Nobody thought to check whether hedgehogs hear ultrasound. They're not echolocators. They don't hunt by sonar. They trundle through gardens eating slugs. The assumption was that their hearing, like most small ground mammals, topped out in the normal audible range.
Researchers at Oxford tested twenty rehabilitated hedgehogs using auditory brainstem response — a non-invasive method that measures electrical activity in the brain when sound is played. The hedgehogs responded to frequencies from four kilohertz up to eighty-five kilohertz, well into the ultrasonic range. Peak sensitivity sat around forty kilohertz, which is above the human hearing ceiling of roughly twenty kilohertz.
The anatomy explains it. Three-dimensional models of the hedgehogs' middle-ear bones reveal structures characteristic of ultrasonic hearing: very small, dense ossicles and a partially fused joint between the eardrum and the first bone in the chain. The fusion stiffens the ossicular system, allowing it to transmit high-frequency vibrations efficiently rather than damping them out. It's the same mechanical solution that echolocating bats evolved, arrived at by a different route.
The conservation application is immediate. Road traffic kills large numbers of European hedgehogs — they freeze rather than flee when headlights approach. Ultrasonic deterrent devices mounted on vehicles could emit frequencies that hedgehogs hear but humans and domestic pets don't, warning the animals before the car arrives. The same principle extends to robotic lawn mowers and garden strimmers, which kill hedgehogs in their nests.
The capability was always there. The ear bones were always that shape. Nobody measured it because nobody expected it, and nobody expected it because hedgehogs don't do anything that obviously requires ultrasonic hearing. The function may be passive — detecting the ultrasonic calls of prey insects, or the high-frequency rustle of predators. The application to conservation is a human afterthought. The hedgehog has been hearing frequencies we can't for millions of years.