphins had been captive their entire lives, they couldn't have picked them up from real whales.

To get a second opinion, the team asked 20 human volunteers to listen to humpback whale sounds and wild dolphin sounds. Then the researchers played the nighttime vocalizations and asked the volunteers whether the sounds came from a whale or a dolphin. About 76% of the time, the volunteers classified the imitations as sounds from real whales, the researchers reported online in December in Frontiers in Comparative Psychology. Until now, dolphins have been known to mimic sounds (including whale sounds) only right after hearing them. Because the dolphins didn't make these noises during the show, the finding suggests that they waited to practice the sounds hours later.

So why would dolphins want to mimic whales? Hausberger believes that it might be because the shows prime the animals to learn and remember information. "The shows are a really special time in the day," she says, because the dolphins get rewards for performing well. During the rest of the day, the park is open and people can view the tank, chatter, and applaud. "There are lots of things they could mimic but don't. ... It's really remarkable the only mimicry [we] found was this one." She wants to find out whether the dolphins are asleep and dreaming when they mimic, which might mean that dreams help dolphins etch new information into their memories, just like in humans. She plans to capture electroencephalogram recordings of the dolphins' brains at night, which would show whether this is the case.

Peter Tyack, a biologist at the University of St. Andrews in the United Kingdom who studies animal vocalization, says that the idea that dolphins might delay their rehearsals for hours is intriguing. But he isn't convinced from the small number of recordings that the researchers obtained that the dolphins were imitating whales. "I have heard sounds in the field that sound quite a bit like the [whalelike sounds]," he says. In addition, he says, dolphins make so many different sounds that it's difficult to pin down one as an imitation of a particular source. But songbirds rehearse their imitations of other noises at night, so he thinks it's not unlikely that dolphins might do the same.

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