We have given names to many bottlenose dolphins, from Flipper to Darwin. But in the wild, these animals have their own badges of identity—signature whistles that they develop during their early years and that are unique to them.

These whistles seem a lot like human names. The comparison isn’t perfect, but it’s now stronger than ever after Stephanie King and Vincent Janik managed to address individual wild dolphins with recordings of their own signatures. The dolphins responded by calling back, but only when they heard their own whistle. “I think our results do present the first case of naming in mammals, providing a clear parallel between dolphin and human communication,” says King.

The evidence that dolphins use signature whistles like names has been building since they were first discovered in the 1960s. They can convey a dolphin’s identity, as well as its mood and motivation. Individuals invent their own whistles at a few months of age, possibly with some influence from their mums. They’ll keep the same call for decades, although males sometimes change theirs to resemble the whistle of a new ally.

The whistles are clearly important, since they account for half of all the calls that wild dolphins make. They can mimic each other’s signatures, but they usually call with their own, perhaps to broadcast their identity and keeping in touch while swimming.

Working out how dolphins actually use these signatures in the wild has been very hard. These are fast-moving animals, whose calls blend together into a cacophonous mess. But Janik has developed a way of identifying individual whistles based on their distinctive rhythms. He can now follow free-swimming pods, record their calls using underwater microphones, and parse out the signatures of different individuals.

In 2011, he used this technique to show that groups of bottlenoses exchange signature whistles when they meet up—a ritualised greeting, like saying hello or shaking hands. When I covered that study, I ended with: “[Janik] also wants to try some playback experiments – the cornerstone of animal communication research – to see if he can provoke a specific response by playing a chosen signature whistle.”

And: check.

The duo worked with a group of Scottish bottlenoses that Janik’s been studying since 1994. Once they spotted the group, they recorded its signature whistles and made computer-generated copies on the spot, filtering out other distinctive features of the animals’ voices. They then played the synthetic whistles back at the pod.

It wasn’t easy—dolphin groups fuse and split all the time, and if that happened, the duo had to start from scratch. “There were times when we had everything ready to start a playback, and the dolphin group separated and the playback had to be aborted,” says King.

But when it worked, it really worked. When the animals heard their own signature, they usually called back with the same whistle type within seconds. This hardly ever happened if they heard the signature of a dolphin from another group. “It was really interesting to see how strong the response was,” says King. “In most cases, when we played back a copy of an animal’s signature whistle, it replied quickly and sometimes multiple times.”

Of course, it’s impossible to say who’s actually replying—it could be the signature’s owner or another dolphin mimicking the same call. However, we know that wild dolphins don’t mimic each other’s signatures that frequently; it happens, but not nearly often enough to explain the strong responses that King and Janik saw.

“Our new study really demonstrates that signature whistles are used like names,” says King. “It’s now clear that signature whistles have meaning, in that they’re labels for particular individuals and can be used by animals to address a social companion.” (Admittedly, there’s still no direct evidence that dolphins are using these whistles to call to their peers, but given the evidence we have, that’s not an unreasonable assumption.)

So, dolphin signature whistles are exclusive to individuals, rather than being part of a shared repertoire. They’re social sounds, unlike bird songs which are largely used to attract mates or defend territories. And they’re learned; many animals like birds and monkeys use distinctive calls to refer to specific objects (like different types of predators), but these are innate and inherited behaviours. “The use of new or learned sounds to label an object or class of objects is rare in the animal kingdom,” says King.

This combination of traits makes the signature whistles unique in the non-human world, at least for now. It’s possible that parrots and other birds might use similar calls. Various species can learn new sounds and use them to label objects, mimic each other’s sounds, tell individuals apart based on their distinctive calls, or refer to family members with specific sounds. If we want to look for other examples of natural animal names, the parrots are a good bet.