We all know that humans cannot breathe under water, but have you ever tried to talk or even make a sound while swimming under the surface? This seems almost impossible as we need air to flow through our larynx to create the vibrations that transmit sound. To protect themselves from drowning, baleen whales evolved nasal and oral plugs to keep the water out of their respiratory system while diving and feeding. In addition, they have an air sac, which might recycle air from the lungs during vocalization, but it wasn't clear how they use air to produce sound in the larynx and whether the larynx was modified in some way to allow these residents of the deep to sing while cut off from the surface. Coen Elemans, from University of Southern Denmark, and a team of international collaborators investigated the structures of the larynges of several baleen whale species, to reveal how they use air to produce their enigmatic sub-marine moans.

The researchers obtained the larynges – the section of the windpipe that produces vocal sounds in most mammals – from three different species of baleen whales (sei, common minke and humpback whales) – and used high-resolution 3D X-ray scans to visualize the larynges’ internal structures. To test whether and how the vocal structures produce sound the team pushed air through each larynx and recorded the resulting sounds with microphones. In addition, the team built a computer simulation of the whales’ voice boxes to understand how air flows through the structure, how muscular contractions change the shape of the larynx to modify the sound, and how tissues in the structure vibrate to produce sound.

The team discovered that baleen whales have several novel laryngeal structures that have never been seen before: muscularized fold structures (which are unlike conventional sound-producing vocal folds and are known as the transverse arytenoid folds) that run along the arytenoid cartilage in the larynx; and a large wedge-shaped pad of fat attached to the cricoid cartilage. The team discovered that when the whales are calling, the arytenoid cartilage in the larynx rotates, closing the gap between the transverse arytenoid folds and the fat pat so that when air is forced out of the lungs through the gap between the two structures, mucus membranes covering them both begin to vibrate, producing sounds pitched at ∼42 Hz in the minke whale and ∼33 Hz in the humpback whale. And when the team took a closer look at the humpback whale's transverse arytenoid folds, they realized that they could also vibrate like conventional mammalian vocal folds in a voice box, probably extending their frequency range to 6000 Hz.

This study shows that baleen whales evolved unique structures in their larynges that enable them to produce sounds powered by air while living in the opaque ocean environment. And the team predicts that baleen whales can only produce calls lasting 15 s at a depth of 30 m because air is compressed under pressure and unable to provide sufficient flow through their larynges to produce long calls at greater depths. These findings are consistent with whale calls recorded in the ocean, suggesting that the majority of baleen whale communication is limited to shallower depths, meaning that whales may not be able to continue singing as they dive deeper to stop the sound pollution generated by sea vessels muffling their messages.

Elemans
,
C. P. H.
,
Jiang
,
W.
,
Jensen
,
M. H.
,
Pichler
,
H.
,
Mussman
,
B. R.
,
Nattestad
,
J.
,
Wahlberg
,
M.
,
Zheng
,
X.
,
Xue
,
Q.
and
Fitch
,
W. T.
(
2024
).
Evolutionary novelties underlie sound production in baleen whales
.
Nature
627
,
123
-
129
.