Almost everyone talks to themselves sometimes. But humans might not be the only animals that do this: researchers have observed this behaviour in a bottlenose dolphin living in the cold waters of the Baltic Sea, near the Danish island of Funen.

For years, Delle has lived alone in Svendborg Sund, far from the areas usually frequented by his fellow dolphins. Researchers at the University of Southern Denmark have been studying his behaviour for several months.

In a study published in the journal Bioacoustics, they explain that they recorded 10,833 sounds produced by Delle over a period of 69 days, between December 2022 and February 2023.

The experts identified 2,291 whistles and 2,288 rapid clicks, sounds that dolphins usually make to interact with other dolphins. These vocalisations, known as “signature whistles”, are unique to each dolphin and play a role similar to that of a first name.

“I thought we might pick up a few distant whistles or something along those lines, but I certainly didn’t anticipate recording thousands of different sounds,” study lead author Olga Filatova, a cetacean biologist at the University of Southern Denmark, told Live Science.

The researchers were puzzled by this. Delle could be making these sounds to attract the attention of a diver or a local paddleboarder, but this explanation seemed implausible, as these noises were also captured in the middle of the night.

It is also possible that he was trying to make contact with his fellow creatures, but this hypothesis is not especially convincing, as Delle lives in an area rarely frequented by others of his species.

Filatova and colleagues believe that Delle produces these sounds to cope with his loneliness – in other words, he is talking to himself. “Similar to burst-pulse sounds and gulps, whistles – at least some of them – might serve as emotional signals lacking first-order intentionality,” the researchers wrote.

This scientific breakthrough sheds new light on the emotional intelligence of dolphins, marine mammals with impressive cognitive abilities. A study published in 2013 in the journal Proceedings B of the British Royal Society demonstrated their ability to recognise one of their fellow dolphins after 20 years apart.

Solitary dolphins are rarely studied, as the scientific community considers them anomalies, but it seems they could help us better understand the subtleties of animal communication.