Biologist blends love of sound and insect communication
A thornbug treehopper (Umbonia crassicornis) female guards her offspring. They will signal her as a group if a predator approaches. Thornbugs can communicate using vibrations they "hear" through sensors throughout their bodies. Below, listen to sounds male treehoppers make to attract females.
In the summer, open your window and listen to the night. A chorus of chirps, whirs, buzzes and hums fills the air. And there’s even more that you’re not hearing.
Rex Cocroft hears music in the rainforest and in his own backyard — worlds of sound, entire systems of communication, all made by insects no larger than the tip of your finger.
Cocroft, a biologist, has been paying attention to the natural world since he was a young boy growing up on a farm in Virginia. While getting a bachelor’s degree in music, he volunteered at the Smithsonian. Then he spent four years working there as a research assistant developing a natural sounds collection to accompany the museum’s collections of frogs and reptiles; he contributed his own recordings of South American frogs.
Cocroft always planned on going back to school to study biology. He studied frogs in graduate school but then found a fascination with insects — the treehopper, to be precise.
Treehoppers measure between half and two centimeters in size, and there are more than 3,000 species. They often spend huge chunks of time quietly feeding on the branches and twigs of trees. Some spend their short lives (two to three months) typically living on one species of plants; others feed on many different species of plants. They live in Missouri, the rainforest and a lot of other places.
Studying the same species of insects in both the wild and the lab allows Cocroft to investigate cause and effect in the bugs' communication.
Vibration or airborne sound?
Treehoppers communicate for a variety of reasons, including enticing a mate or avoiding a predator. They "hear" through a complex set of sensors on their body. Do they perceive airborne sound? Biologists don’t know for sure, but they do know the creatures perceive vibration in their bodies.
Treehoppers have a sensor below the knee and another one in the femur so that, when the leg moves up and down, the structure stretches and nerves fire. Treehoppers also can detect when various other parts of their bodies are moving.
"They are just bristling with sensors really," Cocroft says.
The treehopper’s sounds can’t be heard with the ear. Hearing them requires special instruments — small leaf-mounted microphones, lasers that convert light movement into sound and phonograph cartridges — depending on the environment.
The insects can differentiate between vibrations made by their species and others through rhythm and pitch. "Many species of treehoppers live in groups, and the kind of communication that takes place within the group depends on the ecology of the species," Cocroft says.
In the case of a predator, real or simulated, closely-related species of treehoppers that have maternal care of offspring create a collective warning vibration that in turn initiates a wavelike movement of the insects on the stem of a plant. The offspring produce the signal, which involves a simultaneous vibrational "chirp" and a rocking motion. A few individuals closest to the predator signal first, and then signaling spreads in a wave through the group.
Evolution and aesthetics
A buffalo treehopper (Stictocephala bisonia) feeds on the stem of a plant. The treehopper's communication differs from species to species.
The treehoppers on a redbud differ from those on an elm — in communication and genetics. Does communication play a role in speciation? Quite possibly, Cocroft says. Treehopper communication can provide clues to evolution.
Male and female treehoppers signal to one another as part of the mating process. Some aggressive males use signals to "jam" the communication of another male with a female.
"The communication that precedes mating is the most fun to work with from the aesthetic standpoint. Sexual selection is an evolutionary force that’s extremely creative and goes in lots of unpredictable directions," Cocroft says. "Even closely related species — which you may not be able to tell apart, and which may be genetically very similar — often have extremely different mating signals, showing that the signals are very rapidly evolving."
Cocroft says that although insect signals might not be as attractive to human ears as bird songs, insect sounds can be compelling: "We seem to share enough aesthetic sense that we can also find them very beautiful."
Even with an appreciation for insect sounds, no one, Cocroft says, has any idea what it’s like to be an insect.
"They’re not really any simpler than mammals; they’re just really different," he says.