Their fascinating underwater songs are famous - but until now it was unclear how the baleen whales actually produce their sounds. An experimental study now shows that marine mammals have developed special larynx structures for this purpose. The baleen whales generate the sonorous vibrations from the combination of a U-shaped structure and a fat cushion. According to the modeling, the system can only generate a frequency range that lies in the range of man-made underwater noise, say the researchers.
They are mammals like us - and yet so different: Whales exert an enormous fascination on many people due to their gigantic size, mysterious way of life and their sad history of persecution. An acoustic component also contributes to the almost mystical image of the marine mammals: baleen whales communicate over long distances using underwater sounds. Some species produce complex songs with verses, which is why the vocalizations are called songs. “The first acoustic recordings of humpback whale songs by Roger and Katy Payne in 1970 were well received by humanity, founded the research field of marine bioacoustics and sparked interest in conservation efforts,” says first author Coen Elemans from the University of Southern Denmark Odense. As a result, the whale songs were even added to the recordings that left our solar system on board the Voyager probes.
How do baleen whales sing?
Despite this fame, the basis of the song - the mechanism by which the baleen whales communicate their communication sounds - has remained unclear. “Although the whales were hunted to near extinction, there was little effort to learn details about their physiology,” says co-author Magnus Wahlberg of the University of Southern Denmark. Until now, there was only basic information about the voice-generating structures and assumptions about how they function: Since baleen whales evolved from land mammals and breathe air, they still have a larynx, which serves to protect the respiratory tract and generate sound. But their way of life presented special challenges to this “instrument”.
In order to gain new insights into the sound production system of baleen whales, the researchers examined the larynxes of stranded whales: thanks to early information, they were able to remove the larynxes of a humpback whale, a sei whale as well as a minke whale and examine them in detail in the laboratory before the decomposition processes began. Endoscopic methods and scanners were used. The researchers also artificially caused the structures to vibrate: technically, they created air currents that were supposed to correspond to those in which animals naturally produce sounds.
It now became apparent that sound production is based on a concept that only occurs in baleen whales. Firstly, the tiny cartilages that are used in the larynx in other mammals and also in us to form the voice have changed significantly: “These so-called arytenoid cartilages transformed into large, long cylinders, which at the base became a large U-shaped structure that extends almost the entire length of the larynx,” says Elemans. Senior author Tecumseh Fitch from the University of Vienna explains: “This structure probably serves to keep the airways rigidly open when the animals have to move large amounts of air in and out again as they breathe in bursts on the surface.”
U-shaped structure and a fat cushion
“We found that the U-shaped structure presses against a large pad of fat on the inside of the larynx. When the whales push air from their lungs past this cushion, it creates the vibrations that cause low-frequency underwater noises,” says Elemans. However, the air is not released when producing sound underwater, but rather enters a special air bag. The researchers explain that it can then apparently flow through the larynx in the opposite direction and produce another sound.
To gain further insight, the team used the results to create a computer model of the baleen whales' sound production system. It included the three-dimensional structures of the larynx and its muscles and thus also enabled simulations of how different frequencies can be generated through muscle modulations. “Our model accurately predicted the results of our experiments and we were also able to calculate other acoustic effects,” says co-author Weili Jiang from the Rochester Institute of Technology. The modeling results also reflect the known frequency ranges of natural vocalizations in baleen whales, say the researchers.
However, as the team finally reports, it also revealed a disadvantageous aspect of the system today: the model shows that the baleen whales are physiologically unable to acoustically bypass the acoustic masking of their communication caused by the noise of ship's propellers and the like. “Unfortunately, the frequency range we predicted completely overlaps with the predominant frequency range of human-made noise,” says Elemans. “Our results show that, despite their amazing physiology, baleen whales cannot escape the noise that humans cause in the oceans,” said the scientist.
Source: University of Southern Denmark, specialist article: Nature, doi: 10.1038/s41586-024-07080-1