Bottlenose dolphins are famous for making a wide range of high-pitched noises, but they aren't just whistling Dixie — unless one of them happens to be named Dixie, that is.
A study, published in the Proceedings of the Royal Society B, suggests the gregarious marine mammals not only name themselves with "signature whistles," but they also recognize the signature whistles of other dolphins they know. This has yet to be definitively proven, but the results resemble a linguistic feat known as "referential communication with learned signals," which is traditionally seen as uniquely human.
"This use of vocal copying is similar to its use in human language, where the maintenance of social bonds appears to be more important than the immediate defence of resources," the study's authors write. This helps differentiate dolphins' vocal learning from that of birds, they add, which tend to address one another in a more "aggressive context."
The researchers first tackled this issue in a study published in PNAS, concluding that bottlenose dolphins "extract identity information from signature whistles, even after all voice features have been removed from the signal." These whistles are a big part of the species' "fission-fusion societies," in which they form a variety of different social relationships, especially since it can be hard to recognize individuals by sight or smell underwater.
But despite the possibility that dolphins address friends and relatives by name, the researchers couldn't rule out other explanations for identity-encoded whistling, such as birdlike competition for resources. So in their new study, they examined whistle-copying behavior through the lens of social relationships, hoping to reveal the animals' true motivations. They analyzed acoustic data from wild bottlenose dolphins in Florida's Sarasota Bay, recorded between 1984 and 2009 by the Sarasota Dolphin Research Program, as well as the vocalizations of four captive adults at a nearby aquarium.
The wild dolphins were briefly captured and held in separate nets by the SDRP, allowing them to hear but not see each other. In studying the resulting audio files, the researchers noticed the dolphins were copying their podmates' signature whistles, apparently part of an effort to stay in touch during their ordeal. Most of this took place among mothers and calves, or among males who were close associates, suggesting it was affiliative and not aggressive — sort of like calling out the name of a missing child or friend.
But while the dolphins closely imitated one another's "names," they didn't mimic them exactly. They added "fine-scale differences in some acoustic parameters," the researchers report, which were subtle yet also outside the variations used by the original dolphin. Some even applied aspects of their own personal frequency signatures to other dolphins' whistles, possibly sharing extra information about the speaker's identity.
If confirmed, this would be a level of communication rarely found in nature. Using learned language to represent objects or individuals is considered a hallmark of humanity, replicated only sporadically in captive animals. If dolphins can identify themselves and address friends with just a few squeaks, it's easy to imagine what else they're saying.
Still, as the study's authors point out, all we can do right now is imagine. They suspect they've found evidence of dolphin dialogue, but they advise caution in interpreting their results, citing the need for further research both in dolphins and other animals.
"It is possible that signature whistle copying represents a rare case of referential communication with learned signals in a communication system other than human language," they write. "Future studies should look closely at the exact context, flexibility and role of copying in a wider selection of species to assess its significance as a potential stepping stone toward referential communication."