Researchers at the University of Oxford are investigating the idea that ultrasonic repellers could be fitted to cars to help reduce hedgehog deaths across the UK. The proposal is based on new findings, published in March in Biology Letters, which demonstrate for the first time that hedgehogs can hear high-frequency ultrasound.
Despite their beloved status, West European hedgehogs (Erinaceus europaeus) are in serious decline, listed as Near-Threatened by the International Union for Conservation of Nature after a drop in numbers of at least 30% over the past decade across much of their range. They’re classified as Vulnerable on the Great Britain Red List of Mammals. A National Hedgehog Conservation Strategy was launched in 2024 the by People’s Trust for Endangered Species (PTES) and The British Hedgehog Preservation Society (BHPS) to tackle the 30-75% decline in rural hedgehogs since 2000.
Factors behind the declines include habitat fragmentation, loss of hedgerows, intensive farming and pesticide use (reducing prey), but a major cause of hedgehog deaths is being run over by vehicles, which is thought to kill as many as 335,000 each year on British roads.
New hope for hedgehog conservation
In the new study, researchers at the University of Oxford collaborated with the University of Copenhagen to test the auditory brainstem responses of 20 rehabilitated hedgehogs from Danish wildlife rescue centres. Small electrodes were placed on the hedgehogs to record electrical signals travelling between the inner ear and the brain while short bursts of sounds were played through a small loudspeaker, revealing that hedgehogs can hear in the ultrasound range (which starts at frequencies greater than 20 kHz) up to at least 85 kHz. After being checked by a veterinarian, the hedgehogs were then released back into the wild.
Up to now, it was not known if hedgehogs could hear within the ultrasound range. “I was absolutely thrilled to learn that hedgehogs can hear high-frequency ultrasound,” says lead researcher Assistant Professor Sophie Lund Rasmussen from the Wildlife Conservation Research Unit/Department of Biology, University of Oxford and University of Copenhagen. “Equipped with that knowledge, we could potentially develop sound repellers targeted at hedgehogs that humans and their pets cannot hear, which would mean there’s a much greater chance they would be used.”
The results suggest it would be possible to design ultrasonic repellers that can be heard by hedgehogs, without disturbing people, dogs or cats - humans hear in the range of 20-20,000 Hz, dogs 67- 45,000 Hz and cats 45-65,000 Hz. If proven effective, ultrasound repellers could potentially be used to deter hedgehogs from roads when vehicles are approaching, potentially reducing the number killed by cars, as well as from other potential threats, such as robotic lawnmowers and garden strimmers.
Now the real work begins
There are major challenges ahead before that happens, including bringing the car industry onboard with development and testing. “To me, it’s a complete no-brainer for the car industry to support this research,” says Rasmussen. “I’ve successfully collaborated with the industry previously to create hedgehog-friendly robotic lawnmowers, so I know there’s potential for a good collaboration potentially benefitting hedgehog conservation. It’s my ambition to eventually have these sound repellers built into all new cars and to have any potential earnings from these go directly into financing future research to inform and optimise hedgehog conservation.”
More research is also needed to understand if other animals, such as bats and mice (who can also hear ultrasound frequencies), could be disturbed or driven away by ultrasound repellers on cars, or if hedgehogs could be permanently pushed out from gardens by lawnmowers and garden strimmers fitted with ultrasound repellers. It’s also not yet known if hedgehogs hearing ultrasound repellers on vehicles will curl up or freeze in place on the road, meaning they’re still run over, or move away or avoid crossing the road.
But the team is hopeful the study could be a useful first step. “Now that we’ve discovered what hedgehogs can hear, the real work starts,” says Rasmussen. “We will have to do comprehensive research to create the most efficient and unharmful sound repellers targeted at hedgehogs. If we manage to do that, this could be a game-changer for hedgehog conservation, potentially saving thousands of hedgehog lives.”
