Tests uncover unexpected humpback sensitivity to high-frequency noise

Photo Credit: Mike Doherty

Scientific Frontline: "At a Glance" Summary

• Main Discovery: Humpback whales demonstrate unexpected sensitivity to high-frequency sounds, reacting to frequencies significantly higher than prior anatomical predictions suggested.
• Methodology: Researchers employed behavioural observation audiometry (BOA) over four migration seasons, broadcasting frequency-modulated upsweeps to migrating groups and recording behavioral changes such as course deviation or speed adjustment.
• Key Data: The study confirmed a hearing range extending from 80 Hz to 22 kHz, with specific reactions at the 22 kHz threshold proving sensitivity at the upper end of the human hearing range.
• Significance: This finding overturns the long-held assumption that baleen whales are exclusively low-frequency specialists and validates that wild-setting experiments can match the precision of captive studies.
• Future Application: These insights will refine strategies for mitigating human-induced noise pollution along migration routes, thereby enhancing conservation and protection protocols.
• Branch of Science: Marine Biology and Environmental Science.
• Additional Detail: The research generated the first data-driven audiogram for humpback whales, visually mapping their sensitivity across the tested frequency spectrum.

Image Credit: Courtesy of University of Queensland

University of Queensland hearing tests conducted across kilometers of ocean off the Sunshine Coast show humpback whales react to higher frequency sounds than expected. 

Associate Professor Rebecca Dunlop from UQ’s School of the Environment said the discovery has implications for the mitigation of noise-related human activity along whale migration routes. 

“Until our studies, estimates of humpback hearing sensitivity were inferred based on the anatomy of their ears,” Dr. Dunlop said. 

“In a series of experiments, we exposed whales to upsweep sounds at a range of frequencies. 

“We looked for changes such as slowing their swimming or turning towards or away from the sound as an indication they’d heard it. 

“Against a background of natural noise, we confirmed that the humpbacks’ best hearing range largely overlaps with the estimates from anatomical models. 

“But their responses at 22 kHz provided the first direct evidence humpbacks are highly sensitive to higher frequencies at the upper end of the human hearing range. 

“They are more sensitive to these higher frequencies than the anatomical models predicted. 

“This finding is significant because the baleen whale species – which includes humpbacks – had previously been considered low-frequency hearing specialists. 

“Our experiments also showed their ability to hear quiet signals within natural ocean noise matched data from hearing tests on captive toothed whales, indicating our study in a wild setting was able to mimic hearing experiments in a controlled environment.” 

The UQ team conducted the hearing tests across 4 whale migration seasons, 2021-2024. 

In a process called behavioral observation audiometry (BOA), frequency-modulated upsweeps were broadcast from a boat positioned in the path of a migrating whale group but too far away to be heard. 

As the group approached and came within hearing range, observers watched for deviations in the whales’ course or a change in speed and/or dive behavior to indicate the sound was heard. 

This was repeated for 8 to 10 different groups before the frequency of the upsweeps was changed, and the whole process repeated. 

“The whales typically responded by temporarily slowing down or speeding up and turning away to avoid the source vessel, and this behavioral change-point was noted,” Dr. Dunlop said. 

“Sometimes this was followed by heading directly towards the vessel and circling it or resuming their original course, so we know the experiments were a minor but important disruption to their behavior.” 

The results have been displayed in a graph called an audiogram, which shows how sensitive the animal is to each frequency. 

For these experiments, the most sensitive whale group for each frequency was used to produce the first data-driven audiogram for humpback whales. It shows their hearing range extends from at least 80 Hz at the low end to 22 kHz at the high end. 

“With a better understanding of humpback hearing, we can better protect them and minimize human interference on their lives,” Dr. Dunlop said. 

Published in journal:

1. Communications Biology
2. Current Biology

Title:

1. Humpback whale masked hearing thresholds in noise measured with modified behavioral observation audiometry
2. A predicted humpback whale hearing curve based on modified behavioral observation audiometry data

Authors: Rebecca A. Dunlop, Michael J. Noad & Dorian S. Houser (both papers)

Source/Credit: University of Queensland

Reference Number: mb012826_01

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