Baleen whales evolved a unique larynx to communicate but cannot escape human noise

Baleen whales are the largest animals to have ever roamed our planet and as top predators play a vital role in marine ecosystems. To communicate across vast distances and find each other, baleen whales depend critically on the production of sounds that travels far in murky and dark oceans.

However, since whale songs were first discovered more than 50 years ago, it remained unknown how baleen whales produce their complex vocalizations -- until now.

A new study in the journal Nature reports that baleen whales evolved unique structures in their larynx that enable their low-frequency vocalizations, but also limit their communication range.

The study was led by voice scientists Professor Coen Elemans, at the Department of Biology, University of Southern Denmark and Professor Tecumseh Fitch at the Department of Behavioral and Cognitive Biology, University of Vienna in Austria.

"The toothed and baleen whales evolved from land mammals that had a larynx serving two functions: protecting the airways and sound production. However, their transition to aquatic life 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 evolved novel structures to do so, that only exists in baleen whales. First, the tiny cartilages in the human larynx -- called the arytenoids -- that change the position of our vocal folds, have changed dramatically in whales.

"The arytenoids changed into large, long cylinders fused at the base to form a large U-shaped rigid structure that extends nearly the full length of the larynx," Elemans says.

"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 on the inside of the larynx. When the whales push air from their lungs past this cushion, it starts to vibrate and this generates very low frequency underwater sounds," says Elemans.

Trying to work on the biology and particularly physiology of whales is very challenging.

"Even though humans hunted whales close to the brink of extinction, they made very little effort in trying to learn about their physiology," says Magnus Wahlberg, whale expert at University of Southern Denmark and co-author on the study.

"Strandings are unique and rare opportunities to learn about these amazing animals, but even then, it is very hard to study physiology, because the tissue decays so fast. Whales are known to explode on the beach," adds Wahlberg.

Thanks to Danish and Scottish Marine Mammal Stranding Networks, the researchers could quickly extract the larynx of a sei, minke and humpback whale for close investigation in the lab.

"Our experiments showed for the first time how the whales make their very low frequency vocalizations," says Elemans.

To understand how muscle activity could change the calls, the researchers built a computational model of the entire whale larynx.

"Our model includes accurate 3D shapes of the larynx and its muscles, which made it possible to simulate, for example, how the frequency is controlled through muscle modulation," say Qian Xue and Xudong Zheng, professors at the Mechanical Engineering Department at Rochester Institute of Technology, USA, co-authors on the study.

"Our model accurately predicted the results of our experiments, but we could also calculate acoustic features we could not measure in the lab, such as the frequency range," says Weili Jiang, postdoc at Rochester Institute of Technology, USA, co-author on the study.

The models predicted the natural vocalizations of the whales very well.

However, these newly discovered anatomical features that allowed whales to successfully communicate in the vast oceans also poses unsurmountable physiological limits for many baleen whales.

Combining experiments and models, the researchers provide the first evidence that baleen whales are physiologically incapable of escaping anthropogenic noise, because it masks their voices, and thus limits their communication range.

"Regrettably, the frequency range and maximum communication depth of 100 meters we predict, overlaps completely with the dominant frequency range and depth of human-made noise caused by shipping traffic," Elemans says.

"The first acoustic recordings of humpback whale song by Roger and Katy Payne in 1970 resonated with humanity profoundly, started the flourishing field of marine bioacoustics, and sparked global interest in marine conservation efforts." says Coen Elemans.

"These recordings were so politically important then that they are aboard the Voyager space missions," he continues.

The Payne's made people aware how quiet the seas were before humans started the widespread use of propeller ships and continuously running shipboard generators. Those were the seas whales evolved in.

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Sound reveals giant blue whales dance with the wind to find food

A study by MBARI researchers and their collaborators published today in Ecology Letters sheds new light on the movements of mysterious, endangered blue whales. The research team used a directional hydrophone on MBARI's underwater observatory, integrated with other advanced technologies, to listen for the booming vocalizations of blue whales. They used these sounds to track the movements of blue whales and learned that these ocean giants respond to changes in the wind.

Along California's Central Coast, spring and summer bring coastal upwelling. From March through July, seasonal winds push the top layer of water out to sea, allowing the cold water below to rise to the surface. The cooler, nutrient-rich water fuels blooms of tiny phytoplankton, jumpstarting the food web in Monterey Bay, from small shrimp-like krill all the way to giant whales. When the winds create an upwelling event, blue whales seek out the plumes of cooler water, where krill are most abundant. When upwelling stops, the whales move offshore into habitat that is transected by shipping lanes.

