Porpoises are entirely dependent on their hearing for survival. They navigate, hunt, and communicate by emitting rapid click sounds and listening to the returning echoes. However, with increasingly noisy oceans, it is getting harder for porpoises to "hear their way." Noise from shipping is a particular problem. While ship engines primarily emit low-frequency noise, they also produce high-frequency sounds that can drown out the porpoises' own clicks. These clicks are sharp, brief, and only travel limited distances, making them highly vulnerable to noise sources in their immediate vicinity.
A new study from the Department of Biology at Aarhus University, published in The Journal of the Acoustical Society of America , provides new insight into the function of the hearing system of porpoise's auditory system faced with acoustic disturbance. To the researchers' surprise, the study shows that porpoises are not entirely defenseless against loud sounds.
Kristian Beedholm, a biologist and research programmer at Aarhus University and lead author of the study, explains the findings. He was responsible for the software used to record the porpoise's brain response after exposure to noise.
"A porpoise can likely 'dial down' its hearing sensitivity when it anticipates noise. This functions as a natural hearing protection mechanism and is a vital defense in an increasingly noisy marine environment," says Kristian Beedholm.
Although the brain response is clear, the researchers still need to uncover the precise processes in the guinea pig’s inner ear.
“It’s complicated to look deep into the hearing system of a living animal, so we actually don’t yet know exactly how it works in practice,” says Kristian Beedholm.
Freja the Porpoise Participates in the Study
The study was conducted with Freja, a harbor porpoise at the Fjord & Bælt research center in Kerteminde in Denmark. Freja is trained to remain still in the water, and during the trials, two small electrodes were attached via suction cups to her back: one near the blowhole to record brain activity, and one near the dorsal fin.
First, she was stimulated with echo-like clicks in silence; then, she was exposed to 20 seconds of intense noise. Using the Auditory Evoked Potential (AEP) technique, the researchers could measure changes to her brain's response in real-time before, during, and after the noise exposure.
"Since a porpoise cannot tell us what it hears, this is an effective method to investigate how the auditory system reacts to both clicks and noise as it happens," says Kristian Beedholm.
Previous studies often tested hearing with long, steady tones—similar to testing human hearing with a single long beep. However, porpoises use short, sharp clicks for echolocation. The new study used a faster AEP method to measure the brain’s reaction to these clicks against a background of realistic noise levels, providing a more accurate picture of the porpoise’s sensory reality.
Ship Traffic Causes Masking at Great Distances
"The goal was to determine how high-frequency ship noise affects hearing -specifically if it makes it harder for the animal to detect echoes from its own clicks," explains Beedholm.
When noise occurs in the same frequency range as the sounds an animal is trying to perceive, it can drown out the signal. This is known as masking. The researchers measured the brain's response to clicks at around 125 kilohertz (the porpoise's primary frequency) while playing noise at that same frequency.
The results showed that even at relatively low levels, noise can mask the clicks used for orientation. This masking occurs at ranges of several hundred meters from vessels with cavitating propellers.
"When noise interferes with echoes, the porpoise’s 'acoustic image' becomes blurred. If a speedboat zooms past, the porpoise may lose its orientation and the fish it was chasing—or worse, fail to detect a fishing net and become entangled," says Beedholm.
No Permanent Damage Despite High Exposure
Intense noise can also cause Temporary Threshold Shift (TTS), where hearing remains impaired even after the noise stops. TTS is often used as the benchmark for how harmful noise is to marine life.
"Science is currently divided on this issue. Laboratory results vary so widely that it is difficult to establish a precise limit for when a porpoise's hearing suffers permanent damage," says Beedholm.
To test this, the researchers exposed Freja to noise levels reaching 147 dB, similar to being just tens of meters away from a passing vessel. Remarkably, they found no evidence of TTS. In fact, hearing sensitivity appeared slightly better immediately after the noise stopped—the exact opposite of the expected impairment.
"This is likely because porpoises only remain near a noise source briefly, and because they can actively reduce their hearing sensitivity to protect themselves," explains Beedholm.
"The fact that they dampen their sensitivity while anticipating the noise means their hearing acuity is maintained—or even appears improved—once the noise stops. This result was quite mysterious to us at first," says Beedholm. He hopes the study leads to a more acoustically porpoise friendly marine environment.
"Even if they can protect their hearing to some degree, ship noise still masks their most important senses. Our current criteria for 'acceptable' noise levels are likely too high. I hope this study contributes to better protection for porpoises in Danish waters."
The Journal of the Acoustical Society of America
Experimental study
Animals
Moderate levels of high-frequency noise mask harbor porpoise hearing, but do not cause temporary threshold shift
3-Dec-2025