Delle first appeared in 2019 in the Svendborgsund channel, south of Denmark’s Funen Island, far outside the typical range of bottlenose dolphins. Normally, dolphins are highly social creatures living in pods, but Delle has been swimming solo in these uncharted waters with no sign of others joining him. In late 2022, scientists decided to monitor his impact on local harbor porpoises by deploying underwater recording devices. What they found surprised them.
“Out of curiosity, I decided to add a recorder that captures actual sounds,” explained Olga Filatova, a cetacean biologist at the University of Southern Denmark. “I thought we might pick up a few distant whistles or something along those lines. I certainly didn’t anticipate recording thousands of different sounds.”
Over the course of 69 days between December 2022 and February 2023, researchers documented an astonishing 10,833 distinct noises from Delle, including 2,291 whistles and 2,288 burst-pulses, a rapid clicking sound often associated with aggression or social interaction. In addition, he produced 5,487 low-frequency tonal sounds and 767 percussive sounds.
What made the study even more striking was that some of the sounds typically require a second dolphin to make sense. “These sounds are traditionally considered communicative, meaning there should be at least two dolphins ‘talking’ to each other. But Delle was completely alone,” Filatova said. Among the recorded vocalisations were three unique whistles, which dolphins use as signature calls, akin to names. “If we hadn’t known that Delle was alone, we might have concluded that a group of at least three dolphins was engaged in various social interactions.”
At first, researchers wondered if Delle’s chatter might be directed at nearby humans, such as paddleboarders, but they also recorded his sounds at night when no humans were present. This eliminated the possibility that his vocalisations were a direct response to human activity.
The motivations behind Delle’s vocalisations remain speculative, but several theories have emerged. Filatova posited that the sounds could be involuntary, triggered by emotions, much like humans laugh or talk to themselves when alone. Alternatively, Delle might have been calling out in the faint hope of attracting other dolphins. However, she noted that this seems unlikely given that he has been in the area for three years and would likely know by now that no other dolphins are present.
Other experts, such as Thea Taylor of the Sussex Dolphin Project, weren’t entirely shocked by the findings. “Dolphins are very vocal animals, so I’m not overly surprised that this individual was still making sounds despite being alone,” Taylor explained.
She added that dolphins rely heavily on sound, not just for communication but also for navigating their surroundings and hunting. The idea that Delle’s vocalisations may be an emotional outlet was particularly intriguing to Taylor. “This opens up so many questions about whether we can utilise dolphin communications to understand more about their behaviour and emotions in the wild,” she said.
Delle’s story sheds light on a largely overlooked area of dolphin research: the lives and behaviours of solitary dolphins. As Filatova pointed out, such individuals are often seen as “outcasts” and rarely studied in depth. Yet Delle’s solitary symphony of sounds raises profound questions about loneliness, emotion, and adaptation in these intelligent marine mammals. Whether he is singing out of longing or simply navigating his aquatic world, Delle offers a poignant reminder of how much we still have to learn about the emotional lives of dolphins.
The findings of this research have been published in Bioacoustics.