Dolphin whistles are just what the name implies: a high-pitched tone that goes up and down in a complicated way, and it is that diversity in the whistles that has fascinated scientists, and absorbed huge amounts of time and effort into answering two questions: why are these whistles so diverse and what do they mean? Dolphins whistle pretty much all the time. Not as often as they produce echolocation clicks—they need the clicks to find their way around—but fairly constantly nevertheless. As I mentioned, they don’t whistle with their mouths as we do, but then that’s not surprising, because they don’t breathe with their mouths as we do either. Dolphins make all their sounds through their blowhole—their nose, effectively.

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Another difference between the whistling you’re used to performing and the whistles that dolphins make is that dolphins can recirculate the air they use for making sound, so you don’t necessarily see bubbles coming out of the blowhole of the vocalizing animal. This is terribly frustrating when you’re looking around hoping to figure out who is saying what to whom, but it makes sense, when you think that these animals must hold their breath as long as they’re underwater.

There is something deeply and fundamentally different about creatures that can invent unique sequences of sounds to refer to themselves.

Dolphin whistles and wolf howls have a lot in common. Slowed right down, they sound very similar, and in fact a howl and a whistle differ only in pitch and duration. And it is no coincidence that these two signals have similar properties, and even look the same on spectrograms. Sound might travel very well underwater, but the ocean is still a noisy environment, and there’s a lot of interference—precisely because sound does travel so well. Dolphins aren’t the only animals making use of sound to communicate, and the incessant snapping clicks of different species of shrimp, in particular, is a constant background noise that tends to drown out everything else. Add to that the sound of waves, and myriad other creatures, and you see that getting your message across is even harder for dolphins underwater than it is for wolves in the mountains and valleys of Yellowstone.

If you’ve got something complex to say (and by complex, I mean something more than “I’m here!”), then your signal must be faithfully reconstructed by the listening animal, despite all of the interference that occurs along the way. And as with wolf howls, the way to do that is by using a single note that varies up and down in pitch. Concentrate your sound energy into a single frequency, but vary that frequency up and down to represent different kinds of messages. That’s precisely what a whistle is.

So, as well as simply making a whistling sound, if they want to send complex information, dolphins must have very precise control over the pitch of the whistles that they make. It’s those up-down changes in pitch that will, after all, be the only thing that the listening animal can distinguish about your whistle. And indeed, dolphins have very precise control over their whistles, so that their repertoire can be very large—they can produce a huge variety of whistles and can quite easily copy the whistles produced by other dolphins, or even by humans. Some kinds of complex whistles they can make over and over again and they sound almost the same each time.

Well, not quite the same each time. You’ll notice that the whistles shown in the diagram (all from a single dolphin) differ slightly. Those subtle differences point to one of the biggest problems in studying complex animal communication, not just in dolphins but also in many different species. It’s all very well when an animal produces discretely different—clearly different—kinds of sounds: whistles vs. clicks, for instance. Anyone can tell the difference between a whistle and a click. But more often, an animal’s sounds of different kinds merge into each other; they are graded, rather than discrete.

When does a cat’s cheerful meow become an angry meow? You might be able to tell the difference between your cat happily curling up on your lap, and angrily wanting to be fed—the extremes are easy to distinguish. But between those extremes, the sounds gently morph from “happy” to “angry,” and there’s no clear dividing line. Such graded calls pose a problem to interpreting animal communication, because we don’t know where the animals themselves draw a line (if they do) between one meaning and another. Not yet, anyway.

Dolphin whistles, then, are almost infinitely variable. If no two whistles are exactly the same, and yet we want to investigate whether whistles have meaning, how can we describe whistles in a way that captures their variation in any sensible way? One interesting approach I’ve used to measure this in the past is by comparing whistles to musical melodies. Even those of us who are not particularly musically competent can recognize tunes, without being able to name the notes themselves, or even the key in which the tune is played. Why? We seem to recognize the up-down pattern of the pitch, and a similar thing appears to be happening in dolphins.

Back in 1975, a man named Denys Parsons published a method for classifying tunes, based only on their up-down pattern. Instead of notes (A, C#, etc.), a melody is represented only by the direction of change from one note to the next: up in pitch, down in pitch, or remaining the same. This Parsons code greatly reduces the information in a tune—you’d never be able to reproduce the tune itself from a Parsons code, but you could recognize it.

Humans can recognize a tune from its Parsons code, but what’s amazing is that dolphins seem to recognize that two whistles, which are slightly different but have the same Parsons code, are actually the same whistle. This very neatly underlines that when dolphins hear whistles, they’re doing something similar to us—listening for the up/down pattern of pitch, and using that to interpret what the whistle means.

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So, what are dolphin whistles actually for? There may be many meanings, possibly even meanings that blur into one another, just as the differences between the whistles are graded and blurry. But surely these whistles serve some purpose. Surely they represent something.

We do at least understand one thing about dolphin whistles: we know that each individual dolphin produces a certain kind of whistle that represents its own name.

The scientific community is pretty confident at this stage that we really are talking about an animal with names.

That fact alone is absolutely extraordinary. As far as we know today, there is no other species in the world—none, bar humans and dolphins—that naturally, and as a part of their regular communication, give themselves names. Yes, you can teach your dog to recognize her name. Yes, many animals can recognize individuals by the sound of their voice, which acts as a kind of individual “signature.”

But there is something deeply and fundamentally different about creatures that can invent unique sequences of sounds to refer to themselves—sequences that are also recognized and used by others. At least it should make you question—and we can’t be sure of the answer to this, one way or the other—whether these animals actually understand themselves as individuals, different from other dolphins, who also understand that they themselves are individuals.

Signature whistles, as these name-like vocalizations have been called, have been studied for decades. Although early researchers speculated that these whistles might function as names, it took a long time before that became the scientific consensus—and rightly so. You may be skeptical of what is, after all, an exceptional claim. The first piece of evidence was that, while any particular dolphin tends to make a wide range of sounds, each animal has one particular whistle that they make more than any other—and that whistle is quite distinct from the whistles of other animals.

In itself, this is not particularly strong evidence—birds also sing individually distinctive songs; why do we not think that they use songs as names? It was also fairly interesting that when wild dolphins in the Sarasota Bay in Florida were caught temporarily for health checks and so on, becoming isolated from other dolphins in their pod, they would make their signature whistles over and over again, as if crying out for help, or alerting the other members of their pod that they have become separated from the rest. Distress calls are a well-recognized behavior among many species, but it’s unusual that dolphins cry out using an individually specific whistle, not a general distress call.

Some more convincing evidence came from the fact that mothers that become separated from their calves use their own signature whistle to call for their baby. Adult animals sometimes seem to respond to a friend’s signature whistle with their own—slightly different—version, as if to confirm that they know who is addressing them. And in a remarkable set of experiments, one researcher showed that dolphins responded excitedly when played a recording of the signature whistle of an animal they hadn’t seen for years. Perhaps most convincingly, by looking at the way that baby dolphins develop their own unique signature whistles, we can see that they seem to copy their mothers to some extent, but ensure that their own whistles are sufficiently different to distinguish them as individuals. The scientific community is pretty confident at this stage that we really are talking about an animal with names.

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Excerpted from Why Animals Talk: The New Science of Animal Communication. Used with the permission of the publisher, Penguin Press, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2024 by Arik Kershenbaum.