Hedgehogs can hear high-frequency ultrasound – that knowledge could help save them
The hedgehog is one of Europe’s most familiar and well-loved wild mammals. Many people encounter them in gardens, hear their snuffling at dusk, or glimpse their spiny shapes moving through the night.
But sadly, across Europe, hedgehog populations are shrinking rapidly. The European hedgehog is now listed as “near threatened” on the International Union for Conversation of Nature red list for Europe. Understanding why this is happening – and what can realistically be done to halt or reverse it – has become an urgent priority.
My team’s new research shows that hedgehogs can hear high-frequency ultrasound. With this knowledge, it could be possible to design sound-based deterrents that target hedgehogs specifically, without disturbing people or their pets. In theory, ultrasonic signals could warn hedgehogs away from approaching vehicles or keep them clear of dangerous machines in the future.
This is significant because one of the greatest threats to hedgehogs comes from road traffic. Cars are estimated to kill huge numbers of hedgehogs across Europe every year, with some studies suggesting that up to one in three of these animals may die on roads annually.
Hedgehogs are simply not built for modern infrastructure. Their primary defence strategy evolved to escape natural predators that see movement in the dark. They freeze, assess the threat, and then either flee or curl into a tight ball of spines. Against a fast-moving vehicle, this strategy is fatal.
Roads also fragment landscapes, making it harder for hedgehogs to find food, mates and new habitats. When this is combined with barriers such as massive fences, intensively managed agriculture and gardens, and widespread use of machinery such as garden strimmers and robotic lawn mowers, it becomes clear that the problem is not hedgehog behaviour. It is the environment humans have created.
Could sound offer a solution?
For years, I kept returning to the same question: could humans somehow warn hedgehogs before danger arrives? Could we deter them from roads and machines in a way that does not disturb people?
To explore that possibility, I had to start with a surprisingly basic question: what can hedgehogs actually hear?
I assembled a multidisciplinary team including experts specialising in imaging, bioacoustics (studying what animals hear), animal behaviour, hedgehog ecology, animal experiments and hedgehog anaesthesia.
Using high-resolution micro-CT scans of a hedgehog that had been put to death at a wildlife rescue centre for welfare reasons, the team built a three-dimensional model of the middle and inner ear.
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, enabling it to better process ultrasonic vibrations.
Ultrasound refers to sound frequencies above 20kHz, beyond the upper limit of human hearing. But anatomy alone is not proof. To confirm what hedgehogs could actually hear, we needed direct measurements. But how does one measure hedgehog hearing?
Measuring hedgehog hearing
We tested the hearing of 20 European hedgehogs using Auditory Brainstem Response recordings. Under brief anaesthesia, small electrode needles placed just under the skin of the hedgehogs, measured their brain activity while they were soundly asleep. During this time, we played sounds across a wide range of frequencies and pulses, and if the hedgehogs could hear it, their brain activity would indicate this. They were fit and ready for release back into the wild the night after their tests.
The results were striking. Hedgehogs heard sounds from around 4kHz up to at least 85kHz, with peak sensitivity around 40kHz – well into the ultrasonic range. Hedgehogs can therefore hear sounds that humans, dogs and cats cannot. This could be a game changer for hedgehog conservation.
This would, in theory, make it possible for scientists to use ultrasonic signals to warn hedgehogs away from approaching vehicles or keep them clear of potentially dangerous machines.
Many questions remain though. Which sounds are effective? Do hedgehogs become used to specific noises and start ignoring them? How far do ultrasonic signals travel?
Further research is now needed to design effective and beneficial sound repellents for hedgehogs, but this is a significant step forward. Perhaps the car industry could help fund this important research?
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Sophie Lund Rasmussen does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.