"This research and its underlying technologies are opening new windows into the complex, and beautiful, ecology of these endangered whales," said John Ryan, a biological oceanographer at MBARI and lead author of this study. "These findings demonstrate a new resource for managers seeking ways to better protect blue whales and other species."

The directional hydrophone is a specialized underwater microphone that records sounds and identifies the direction from which they originate. To use this technology to study blue whale movements, researchers needed to confirm that the hydrophone reliably tracked whales. This meant matching the acoustic bearings to a calling whale that was being tracked by GPS. With confidence in the acoustic methods established, the research team examined two years of acoustic tracking of the regional blue whale population.

This study built upon previous research led by MBARI Senior Scientist Kelly Benoit-Bird, which revealed that swarms of forage species -- anchovies and krill -- reacted to coastal upwelling. This time, researchers combined satellite and mooring data of upwelling conditions and echosounder data on krill aggregations with the acoustic tracks of foraging blue whales logged by the directional hydrophone.

"Previous work by the MBARI team found that when coastal upwelling was strongest, anchovies and krill formed dense swarms within upwelling plumes. Now, we've learned that blue whales track these dynamic plumes, where abundant food resources are available," explained Ryan.

Blue whales recognize when the wind is changing their habitat and identify places where upwelling aggregates their essential food -- krill. For a massive animal weighing up to 150 tonnes (165 tons), finding these dense aggregations is a matter of survival.

While scientists have long recognized that blue whales seasonally occupy Monterey Bay during the upwelling season, this research has revealed that the whales closely track the upwelling process on a very fine scale of both space (kilometers) and time (days to weeks).

"Tracking many individual wild animals simultaneously is challenging in any ecosystem. This is especially difficult in the open ocean, which is often opaque to us as human observers," said William Oestreich, previously a graduate student at Stanford University's Hopkins Marine Station and now a postdoctoral fellow at MBARI. "Integration of technologies to measure these whales' sounds enabled this important discovery about how groups of predators find food in a dynamic ocean. We're excited about the future discoveries we can make by eavesdropping on blue whales and other noisy ocean animals."

Background

Blue whales (Balaenoptera musculus) are the largest animals on Earth, but despite their large size, scientists still have many unanswered questions about their biology and ecology. These gentle giants seasonally gather in the Monterey Bay region to feed on small shrimp-like crustaceans called krill.

Blue whales are elusive animals. They can travel large distances underwater very quickly, making them challenging to track. MBARI researchers and collaborators employed a novel technique for tracking blue whales -- sound.

MBARI's MARS (Monterey Accelerated Research System) observatory offers a platform for studying the ocean in new ways. Funded by the National Science Foundation, the cabled observatory provides continuous power and data connectivity to support a variety of instruments for scientific experiments.

In 2015, MBARI researchers installed a hydrophone, or underwater microphone, on the observatory. The trove of acoustic data from the hydrophone has provided important insights into the ocean soundscape, from the migratory and feeding behaviors of blue whales to the impact of noise from human activities.

In 2019, MBARI and the Naval Postgraduate School installed a second hydrophone on the observatory. The directional hydrophone gives the direction from which a sound originated. This information can reveal spatial patterns for sounds underwater, identifying where sounds came from. By tracking the blue whales' B call -- the most powerful and prevalent vocalization among the regional blue whale population -- researchers could follow the movements of individual whales as they foraged within the region.

Researchers compared the directional hydrophone's recordings to data logged by tags that scientists from Stanford University had previously deployed on blue whales. Validating this new acoustic tracking method opens new opportunities for simultaneously logging the movements of multiple whales. It may also enable animal-borne tag research by helping researchers find whales to tag. "The integrated suite of technologies demonstrated in this paper represents a transformative tool kit for interdisciplinary research and mesoscale ecosystem monitoring that can be deployed at scale throughout protected marine habitats. This is a game changer and brings both cetacean biology and biological oceanography to the next level," said Jeremy Goldbogen, an associate professor at Stanford University's Hopkins Marine Station and a coauthor of the study.

This new methodology has implications not only for understanding how whales interact with their environment and one another but also for advancing management and conservation.

Despite protections, blue whales remain endangered, primarily from the risk of collisions with ships. This study showed that blue whales in Monterey Bay National Marine Sanctuary regularly occupy habitat transected by shipping lanes. Acoustic tracking of whales may provide real-time information for resource managers to mitigate risk, for example, through vessel speed reduction or rerouting during critical periods. "These kinds of integrated tools could allow us to spatially and temporally monitor, and eventually even predict, ephemeral biological hotspots. This promises to be a watershed advancement in the adaptive management of risks for protected and endangered species," said Brandon Southall, president and senior scientist for Southall Environmental Associates Inc. and a coauthor of the research study.

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