Two dolphins have been recorded for the first time having a conversation,
after scientists developed an underwater microphone which could distinguish the animals' different "voices".
Researchers have known for decades that the mammals have an advanced form of communication, using distinctive clicks and whistles to show they are excited, happy, stressed or separated from the group.
But they have now shown that dolphins alter the volume and pitch of pulsed clicks to form individual "words" which they string together into sentences in much the same way as humans speak.
Researchers at the Karadag Nature Reserve, in Feodosia, Russia, recorded two Black Sea bottlenose dolphins, Yasha and Yana, talking to each other in a pool. They found that each dolphin would listen to a sentence of pulses without interruption, before replying.
Dr Vyacheslav Ryabov, the lead researcher, said: "Essentially, this exchange resembles a conversation between two people. 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.
"In this regard, we can assume that each pulse represents a phoneme or a word of the dolphin's spoken language. The analysis of numerous pulses registered in our experiments showed that the dolphins took turns in producing [sentences] and did not interrupt each other, which gives reason to believe that each dolphin listened to the other's pulses before producing its own.
"This language exhibits all the design features present in the human spoken language. This indicates a high level of intelligence and consciousness in dolphins, and their language can be ostensibly considered a highly developed spoken language, akin to the human language."
Dolphins have possessed brains that are larger and more complex than human brains for more than 25 million years. The researchers found that Yasha and Yana could create sentences of up to five "words", but the scientists do not understand the content.
Dr Ryabov said it was now beyond doubt that dolphins speak their own language and it is time to start studying how to communicate directly with them.
"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," he added.
Scientists already knew that dolphins use more than 1,000 different types of whistle depending on social context, but it was unclear whether they could communicate one-to-one.
The research was published in the journal Mathematics and Physics.