London: Dolphins are more intelligent than you thought. They are multilingual -- at least in their sleep, scientists have found.

A team at University of Rennes claims to have recorded captive dolphins at an acquatic park in France talking in their sleep -- in fact, the aquatic mammals have been found to be making whale-like noises, and not dolphin sounds.

In their study, the scientists focussed on five dolphins -- Peos, Mininos, Cecil, Teha, and Amtan -- who were born in captivity and have only ever heard whale sounds as recordings, media reports said.

According to the scientists, if the sounds are confirmed to be "whale", it'll be the first known instance of dolphins remembering a particular noise and repeating it "later".

For their research, the team hung underwater microphones in the tank of the five dolphins who perform everyday at the French aquatic park Planete Sauvage.

The unusual noises -- which make up just one per cent of all the sounds recorded -- strongly resemble whale song and occur only during "rest periods", mainly between midnight and 3 am, a newspaper reported.

The scientists recruited 20 volunteers to compare dolphin whistles and whale songs and found the dophin's "whale" is so good that listeners mistook it for real whale song 72 per cent of the time. The noises suggest that the dolphins could be practising their daily shows in their minds at night.

Martine Hausberger, who led the team, told the 'Science' magazine: "The shows are a really special time in the day, because the dolphins are rewarded for their performance. There are lots of things they could mimic but don't... It's really remarkable the only mimicry (we) found was this one."

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 'Frontiers in Comparative Psychology' journal.

"This finding opens very large perspectives for future investigations on dolphin learning processes and 'mental representations'," they added.

PTI