Coen Elemans was waiting for his flight to take off at the Copenhagen Airport. Numerous safety and passenger announcements cascaded out of the intercom.

"They normally never say anything at this airport!" observed Elemans, a bioacoustician at the University of Southern Denmark. "They're very vocal today."

He says the airport is as good a place as any to listen to the different ways people vocalize.

"What I hear is a lot of people talking," he notes. "They're mostly using what's called the chest register." That's our typical speaking voice.

Then, Elemans notices some music playing. It's the situation where we most often hear our higher-pitched vocal register on display — falsetto.

We also have a lower register, below the frequency range where we usually talk. That's vocal fry. In English, it's usually regarded as an affect — something that changes the emotion or attitude of what's said. Certain people are known for it, like Kim Kardashian or Leonard Cohen. In other languages like Danish, Elemans says it alters the meaning of words.

We produce all these sounds — the vocal fry, the chest voice, and the falsetto — by sending air across our vocal folds in the larynx (a structure that allows air to pass from your throat to the rest of your respiratory system). And the vocal folds vibrate differently for each register.

"In a vocal fry register," explains Elemans, "your vocal folds are mostly slack, so they're thick and heavy, and they vibrate at their lowest frequencies. And in the falsetto register, they're stretched very long and the tension is very high. And this leads to the highest frequencies."

Elemans wondered whether a similar thing might be at play in toothed whales (like bottlenose dolphins, orcas, and pilot whales) to allow them to produce their diverse array of vocalizations. Such sounds range from whistles to bursts (the sounds we associate with Flipper) to echolocation clicks — the sound pulses used to hunt for prey. These clicks operate "more or less like a flashlight," says Elemans, "to search their environment with a very focused beam."

Toothed whales have a larynx but it doesn't produce sound. Rather, they evolved a "new structure that's located in their nose that generates the sounds — what's called phonic lips," says Elemans.

For decades, it's been hard to observe the phonic lips in action. The technology hasn't been up to the task and we can't yet observe the whales at the depths where they often feed. But Elemans and his colleagues developed several experiments to peek inside these animals. They report their findings in the latest edition of the journal Science.

First, they lowered an endoscope into the blowholes of a few trained, captive dolphins and porpoises. The small camera didn't need to go deep; it just had to get to where it could film the phonic lips at high speed. "And we show that there's definitely movement of the [lips] while they make echolocation clicks," summarizes Elemans.

For their next experiment, they needed animals that had recently died.

"That's really difficult," explains Elemans. "Typically when they die, they sink. So it's very hard to study their physiology because you don't have access to fresh tissue."

But Elemans and the others worked with marine mammal stranding networks, especially in Germany, to collect harbor porpoises that had died in the wild. Then then blew air across their phonic lips.

"What we've been able to show," says Elemans, "is that these phonic lips [are] not moved by muscle control like, for example, in cat purring. But instead, they're made just like a human voice by airflow. And that's a really striking parallel."

Additional experiments involving vocalization analysis and a kind of CT scan suggested that toothed whales likely have separate vocal registers that generate their numerous sounds, just like we do.

On top of that, the different registers have different functions. For instance, through audio recording the sounds of wild animals during their dives (by attaching acoustic tags to individual animals) as well as those of trained porpoises and dolphins, the research team determined that it's the vocal fry register that's responsible for echolocation in toothed whales.

"The strength of this work," says Kelly Benoit-Bird, Science Chair at the Monterey Bay Aquarium Research Institute, "is that it reconciles field observations of [toothed whale] sounds and laboratory studies of physiology with our understanding of the evolution of marine mammals to provide a clear, complete picture of how dolphins produce the wide repertoire of sounds that is critical for their survival."

Benoit-Bird, who wasn't involved in the study, points to the way the researchers tackled this scientific challenge from a variety of angles.

"This work took all the fragments of the puzzle, figured out exactly how they fit together, and filled in the gaps, finally making the picture of dolphin sound production clear," she says.

Agnese Lanzetti, an evolutionary biologist at the University of Birmingham who was not part of the research, agrees.

"This is the very best research that shows how the sounds are made mechanically," she says, "and to prove that these sounds are generated by air."

The physics of air in the bodies of toothed whales plays out differently than it does for us on land. When an animal like a sperm whale dives a few thousand feet below the surface of the water, its lungs collapse under the pressure. But inside the bony structure of the nose, air can continue to move around and power echolocation.

"By moving all the air into the nose," says Elemans, "these toothed whales are able to generate much higher pressures to drive the system. And with that, they can make basically the loudest sounds any animal can make on the planet."

And more importantly, feed themselves in the process — turning vocal fry into fish fry.

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