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Baleen whales play a vital role in ecosystems. To communicate across vast distances and find each other, baleen whales depend critically on the production of sounds that travels far in dark oceans. However, since whale songs were first discovered more than 50 years ago, it remained unknown how baleen whales produce their complex vocalizations（发音）.

A new study in the journal Nature reports that baleen whales developed unique structures in their larynx（喉） that enable their low-frequency vocalizations, but also limit their communication range. The study was led by voice scientists Professor Coen Elemans and Professor Tecumseh Fitch. "The toothed whales and baleen whales were initially land animals that had a larynx serving two functions: protecting the airways and sound production. However, their switching to living in the water placed new and strict demands on the larynx to prevent choking underwater," says Tecumseh Fitch.

The study shows that baleen whales nevertheless can still produce sound with their larynx, but they have developed new structures to do so, which only exists in baleen whales. "This is probably to keep a rigid open airway when they have to move huge amounts of air in and out during explosive surface breathing," states Fitch. "We found that this U-shaped structure pushes against a big fatty cushion（垫） on the inside of the larynx. When the whales push air from their lungs past this cushion, it starts to shake and this generates very low frequency underwater sounds," says Elemans.

To understand how muscle activity could change the calls, the researchers built a computational model of the entire whale larynx. The model predicted the natural vocalizations of the whales very well. However, these newly discovered features that allowed whales to successfully communicate in the vast oceans also poses huge physiological（生理的） limits for many baleen whales. Combining experiments and models, the researchers provide the first evidence that baleen whales are physiologically incapable of escaping noise caused by humans, because it covers up their voices, and thus limits their communication range. "Unfortunately, the main frequency and depth of man-made noise caused by shipping traffic cover the frequency range and maximum communication depth of 100 meters that we predicted," Elemans says.

The first voice recordings of a certain whale song by Roger and Katy Payne in 1970 attracted global interest in sea conservation efforts. The Payne's made people aware how quiet the seas were before humans started the widespread use of machine ships. Elemans adds: "Compared to the seventies, our oceans are now even more filled with human-made noise, which affects the whales, because they are dependent on sound for communication. Now we show that despite their amazing physiology, they literally cannot escape the noise humans make in the oceans."