Hearing tests show that minke whales can hear high-frequency sounds

For the first time, scientists have directly measured the hearing range of minke whales, discovering that the species can detect high-frequency sounds as high as 90 kilohertz (kHz), according to a new study, demonstrating hearing sensitivity far greater than previously believed.

The findings suggest that baleen whales — the planet’s largest mammals — may be even more impacted by anthropogenic ocean noise than currently recognised but have been excluded from regulatory consideration due to underestimated hearing ranges. There have been concerns about the impacts of anthropogenic noise on marine mammals like high-profile whale stranding events linked to naval sonar noise activity. As a result, substantial progress has been made in developing criteria and thresholds for evaluating impacts on marine mammals exposed to anthropogenic noise.

Although researchers have suggested using behavioural changes, hearing damage, and other physical effects as ways to measure the impact of noise on marine mammals, setting clear thresholds is challenging because different species of marine mammals respond to noise in widely varying and poorly understood ways. Audiograms — graphs illustrating an animal’s hearing sensitivity — are essential for identifying the sound frequencies that affect marine mammals. However, while audiograms are available for at least one representative species in most major marine mammal groups, none exist for baleen whales.

To better understand how ocean noise impacts baleen whales, researchers developed a novel catch-and-release method to temporally hold adolescent minke whales (Balaenoptera acutorostrata) for auditory evoked potential (AEP) tests, which estimate hearing sensitivity by measuring electrical signals produced in the brain in response to sound. Leveraging a natural channel between two islands in Norway, and a system of net barriers, the researchers contained two adolescent minke whales making their northward migration.

While it has been assumed that baleen whales are exclusively low-frequency hearing specialists, the authors discovered that minke whales can detect frequencies as high as 45 to 90 kHz — a range much higher than previously believed based upon their ear anatomy and the frequencies at which they vocalise.