Researchers have found a lonely dolphin in the Baltic Sea that appears to be talking to himself, according to a study published in the journal Bioacoustics last month. The solitary bottlenose dolphin is known as Delle by the locals and has been hanging around the Svendborgsund channel, south of Funen Island in Denmark since September 2019. The bottlenose dolphins are social animals and usually live in pods but strangely the region Delle was found in, remains outside the usual range of the species as no other dolphins were spotted nearby.
Scientists usually don't record the sounds made by solitary dolphins as they are perceived as outcasts and because they would not produce any sound worth documenting. However, the researchers involved in the study took the gamble and put down underwater recording devices to see how the solitary dolphin behaved. The research took place across 69 days between December 8, 2022, and February 14, 2023.
"Out of curiosity, I decided to add a recorder that captures actual sounds. 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," lead author Olga Filatova was quoted as saying by LiveScience.
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Study findings
The findings revealed that Della produced 10,833 sounds, which included 2,291 whistles, 2,288 burst pulses, 5,487 low-frequency tonal sounds, and 767 percussive sounds. The study added that "three different types of stereotyped whistles, deviating from the typical pattern of one individually distinctive signature whistle per dolphin" were discovered. Additionally, three different biphonic sound categories were identified: combinations of two whistle components, a whistle and a low-frequency tonal component, and two burst-pulse components.
Notably, whistles are narrow-band, frequency-modulated signals used to maintain group cohesion while burst-pulse sounds consist of a series of rapid impulses that are emitted in quick succession.
"Bottlenose dolphins have what are known as signature whistles, believed to be unique to each individual, much like a name. 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," Ms Filatova added.
It remains a mystery why Della was talking to himself but one theory suggests that he was making involuntary sounds, akin to humans who laugh when reading something funny even when no one is around. Another theory suggests that Della may have been calling out in the hope of attracting the other dolphins nearby.
