The world is always abuzz with sounds, many of which human ears simply can't hear. However, other species have extraordinary adaptations that grant them access to realms of sonic extremes. And some of them don’t even have ears— at least, not like we typically imagine.
To understand how the animal kingdom’s best listeners do it, we need to know the rules of their game. When an object in a medium like air or water moves, it sends out physical waves. The basics of hearing involve structures that vibrate in response to these waves and excite sensory cells, generating signals that nerves transmit to the brain, where they’re processed.
But despite the assemblage of sound-absorbing and -amplifying structures in our ears, many noises are too quiet for us to detect. Owls, however, have some workarounds. Our external ears funnel sounds inward— but many owls use their whole faces to do this. Their ears, hidden beneath a flap of feathers, have eardrums proportionally much larger and more sensitive than humans’. And because many owls ears are positioned asymmetrically, sound waves reach them at different times. This slight delay helps their brains determine the direction of the sound’s source. And great grey owl wings have especially thick velvety coatings and long feather combs and fringes, which are thought to help reduce their flight sounds. So, while hovering, they can go undetected and concentrate on the subtle sounds of their prey. All these adaptations enable a great grey owl to hear a vole tunneling under 18 inches of snow— and make a fatal strike.
Other animals are almost all ears, like the aptly named long-eared jerboa, which is the animal kingdom’s largest ears in proportion to body length. These sizable sound-collectors help the jerboas sense low frequency noises— and keep cool by radiating heat. Fennec foxes use their large, swiveling ears to rapidly home in on activity beneath Sahara sands, while bat-eared foxes can pick up savanna sounds as slight as termites crawling and munching on grasses. Ogre-faced spiders, meanwhile, might not have ears in the traditional vertebrate sense, but their legs are covered by receptors sensitive to sound waves as soft as those generated by mosquito flight. This allows them to catch airborne prey— even after being blindfolded by scientists.
Lots of different features also help animal ears hit especially high notes, like the extra hard, stiff middle ear bones of toothed whales; like dolphins and sperm whales, which efficiently propagate high-frequency vibrations. Indeed, some toothed whales and bats emit sound pulses around 200,000 hertz and listen for the reflections. These high-frequency wavelengths— more than 10 times higher than what we can hear— are small enough to generate strong reflections from objects as tiny as the insects many bats are after, which would be missed altogether by lower ones.
But many insects are also in on the conversation— and vigilant to ultrasonic onslaughts. The greater wax moth can register the highest frequencies of any animal recorded— up to 300,000 hertz, thanks to thin, vibration-sensitive, eardrum-like membranes on their abdomens. In fact, hearing organs have evolved independently more than 20 times among insects. Katydids sense ultrasonic sounds with their front legs; certain hawkmoths can hear with their mouthparts; a parasitic fly registers cricket chirps from organs behind its head; and the praying mantis has just one hearing organ, which sits smack in the midline of its thorax.
But how low can animals go? Well, baleen whales emit sounds around 14 hertz, the deepest among mammals. These vibrations can travel thousands of kilometers. And they get picked up by other baleen whales— possibly via their skulls, which conduct the vibrations along to their ear bones. Snakes pick up ground vibrations by way of their jawbones, which connect directly to their middle ear bones. And Namib Desert golden moles regularly stick their heads into the sand, which likely helps them use their large, club-like middle ear bones to sense low frequency activity in mounds more than 20 meters away.
So, odds are: if a tree falls in a forest, someone’s bound to hear it.
