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). This groundbreaking study reveals that minke whales have a hearing sensitivity far greater than previously believed.
The findings suggest that baleen whales, the planet's largest mammals, may be even more affected by anthropogenic ocean noise than currently recognized. Historically, they have been excluded from regulatory considerations due to underestimated hearing ranges.
Concerns have been raised about the impacts of anthropogenic noise on marine mammals, particularly with high-profile whale stranding events linked to naval sonar noise activity. Consequently, significant progress has been made in developing criteria and thresholds for evaluating the effects of such noise on marine mammals.
Researchers have proposed using behavioral changes, hearing damage, and other physical effects as indicators of noise impact. However, setting clear thresholds has proven challenging due to the varied responses of different marine mammal species to noise exposure.
Audiograms, which illustrate an animal's hearing sensitivity, are crucial for identifying sound frequencies that affect marine mammals. While audiograms exist for at least one species in most major marine mammal groups, none have been created for baleen whales.
To better understand how ocean noise impacts baleen whales, researchers developed a novel catch-and-release method to temporarily hold adolescent minke whales (Balaenoptera acutorostrata) for auditory evoked potential (AEP) tests. These tests estimate hearing sensitivity by measuring electrical signals produced in the brain in response to sound.
Utilizing a natural channel between two islands in Norway and a system of net barriers, the researchers contained two adolescent minke whales during their northward migration. Contrary to the assumption that baleen whales are exclusively low-frequency hearing specialists, the study found that minke whales can detect frequencies as high as 45 to 90 kHz, a range much broader than previously understood based on their ear anatomy and vocalization frequencies.