Human noise threatens the communication of baleen whales

Baleen whales, the largest creatures on Earth, have roamed the oceans for millions of years. Their symphony has long captivated humanity. These marine giants, such as blue, grey and humpback whales, rely on complex vocalisations for surviving in the vast, murky depths of the oceans. Their songs help them to navigate, locate peers, coordinate movements and find mates.

New research has now for the first time unveiled the evolution that has enabled these whales to communicate over such great distances: a uniquely structured larynx.

Formidable challenges

“The evolutionary transition from land to water reshaped the whale’s anatomy, especially the larynx, to enable vocalisation without the risk of drowning. Baleen whales developed unique laryngeal structures that are essential for their distinctive low-frequency calls. Nevertheless, this evolutionary achievement is threatened by human noise, because whale communication frequencies clash with the prevalent frequencies of human-made sounds, such as those from shipping traffic,” explains Coen Elemans, Professor, Department of Biology, University of Southern Denmark, Odense.

Since whale songs were first discovered more than 50 years ago, it had remained unknown how baleen whales produce their complex vocalisations. Even though people hunted whales to the brink of extinction, they made little effort to try to learn about their physiology.

“Despite increased recognition of the importance of their verbal communication, the mechanisms by which these whales were able to produce powerful sounds had been unknown,” says Coen Elemans.

This results from the challenges in studying these colossal creatures in their natural habitat.

“The challenges were formidable: their massive size, the difficulty in observing them in their natural settings and the rapid decay of tissue after death made the research particularly demanding,” explains Coen Elemans.

Really fresh animals

Despite historical whale hunting, little was known about their physiology until recent breakthroughs facilitated by marine mammal stranding networks enabled the larynx to be examined in sei, minke and humpback whales.

“Strandings are unique and rare opportunities to obtain experience about these amazing animals, but even then, studying the physiology is ridiculously hard, because the tissue decays so fast. Whales are known to explode on the beach,” notes Coen Elemans.

Thanks to marine mammal stranding networks in Denmark and Scotland, the researchers rapidly extracted the larynxes of sei, minke and humpback whales for close investigation in the lab.

“Getting these fresh animals was completely unique. And that has been crucial. And we were super lucky because two animals beached next to a harbour in a very densely populated country where we could get there fast – within two hours’ drive from my lab,” says Coen Elemans.

Life imposed new demands

A significant advance came with the development of a computational model of the whale larynx. Qian Xue, Xudong Zheng and Weili Jiang of the Rochester Institute of Technology played a key role. Their model includes accurate 3D shapes of the larynx and its muscles, which enabled researchers to simulate how the frequency is controlled through muscle modulation.

“The model successfully predicted the whales’ natural vocalisations and their frequency range, unveiling how baleen whales produce sound with evolved laryngeal structures, notably the transformation of arytenoids into large, cylinder-like structures forming a U-shaped structure that is vital for sound production,” explains Coen Elemans.

Coen Elemans, together with Tecumseh Fitch from the University of Vienna, focused on the larynx’s anatomical adaptations, necessitated by the transition from land to water.

“The evolution of the whale larynx is a tale of adaptation and survival. As descendants of land-dwelling mammals, the transition to aquatic life imposed new demands on the whales’ anatomy,” says Coen Elemans.

Cannot even see its own tail

In terrestrial mammals, the larynx has dual functions: protecting the airway during eating and facilitating vocalisation. How this organ adapted to meet the unique demands of underwater life, in which the risk of drowning and the physical properties of sound transmission differ markedly from transmission in air, was therefore a central question.

“The transition necessitated changes to the larynx, the organ that produces sound in mammals,” explains Coen Elemans. “From an evolutionary viewpoint, it is super interesting to see what adaptations are required to communicate.”

The baleen whale turns out to have a larynx that is completely different from that of other animals. It has no vocal folds in it anymore.

“It is just really hard for us to understand, but if they cannot make sounds, they cannot do anything. So, if you have ever snorkelled, you would know that the maximum visibility is 30 metres. A blue whale is bigger than that. It cannot even see its own tail. So, in most waters, for these animals to be able to communicate, sound is the only thing they have,” says Coen Elemans.

Measures urgently needed

Instead of the lost larynx, the whales have re-evolved other structures to make up for the loss.

“The study shows that novel structures evolved to produce low-frequency sounds that only exist in baleen whales. The arytenoids, small cartilages in the human larynx that adjust vocal fold positions, have transformed into large, long cylinders fused at the base in whales, creating a rigid, U-shaped structure,” explains Coen Elemans.

This adaptation is believed to help to keep a broad, open airway that is crucial for the whales’ forceful breathing at the surface and for producing sounds. Further, this arrangement works in conjunction with a fat pad located in the larynx that vibrates to produce the whales’ deep, resonant vocalisations.

“And so that is how they find each other. That is how they can mate and that is how they can reproduce and migrate, for example. This is important biologically for them. This research not only advances understanding of marine biology but also highlights the urgent need for measures to mitigate the impact of noise pollution on these marine inhabitants,” elaborates Coen Elemans.

We need to get out of there

By integrating experimental work and modelling, the researchers have presented groundbreaking evidence demonstrating that baleen whales cannot physiologically evade the impact of human-made noise.

“Compared with the 1970s, the oceans are now even more plagued by human-made noise from shipping lanes, drilling activity and seismic guns. We need strict regulations for such noise because these whales depend on sound for communication. Now we have shown that, despite their amazing physiology, they literally cannot escape the noise humans make in the oceans,” says Coen Elemans.

The noise effectively conceals their vocalisations, constraining their ability to communicate over distances. The frequency range of the whales combined with their maximum communication depth of 100 metres make the researchers predict complete overlaps with the dominant frequency range and depth of human-made noise caused by shipping traffic.

“They cannot make higher frequencies, they cannot be louder and they cannot go deeper than where we make the most noise. And then they stay, but they also cannot leave. We need to get out of there if we want to save them,” points out Coen Elemans, stressing the dire need for stringent regulations on ocean noise pollution to protect these marine giants.

“Only through concerted efforts to regulate noise pollution can we hope to preserve the natural symphony of the seas and ensure the survival of its most iconic composers, the baleen whales,” concludes Coen Elemans.

“Evolutionary novelties underlie sound production in baleen whales” has been published in Nature. The project was funded by the Carlsberg Foundation, the Austrian Science Fund and the Novo Nordisk Foundation.