Common bottlenose dolphins identify themselves with a unique call, but these whistles may carry extra information through variations in volume
28 December 2021
Common bottlenose dolphins alter their volume throughout their signature whistles, perhaps as a way to communicate additional information besides just their identity.
A signature whistle is a unique combination of sound frequencies – like musical notes – held for specific lengths of time to create a special call that each dolphin uses to identify itself. Created during their infancy, dolphins’ signature whistles hardly change throughout their lifetimes. But a new study has revealed that the cetaceans vary the amplitude – the volume – of their whistles, and that the amplitude pattern changes with nearly every call.
“This seems a lot like the kind of non-word communication that we humans use, based on inflections, like putting emphasis in a certain area or how we sort of fade out and fade in so we know when it’s each other’s turn to talk,” says Brittany Jones at the National Marine Mammal Foundation in San Diego, California.
The findings suggest that the dolphins might be using frequency as an identity stamp and amplitude to convey additional information to each other, says Jones.
Scientists have been studying the frequency of signature whistles in common bottlenose dolphins (Tursiops truncatus) for nearly 60 years. But it is more difficult to capture differences in amplitude in underwater animals that are constantly moving around in different directions.
Now, improved technology in underwater microphones, called hydrophones, along with more advanced sound analysis equipment and a more controlled recording environment have made it possible to study dolphins’ whistle amplitudes with greater accuracy, says Jones.
Her team used this technology to record about 50 repetitions of the signature whistles of eight adult dolphins living in natural seawater enclosures in the San Diego Bay as part of the US Navy Marine Mammal Program.
Consistent with previous research, the dolphins all had very set frequency patterns in their unique signature whistles. However, the dolphins’ volume patterns changed considerably from whistle to whistle.
There was a general tendency for them to increase their volume when they were whistling at a lower frequency. But aside from that, there were no clear amplitude patterns across dolphins or within the whistles from the same dolphin. In each whistle, they made some parts of their call louder and other parts softer.
Jones says she was surprised at first by how variable the amplitude patterns were, and initially wondered if they might just be random. “But then I thought, well, if the signature is so constant all the time, it makes it hard to communicate different things, sort of like saying the same word over and over,” she says. “So maybe this is a new avenue that lets [dolphins] code additional information.”
For now, we can only imagine what the volume patterns could mean to other dolphins, if anything, but there are plenty of things these highly social creatures might want to say.
“Oh gosh, what wouldn’t they want to be telling each other?” says Jones. “Location, arousal level, reproductive status, whether there’s a predator around… The sky is the limit in terms of what dolphins might communicate about.”
Journal reference: Behavioural Processes, DOI: 10.1016/j.beproc.2021.104561
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