Scientists at the University of Oxford say ultrasonic sound devices might one day help reduce the number of hedgehogs killed by cars. Their idea stems from new research published March 11 in Biology Letters showing for the first time that hedgehogs are capable of hearing high-frequency ultrasound.
The European hedgehog is one of Europe’s most familiar wild mammals, yet its numbers have been falling sharply. In 2024, the International Union for Conservation of Nature reclassified the species as “near threatened.” Road traffic is a major contributor to these losses, with vehicle collisions estimated to kill as many as one in three hedgehogs in some local populations.
The new findings suggest that ultrasonic repellents might be used to discourage hedgehogs from entering roads and other dangerous areas. Until now, researchers did not know whether hedgehogs were able to hear sounds in this frequency range.
Lead researcher Assistant Professor Sophie Lund Rasmussen (Wildlife Conservation Research Unit/Department of Biology, University of Oxford and University of Copenhagen) said: “Having discovered that hedgehogs can hear in ultrasound, the next stage will be to find collaborators within the car industry to fund and design sound repellents for cars. If our future research shows that it proves possible to design an effective device to keep hedgehogs away from cars, this could have a significant impact in reducing the threat of road traffic to the declining European hedgehog.”
Testing Hedgehog Hearing
To investigate hedgehog hearing, researchers at the University of Oxford worked with collaborators in Denmark to measure the auditory brainstem response of 20 rehabilitated hedgehogs from Danish wildlife rescue centers. This technique records electrical activity between the inner ear and the brain using small electrodes placed on the animals while short bursts of sound are played through a small loudspeaker.
The measurements showed that the hedgehogs’ brainstems responded to sounds ranging from 4-85 kHz, with their strongest sensitivity around 40 kHz. This confirms that hedgehogs can detect ultrasonic sounds (which starts at frequencies greater than 20 kHz) and can hear frequencies up to at least 85 kHz. After the experiments, a veterinarian checked the animals and they were returned to the wild the following night.
Ear Structure Built for High Frequency Sound
The researchers also used high-resolution micro CT scans to examine the ear of a deceased hedgehog (which had been euthanized after being critically injured by a rat trap). Using these scans, the team created a detailed interactive 3D model of the hedgehog ear that revealed previously unknown features.
The model showed that hedgehogs possess very small, dense middle ear bones along with a partially fused joint between the eardrum and the first of these bones. This arrangement stiffens the chain of bones and allows it to transmit very high-pitched sounds more efficiently, a characteristic often seen in animals such as echolocating bats that can detect ultrasound.
The scans also showed that hedgehogs have a small stapes (the smallest middle-ear bone that connects the chain of ear bones to the inner ear’s fluid-filled cochlea). Because a smaller and lighter stapes can vibrate more rapidly, it can transfer high-frequency sound waves more effectively. Researchers also found that the cochlea itself is relatively short and compact, which helps it process ultrasonic vibrations.
Ultrasonic Repellers Could Reduce Hedgehog Deaths
Taken together, these findings indicate that ultrasonic repellents could potentially be designed so hedgehogs can hear them while humans and pets cannot.* If future research confirms that such devices are effective, they could be used to keep hedgehogs away from roads as well as other dangers like robotic lawnmowers and garden strimmers.
Dr. Rasmussen added, “Our novel results revealed that European hedgehogs are designed to, and can, perceive a broad ultrasonic range. A fascinating question now is whether they use ultrasound to communicate with each other, or to detect prey – something we have already begun investigating.”
Co-author Professor David Macdonald (Wildlife Conservation Research Unit/Department of Biology, University of Oxford) said: “It is especially exciting when research motivated by conservation leads to a fundamental new discovery about a species biology which, full circle, in turn offers a new avenue for conservation. The critical question now is whether the hedgehogs respond to ultrasound in ways that might reduce the risks of collisions with robotic lawnmowers or even cars.”
The research involved scientists from the University of Oxford, The Natural History Museum Denmark at University of Copenhagen, City Dyreklinik (Copenhagen), Aarhus University, Aarhus University Hospital, and the University of Southern Denmark.
* Humans hear in the range of 20-20,000 Hz, dogs 67- 45,000 Hz, and cats 45-65,000 Hz.