In 2019, the quiet waters of Denmark’s Svendborg Sound got an unexpected visitor: a lone bottlenose dolphin nicknamed “Delle”. Delle became a local fixture, appreciated and cherished by the locals — but for researchers, this was surprising. These dolphins don’t normally live in the area and don’t typically live alone, as bottlenose dolphins are social creatures.

After studying him for months, scientists now suspect Delle has been carrying on a conversation—with himself. For over three years, this solitary dolphin has been emitting whistles, clicks, and unique acoustic sequences, despite being entirely cut off from his kind. Researchers believe this might represent a form of “self-talk,” an extraordinary behavior that raises profound questions about dolphin cognition and the emotional roots of communication.

Delle the dolphin

When researchers entered the picture, they quickly learned that they already knew who the dolphin was. As it turns out, Delle has some distinctive notches on its dorsal fin which allowed the researchers to identify him as individual #1022 — initially named Yoda. However, the researchers went with Delle as the name caught on with locals.

Delle the dolphin was born in 2007, in Scotland and seemed to be having a normal life. But something happened because, around the year 2019, he was spotted alone, hundreds of kilometers from his usual range. He ultimately ended up near Denmark.

It's not clear why Delle left his normal range. It could be due to disorientation, injury, or behavioral anomalies. Some researchers speculate that these movements could be linked to social stress, environmental changes, or innate exploratory tendencies. But researchers wanted to explore this unusual behavior so they lowered a microphone into the water to see what Delle had to say.

A microphone for a dolphin

The data for this study was collected through passive acoustic monitoring. An autonomous underwater recorder was deployed in Svendborg harbor for 2 months, from December 2022 to February 2023.

Delle was making a lot of noise. Over the period, researchers detected 10,833 voicings from playful and social to seemingly aggressive sounds.

Recordings revealed that Delle used a combination of stereotyped and unique whistles, low-frequency tonal sounds, and burst-pulse sequences. Some sounds resembled signature whistles often used to identify individuals in dolphin groups. Intriguingly, Delle produced three distinct stereotyped whistles—an anomaly since most dolphins emit only one signature whistle.

But there were no other dolphins around, so who was Delle calling out to? Researchers suspect that these sounds might reflect emotional states or fulfill an intrinsic need for expression rather than intentional communication.

What does this all mean?

Bottlenose dolphins are one of the most intelligent and social creatures in the world. Their communication is complex and nuanced. It's not random -- they send out a call with a specific purpose.

Delle’s isolated vocalizations, however, suggest a different possibility. Like human self-talk, these sounds may fulfill psychological needs, mirroring behaviors seen in other social animals. The researchers also considered that the dolphin could be communicating with humans, but Delle continued when no humans were around, so the dolphin could be "talking" to itself.

The rhythmic patterns and diversity of Delle's sounds also seem to suggest a level of spontaneity. They could serve as a mental stimulation mechanism, similar to how humans use inner dialogue for focus or emotional regulation. It could be an emotional outburst, similar to a human laughing or mumbling to himself, or simply sounds to distract oneself.

Simply put, Delle could be lonely and distracting himself.

Delle’s story is a poignant reminder of how much we still have to learn about the emotional and cognitive lives of dolphins. Researchers say we need to study these animals in more detail before drawing any clear conclusions.

The study was published in the journal Bioacoustics.