Secret of hedgehog hearing discovered at far beyond human range

Hedgehogs may be communicating with more than just snuffles, grunts and snorts, according to scientists who have discovered the animal's hearing range.

The creatures hear sounds at a much higher frequency than humans, meaning they could be signalling to each other with noises that we simply cannot hear.

The discovery could be a "game-changer" for the animal which is in decline in the UK and Europe, according to Dr Sophie Rasmussen at the University of Oxford.

It could help scientists work out how to protect the animals from stressful noises when in rescue centres, or develop sound repellants to deter hedgehogs from going near machines that can kill them, such as cars, lawnmowers or strimmers.

"I wanted to investigate whether it would be possible to make targeted sound repellers for hedgehogs, because they are declining all over Europe. So we set out to measure the hearing of the European hedgehog for the first time, " says Rasmussen.

Working with bio-acousticians who measure hearing in animals, she developed a soundtrack of pulses and beeps to play to anaesthetised hedgehogs from a rescue centre.

The researchers measured the animals' brain responses to pinpoint the frequency of sounds they could hear.

"We discovered, to my great surprise and joy, that hedgehogs can hear up to 85kHz" Rasmussen said.

Humans can hear up to 20kHz, and all sounds above that are called ultrasound. Cats hear up to 45kHz, while dogs' range extends to 65kHz.

After building a 3D model of a hedgehog's ear, the scientists found it has a stiff chain of bones that allows high-pitched sounds to pass through efficiently, in a similar way to echolocating bats.

"It's amazing because this allows us to actually build targeted sound repellents that wouldn't bother us or our pets," she says.

Thousands of hedgehogs are thought to die in road collisions in the UK and other European countries each year, and the animal is classed as near-threatened by the International Union for Conservation of Nature.

Rasmussen hopes to work with companies to understand if audio devices could be designed that would repel hedgehogs from cars, lawn mowers or strimmers.

Some cars already have devices fitted to deter large animals like deer.

The Society of Motor Manufacturers and Traders (SMMT) says that ultrasonic whistles are "widely available" for consumers to buy.

"Although this study suggests they could offer greater protection for hedgehogs on our roads, more detailed research is needed to understand their effectiveness," SMMT Chief Executive Mike Hawes told BBC News.

Rasmussen says more research is needed to understand which specific noises would put off hedgehogs.

"Are they scared of screams or should it be like a pulsating sound?" she says.

It would also be important to make sure the animals were not dissuaded from gardens, which have become an essential habitat as the countryside becomes less suitable.

Rasmussen says the work opens up our understanding of the hedgehog's audio world.

Thousands of hedgehogs are taken into rescue centres in the UK every year where they could be exposed to stressful sounds such as machinery humming that humans cannot hear.

The hedgehogs could also be communicating.

"When hedgehogs pass by each other, you can tell that they are interacting. Maybe that's just chemical reactions to smelling each other," she says.

"But imagine that they're actually blabbering all the time and we just couldn't hear it," she says.

The research is published in Biology Letters.