I sit on the table in the backyard on a London summer evening staring up at the slowly darkening sky. I can hear voices floating gently over the fence from the neighbours, see the glow of warm light escaping from my window. I quietly lie on my back on our rickety picnic table and watch the flits of movement from the sky, disappearing almost as quickly as it takes to notice them. Tiny little bats, dipping and spinning as they catch insects in mid air.
All around me are noises which I can’t hear.
Animals that echolocate use ultrasound, which is above 20kHz. Human hearing registers frequencies between 20 Hz and 20 kHz. All those bats chasing insects are using frequencies above my hearing. And they are loud, somewhere between 50 and 120 decibels. For those that reach 120 dB it’s like standing next to a chainsaw and knowing it’s making a sound, but not being able to hear it. It’s like in His Dark Materials, where Will takes the knife and opens a knife to a parallel world. A layer has been drawn over the space in front of you, and it’s right there in front of you, all around you even, you just can’t access it.
Moths, of all things, can make sound. They evolved it in tandem with 50 million years of being eaten by bats. Some species of tiger moth (Arctiidae) click to alert bats of their toxicity. What’s more, one species from North America, Bertholdia trigona, have the ability to produce a stream of clicks that can jam the bats echolocation signal.
There are also a bunch of animals that produce sounds that are lower than human hearing. Lower sounds produce longer sound waves, which are less easily dissipated underwater, or through dense forests. The Okapi, a relative of the giraffe, which looks like a cross between a giraffe and a zebra, produce infrasonic sounds to communicate with their young, the theory is so predators can’t hear them. Elephants can hear infrasonic sounds of approaching thunderstorms. Hippos make sounds underwater that reach the surface in bubbles. Sumatran rhinos sing a song, much like whales. The rhinos recorded for the study were found to vocalise near constantly, even though in the wild they are solitary.
Sumatran rhinos are critically endangered, there are 30 mature adults left in the wild. Until recently, we didn’t know they sang. Think about all the things we don’t know, that we can’t hear, those worlds we can’t access. We could lose them without ever knowing that they existed.