Wood Vibrations

Take a stroll through nature on a sunny day and you might hear a variety of birds calling, bees buzzing, goats yelling, and squirrels making squirrly noises. But what you probably won’t hear is this little guy talking to his friends:

“I really do have friends”. Tiny treehopper on a stem. (Photo found on Flickr Creative Commons)

Treehoppers (family Membracidae), are actually quite social. But instead of shouting, “Hey you!” or “Watch out for that spider!”, treehoppers talk to each other by vibrating the plants they live on, an almost otherworldy form of communication that some scientists have called a secret society of sound.

While vibrations might seem like a strange way of communicating, in fact many reptiles, birds, and even a few mammals (elephants included!), have been found using the same technique. A particularly large percentage of insect species communicate via vibrations as well, but treehoppers, small and dorkily camouflaged like thorns, are a fascinating group.

“oh hai”. Stylocentrus ancora sp. Family Membracidae. (Photo by Art, Flickr)

Whatcha doin’ over there? Treehopper on a stem. Family Membracidae. (Photo by Yogendra Joshi, Flickr)

Treehopper tremulatin’. Family Membracidae. (Illustration by Rachel Diaz-Bastin)

The vibrations this produces are quite low in frequency, often below the threshold of human hearing.

But fortunately, there are some wonderful people in the world, like Rex Cocroft, a researcher and professor at the University of Missouri, Columbia, who has found a way to tap into this vibrational soundscape. Using a hairclip, he is able to attach phonograph needles and laser vibrometers (which can measure vibrations based on reflected light…don’t ask me how!), to a plant. Once recorded, the signals can be played back as airborne sounds. So now, at long last, we can actually listen to some treehopper conversations.

Here’s the translation for those that don’t speak treehopper:

(Illustration by Rachel Diaz-Bastin)

 

But why vibrate when you can just make noise??! There are actually a number of advantages. Seismic waves don’t weaken as rapidly as air-borne sound waves and can therefore travel further (believed to be up to 16 km in elephants!). That’s handy for big animals that need to communicate over vast distances, but if you are very very small and find it difficult to produce loud airborn sounds, especially in a loud and squawky rainforest, it’s also super handy because even you can produce low-frequency signals capable of traveling the length of a plant or plant stem.

Buffalo Treehopper: Stictocephala bisonia, Family Membracidae. (Photo by Brad Smith, Flickr)

 

But do treehoppers really have much to say? Studies reveal that, yes! They do! Treehoppers use vibrations for a variety of reasons, such as attracting mates, or to announce the discovery of a good feeding site to their hungry compatriots. Baby treehoppers (nymphs), often feed and congregate together for safety. If they sense an approcahing predator, they will signal vibrational alarms, to which adult treehoppers respond by rushing to their defense.

“Halp!” Umbonia crassicornis nymph. Family Membracidae. (Photo by Carlos De Soto Molinari, Flickr)

 

Vibrational signals might also have the benefit of reducing predation risk, because the messages being sent around are directed along specific plant stem pathways rather than being broadcast throughout the forest. But of course nature is never that simple, and many predators (particularly other insects and spiders), can also sense the treehopper’s vibrations and may eavesdrop on them, most likely in hopes of making them their next meal, but possibly also to catch up on the latest treehopper gossip.

A beautiful treehopper: Platycotis vittata. Family Membracidae. (Photo by Matthew Cicanese, Flickr)

 A treehopper’s world is a hard place to imagine. Not only are they talking to each other most of the time via vibrational signals, they also live in a complex environment where natural vibrations from wind and rain and other animals are common, and perhaps quite noisy. The howling wind, the creaking of old branches, the thud of heavy raindrops against leaves, even the songs of birds and the pitter patter of insect feet on plant stems. Treehoppers can likely hear (or feel), all of it. But there is still much to learn, and study, about these ancient insects.

 According to Rex Cochroft, in a wonderful piece on the subject on NPR, “because forest leaves tremble, even with the sound that we make even when we speak, treehoppers have always been listening to us. We have just begun listening to them.”