The European hedgehog is one of our best-loved mammals, but populations are in grave decline, with the species being newly classed as “near threatened” by the International Union for Conservation of Nature in 2024.
A major cause of hedgehog deaths is road traffic accidents, which are thought to kill up to one in three hedgehogs in local populations.
A new study from the University of Oxford now suggests that ultrasound repellers could be used to deter hedgehogs from roads, reducing the number killed by cars. Up to now, it was unknown whether hedgehogs could hear this 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.”
In the study, researchers at the University of Oxford collaborated with colleagues in Denmark to test the auditory brainstem response of 20 rehabilitated hedgehogs from Danish wildlife rescue centres.
This method uses small electrodes placed on the animals to record electrical signals travelling between the inner ear and the brain, while short bursts of sounds are played through a small loudspeaker.
The electrodes detected that the brainstem fired when signals were played across a range of 4-85kHz, with a peak sensitivity around 40kHz. This demonstrates that hedgehogs can hear in the ultrasound range (which starts at frequencies greater than 20kHz), up to at least 85kHz.
After being checked by a veterinarian after the experiments, the hedgehogs were released back into the wild on the following night.
The team also carried out high-resolution micro-CT scans of a dead hedgehog (which had been euthanised after being critically injured by a rat trap).
The scans were used to build an interactive 3D model of the hedgehog’s ear, revealing features never seen before.
The model showed that hedgehogs have very small, dense middle-ear bones and a partly fused joint between the eardrum and the first of these bones.
“This makes the whole chain of bones stiffer, helping it pass very high-pitched sounds efficiently – a hallmark of animals, such as echolocating bats, that can hear ultrasound."
The scans also revealed 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). A smaller, lighter stapes can vibrate more quickly, enabling it to transmit high-frequency sound waves.
The cochlea was also found to be relatively short and compact, allowing it to process ultrasonic vibrations more efficiently.
The results suggest that it would be possible to design ultrasonic repellents that can be heard by hedgehogs, but not humans or pets.
If proven effective, these could potentially be used to deter hedgehogs from roads and other potential threats, such as 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, 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.”
Image courtesy of Tine Reinholt Jensen