Brainwave testing of two young baleen whales has revealed they can hear higher frequency sounds than previously thought, forcing researchers to rethink how the ocean’s largest animals respond to predators and human noises.

“This is truly groundbreaking work,” says Susan Parks at Syracuse University in New York, who wasn’t involved in the new study. “Directly measuring the hearing of a wild baleen whale is something the researchers in the field have been working towards for decades… This is, to my knowledge, the first successful test of this method with a baleen whale.”

But baleen whales include the largest animals on Earth, and the study method of temporarily restraining them for a hearing test isn’t easy. “The body size of most baleen whales is too large for the approach to be effective,” says Dorian Houser at the National Marine Mammal Foundation, a nonprofit organisation based in California. So Houser and his colleagues turned to a relatively small baleen species called the minke whale.

The researchers examined the migratory route of minke whales along the coast of Norway and found a natural channel between two islands, where they used net barriers and boats to guide two whales – each about 3 to 5 metres in length – into a fish farm enclosure with a drop-down net door. Researchers then used a roller system to pull up a net and hold the teenage animals partially submerged at the water’s surface.

The hearing test involved placing two gold-plated electrodes with silicone suction cups on each whale’s skin near its blowhole and dorsal fin, which enabled the researchers to record brainwave signals. They measured how the whales’ brains responded to sounds played from an underwater speaker for about 30 minutes for one whale and 90 minutes for the other.

Such experiments revealed that the whales’ auditory brainstems respond to ultrasonic sounds, which are beyond those the human ear can detect, at frequencies as high as 45 to 90 kilohertz – a much broader hearing range than previously believed possible based on ear anatomy and vocalisations.

The corralling and restraining of wild marine mammals is “quite controversial” because of the potential for “significant stress” in the animals, says Oliver Boisseau at Marine Conservation Research, a nonprofit organisation based in the UK. But he described the findings as “very important” in helping understand how baleen whales may evade predators such as killer whales, which hunt using high-frequency echolocation clicks.

Researchers should also rethink how baleen whales are affected by military sonar and commercially available echo sounders used for mapping the seafloor, says Boisseau. “It seems the more we study the hearing of marine mammals, the more we confound our initial assumptions,” he says.