Oxford researchers find ultrasound could help save hedgehogs

A sound too high for you to hear may one day help keep hedgehogs off the road.

That possibility comes from a new study in Biology Letters reporting that European hedgehogs can hear ultrasound, with their strongest sensitivity around 40 kilohertz. The finding matters because road traffic is thought to kill up to one in three hedgehogs in local populations. This adds pressure to a species the International Union for Conservation of Nature newly classified as near threatened in 2024.

The work gives conservation researchers something they did not have before: evidence that hedgehogs can detect frequencies above the range of human hearing. As a result, this opens the door to ultrasonic warning devices that could steer them away from cars, robotic lawnmowers and garden strimmers.

Lead researcher Assistant Professor Sophie Lund Rasmussen, of the Wildlife Conservation Research Unit in Oxford’s Department of Biology and the University of Copenhagen, said the next step is practical. “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.”

To test the animals’ hearing, researchers worked with 20 rehabilitated European hedgehogs from wildlife rescue centres in Denmark, including 10 males and 10 females. Fourteen were adults and six were recently independent juveniles. All had been treated for sickness, injury or orphaning and were considered fit for release when they entered the study.

The team used auditory brainstem response testing, a method that records electrical activity traveling between the inner ear and the brain while sounds are played. As part of the procedure, small electrodes were placed on the hedgehogs while short bursts of sound ranging sign from 4 to 85 kilohertz were delivered through speakers.

All 20 hedgehogs responded to sounds in that range. Seven adults stayed under anesthesia long enough for the researchers to collect full audiograms. These showed the animals were most sensitive around 40 kilohertz. The researchers said the species can hear at least up to 85 kilohertz. Possibly they can hear beyond that, since the tests did not go higher.

After the experiments, the hedgehogs recovered, were checked again, and were later released back into the wild.

The hearing tests were only half the story.

Researchers also examined the ear anatomy of a dead adult hedgehog using high-resolution micro-CT scans. The animal had been euthanized after suffering a severe leg injury caused by a rat trap, not for the purposes of the hearing experiment. From those scans, the team built a three-dimensional model of the ear.

What they found helps explain the hearing results. The middle-ear bones were small and dense, and part of the connection between the eardrum and the first ear bone was partly fused. That stiffness is a known feature in mammals able to handle very high-pitched sounds. The stapes, the smallest middle-ear bone, was also small and light. This made it better suited to transmit fast vibrations. The cochlea, meanwhile, was short and compact.

Taken together, those features suggest the European hedgehog is not merely capable of detecting ultrasound by accident. Its ear appears physically suited for it.

Dr Rasmussen said, “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.”

The conservation appeal is obvious. Humans hear roughly from 20 to 20,000 hertz, while dogs hear up to about 45,000 hertz. In addition, cats hear up to about 65,000 hertz, according to the source material. That means an ultrasonic deterrent above 65 kilohertz could, in theory, target hedgehogs without bothering people or common pets.

Still, the study did not test whether hedgehogs actually move away from those sounds in real-world danger. It showed they can hear ultrasound, not that a car-mounted repeller would work on a dark roadside.

That gap matters. The researchers noted several open questions, including which ultrasonic sounds hedgehogs would find unpleasant, whether they would get used to them over time, and whether a warning sound could reach far enough ahead of an approaching car to give the animal time to escape.

They also acknowledged a limitation in their data: only seven full audiograms were completed because many hedgehogs woke before the full testing sequence ended.

Moreover, the team used auditory brainstem response testing rather than behavioral hearing tests, a tradeoff they said was necessary for a wild, protected species that does not tolerate long-term captivity well.

Co-author Professor David Macdonald 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”.

This study gives wildlife researchers and engineers a starting point for designing hedgehog-specific warning systems.

If future tests show that certain ultrasonic sounds reliably make hedgehogs stop, turn, or retreat, those sounds could be built into devices for cars or garden machinery.

That would not solve every threat facing the species. However, it could offer a targeted way to reduce deaths from roads and other machines in places where hedgehogs are already in decline.

Research findings are available online in the journal Biology Letters.

The original story "Oxford researchers find ultrasound could help save hedgehogs" is published in The Brighter Side of News.

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