Scientists have recorded two Black Sea bottlenose dolphins in conversation with each other in a pool.
Like humans, dolphins are able to communicate emotions such as stress or happiness, Ukrainian researchers have found.
The breakthrough came after the researchers created an underwater microphone, which was specifically designed to pick up on the animals' harmonics and track their "whistles".
Scientists at the Karadag Nature Reserve, in Feodosia recorded two dolphins, Yasha and Yana, listening to each other 'speak' up to five "words" without interruption.
Dr Vyacheslav Ryabov, told the Telegraph UK that the recordings resembled a conversation between two people.
Dr Ryabov said each frequency "represents a phoneme" of dolphin spoken language. The animals in the recording did not interrupt each other, which suggests they listen during conversation, similar to humans.
“Each pulse that is produced by dolphins is different from another by its appearance in the time domain and by the set of spectral components in the frequency domain,” he said.
“Humans must take the first step to establish relationships with the first intelligent inhabitants of the planet Earth by creating devices capable of overcoming the barriers that stand in the way of using languages and in the way of communications between dolphins and people."
The research was published in a Russian Mathematics and Physics journal titled, The study of acoustic signals and the supposed spoken language of the dolphins on August 21.
In 2007, Australian scientists discovered that dolphins have developed a sign language where they are appear to communicate with their flippers.
Scientists in Florida also revealed earlier this year showed that the communication between dolphins increases when they are taking part in a difficult task.