Hedgehogs can hear ultrasound, offering hope for conservation

Each night, hedgehogs roam through gardens, parks and fields looking for food. On these journeys, many of the small animals have to cross roads.

Sadly, large numbers are killed by cars every year. Scientists are now investigating a new idea that could offer them protection.

A recent discovery has shown that hedgehogs can hear extremely high-pitched sounds known as ultrasound. Researchers may be able to use the finding to develop warning devices that keep hedgehogs away from roads and other hazards.

The study was led by researchers at the University of Oxford and several European institutions. They investigated the hearing and ear structure of European hedgehogs, with results that could help experts protect the declining species.

Hedgehog numbers are falling

The European hedgehog is a familiar wild animal throughout much of Europe. People often spot hedgehogs visiting gardens or travelling through bushes at night, but the species faces serious difficulties.

In recent years, hedgehog populations have fallen sharply for several reasons. Road traffic is among the greatest threats, with many hedgehogs killed while crossing roads in search of food.

Scientists estimate that road accidents may kill up to one third of hedgehogs in some areas. The danger rises during the mating season, when hedgehogs travel farther to find partners and are more likely to encounter cars.

Other machinery poses a risk too. Hedgehogs hiding in grass can be injured or killed by garden strimmers and robotic lawnmowers.

Hedgehogs can hear ultrasound

Scientists have recently confirmed that hedgehogs can hear ultrasound, including sounds beyond the range of human hearing. People can usually hear frequencies from about 20 to 20,000 hertz.

The researchers found that European hedgehogs can hear sounds from about 4 to 85 kilohertz, and that their hearing is most sensitive around 40 kilohertz.

As a result, hedgehogs can detect many high-frequency sounds that are inaudible to humans.

What the hedgehog discovery means

Professor Sophie Lund Rasmussen, of the Wildlife Conservation Research Unit at the University of Oxford and the University of Copenhagen, explained the importance of the discovery.

“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,” noted Rasmussen.

“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.”

The ability could enable scientists to develop warning sounds that hedgehogs will avoid.

Testing hedgehog hearing

For the study, researchers tested 20 hedgehogs recovering at wildlife rescue centres in Denmark.

All had previously been rescued because they were ill, injured or abandoned. Once they had recovered, they were ready to be returned to the wild.

The scientists carried out an auditory brainstem response test, placing small electrodes on each hedgehog to record electrical signals between its ear and brain.

During the test, a loudspeaker played short sounds, while the recordings revealed the brain’s response to each one. The experiments were conducted at night, matching the hedgehogs’ natural period of activity.

Veterinarians examined every animal after testing, and the hedgehogs were released back into the wild shortly afterwards.

Structure of the hedgehog ear

The team also examined the structure of the hedgehog ear. Using high-resolution micro-CT scans of a hedgehog skull, scientists produced a detailed three-dimensional model of the ear.

The images revealed several features that help hedgehogs hear high-pitched sounds. Their middle ear bones, which transmit sound vibrations from the eardrum to the inner ear, are small and dense.

The cochlea within the ear is compact as well. This spiral-shaped structure helps process sound vibrations and, in hedgehogs, forms about 1.7 turns. Its design supports the detection of very high frequencies.

These features work together to enable hedgehogs to hear ultrasonic sounds.

Why hedgehogs may hear ultrasound

Scientists are still investigating why hedgehogs evolved this ability. One possible explanation is directional hearing: ultrasound may help them identify the direction of sounds more accurately.

Hunting offers another possibility. Some insects make ultrasonic sounds, which hedgehogs might detect when looking for prey.

Researchers are also examining whether hedgehogs use ultrasound to communicate. Many small mammals communicate using ultrasonic sounds that humans cannot hear.

“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,” noted Rasmussen.

A new tool for hedgehog conservation

Scientists believe the ability of hedgehogs to hear ultrasound could be used to help protect them.

Special devices might generate ultrasonic sounds that hedgehogs avoid, warning the animals before they reach roads or dangerous machinery.

People would not hear the sounds, and many pets would not notice them either. This makes ultrasound a useful conservation tool.

“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,” said Professor David Macdonald from the University of Oxford.

“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.”

Scientists will now investigate how hedgehogs respond to ultrasonic sounds. If the approach works, it could save many hedgehogs each year.