Hedgehogs can hear ultrasound up to 85 kHz, and that could save them from cars

The European hedgehog can hear ultrasound. That sentence would not have been written with confidence before this month, because nobody had tested it. Now researchers at the University of Oxford, working with Danish collaborators, have demonstrated that hedgehogs respond to frequencies up to at least 85 kHz, with peak sensitivity around 40 kHz, well above the 20 kHz upper limit of human hearing.

The finding, published in Biology Letters, has an immediate practical implication. Road traffic is one of the leading causes of hedgehog deaths, estimated to kill up to one in three animals in local populations. The European hedgehog was classified as "near threatened" by the International Union for Conservation of Nature in 2024, and population declines across the continent have been steep. If hedgehogs respond behaviorally to ultrasound, car-mounted repellers could warn them off roads before a collision.

Twenty hedgehogs and a tiny loudspeaker

The team tested auditory brainstem responses in 20 rehabilitated hedgehogs from Danish wildlife rescue centers. Small electrodes placed on each animal recorded electrical signals traveling between the inner ear and the brain while short bursts of sound were played through a miniature loudspeaker.

The brainstem fired in response to frequencies across a range of 4 to 85 kHz. The peak sensitivity at approximately 40 kHz is notable because it falls in a range that is inaudible to humans (who top out around 20 kHz), largely inaudible to dogs (whose hearing extends to about 45 kHz), and below the upper range of cats (about 65 kHz). This means an ultrasonic repeller tuned to the hedgehog's peak sensitivity could potentially operate without disturbing humans or common pets.

All hedgehogs were checked by a veterinarian after the experiments and released back into the wild the following night.

An ear built for high frequencies

The researchers also performed high-resolution micro-CT scans of a deceased hedgehog's ear, building an interactive 3D model that revealed anatomical features never previously documented. The middle-ear bones are very small and dense, with a partly fused joint between the eardrum and the first bone in the chain. This stiffness helps transmit high-pitched sounds efficiently, a hallmark of animals that hear ultrasound, such as echolocating bats.

The stapes, the smallest bone connecting the chain to the inner ear's fluid-filled cochlea, is particularly small and light. A lighter stapes vibrates more quickly, enabling it to transmit high-frequency sound waves. The cochlea itself was found to be relatively short and compact, a geometry that favors processing ultrasonic vibrations.

Together, these features paint a picture of an ear that is structurally adapted for ultrasonic hearing, not merely able to detect it incidentally.

From hearing to behavior

The study establishes that hedgehogs can hear ultrasound. What it does not establish is whether they react to it in ways that would keep them off roads. Hearing a sound and being deterred by it are different things. The next critical step is behavioral testing: playing ultrasonic signals near roads or robotic lawnmowers and observing whether hedgehogs change direction, freeze, or flee.

Lead researcher Sophie Lund Rasmussen has called for collaborators within the car industry to fund and design sound repellents that could be tested in the field. If effective, such devices could also address other threats, including robotic lawnmowers and garden trimmers, which kill hedgehogs in suburban environments.

The researchers are also investigating whether hedgehogs use ultrasound for communication with each other or for detecting prey, questions that have already begun to be explored.

Unanswered questions

The sample of 20 hedgehogs, all from Danish rescue centers, is modest. Whether the hearing range varies across European hedgehog populations, or with age and health status, is unknown. The deceased hedgehog whose ear was scanned had been euthanized after a rat trap injury, and it is unclear whether this individual's ear anatomy is representative of the species.

Ultrasonic car-mounted repellers for deer have been marketed for decades with mixed evidence of effectiveness. The fact that an animal can hear a frequency does not mean a device emitting that frequency will reliably alter its behavior in the chaotic environment of a road at night. Field trials with rigorous controls will be essential before any conservation benefit can be claimed.

But the foundational discovery is solid: hedgehogs have ears designed for ultrasound, and they demonstrably respond to it at the neurological level. That is a starting point the conservation community can build on.