At least 200,000 species sing in ways that are silent to humans. Listen to one of them now

Some creatures are so quiet, they appear to make no sound at all.

When a male treehopper calls out for a mate, he shakes his abdomen 100 times a second to produce a low sound that vibrates through the stem of the plant he is standing on. While that sound is audible to other treehoppers — particularly the female treehoppers he's trying to attract — it's outside the frequency that humans can hear.

In Love and Rivals, the second episode of Secret World of Sound, we listen in on dime-sized treehoppers in conversation. But how did the crew manage to record the sound of a creature we can't even hear?

"There are at least 200,000 species out there singing in ways that are silent to us, and that's way more songs than if you add up all of the birds and the fish and the whales and the frogs out there," says Rex Cocroft, a biologist at the University of Missouri who is fascinated by creatures that only use vibrations to communicate.  

The treehoppers' inaudible songs made them a bit of a challenge for Cocroft study, but a specialized piece of equipment changed that. It's called a vibrometer, and it uses a laser to detect minute vibrations on the surface of an object. 

By measuring the vibrations on the stem of a plant and amplifying these sounds, he is able to listen in on male and female treehoppers. 

The discovery of these tiny treehopper sounds was irresistible to the Secret World of Sound team. Using an advanced vibrometer to capture the courtship duet between a male and female treehopper, Cocroft invites The Nature of Things viewers into a magical, not-so-silent world.

The variety of their calls stretches the imagination. In the video above, Cocroft describes what he hears from different species — one sounds like a "tap-dancing monkey," another like "an owl laughing at its own joke." 

Watch the video above for the full story.