Each Tree Species Has Its Own Sonic Qualities

The environment seems to shape human linguistic diversity at a regional scale. Productive areas support human cultural groups with smaller geographic ranges, favoring differentiation and high regional diversity of speech. What is true of the air is also true for water and solid matter. Each medium has its acoustic properties. Animals that live underwater or that communicate through wood or soil each find their own voices within the material properties of their homes. Marine mammals like humpback, bowhead, and right whales that feed in coastal waters therefore tend to vocalize at a higher pitch than those of open oceans such as blue and fin whales. Fish in these habitats sing to one another using highly repetitious pulses of knocks, buzzes, or whines, often pitched at the frequency least likely to be masked by the hiss and rush of water noise. Each pulse contains many frequencies and a distinct start and stop. The wide spectrum and repeated onset and offset of the calls increase the chance that the sound will be detected by mates and rivals in a challenging acoustic environment. Sonic communication in these species often happens only at very close range, after a mate or rival has been seen. Levels of background noise also seem to have shaped the hearing abilities of fish. Some have expanded the range of hearing into higher frequencies and evolved finer frequency discrimination.

Lay Your Burden Down

These species with excellent hearing, such as catfish, carp, and freshwater elephant fish, are mostly found in calm waters like sluggish rivers and ponds. To human eyes, the open ocean seems uniform. We might imagine this sameness penetrating all the way to the ocean bottom. Yet for sound, the ocean contains an invisible conduit, a passageway through which sounds can travel for thousands of kilometers. This deep sound channel is about eight hundred meters below the surface. When sound waves veer up or down, they are bent back into the channel by either warmer water above or denser water below. This watery lens transmits sounds across entire ocean basins, especially low sounds whose passage in water is unhindered by water’s viscosity. Whales take advantage of this channel, and their rumbling, moaning, throbbing calls were, until humans invented the telegraph, the only animal signals capable of crossing oceans. Sound also travels through solid matter, zipping through wood or rock ten or more times faster than in air. We use these waves in all our musical instruments, but these vibrating sheets and strings of wood, skin, and metal are designed to send their sounds to the air. For many other species, though, solid matter is the primary or only acoustic medium. All land invertebrates like insects and spiders sense vibrations through nerves in their external skeletons and, especially, in the soft tissues of their leg joints.

A Legitimate Viewpoint

Imagine if every human toe, foot sole, and finger were an ear. This is the insects’ world. They hear the vibratory energies around them through receptors on their body surfaces and inside their appendages. Most also use this ability to communicate. Spiders tap the ground with the feet, signaling to mates and competitors. These signals are usually inaudible in air but transmit with speed and clarity to the listening feet and limb joints of companions. Legs are, for these species, the organs of speech and hearing. Insects live in a parallel world of sound, running alongside the aerial sounds that we humans hear. Only recently has the magnitude and diversity of this soundscape of solids become known. By attaching electronic sensors to vegetation, scientists have discovered that up to 90 percent of insects communicate using some form of vibration through vegetation or the ground. My own initiation into this strange world of insect buzzes, squeaks, and clicks came when I was gathering recordings for an exhibition of tree sounds. I hooked a tiny sensor to a cottonwood twig, capturing the many tremors and bangs that flow inside the windblown tree.

Big Daddy of Them All

I sent the sound file to Rex Cocroft at the University of Missouri, a pioneer in the exploration and study of insect communication, and he confirmed that the sounds were of an insect, likely a leafhopper. For naturalists with an exploratory bent, the insect vibroscape offers fertile ground for discovery. Every plant species and part of a plant has a different physical character. Young leaves are soft and spongy. Mature twigs are brittle and stiff. Bark is a wide sheet, but a leaf petiole, the fine stem that holds the leaf, is a tube of dense material around a more open core. Each of these materials transmits vibrations in a different way, favoring some frequencies over others. We get a crude sense of this when we hear our neighbors in an apartment building. The hardwood floor of the people living upstairs filters out nearly all high frequencies, but acts as an excellent transmitter of midrange footsteps. These varied properties of plant matter are the sonic world in which insects live. Such differences have created sonic diversity in insect sounds, just as differences in vegetation have done for airborne bird and mammal sounds. Treehoppers in eastern North America offer a clear example of how physical differences in vegetation shape vibratory sounds. These diminutive relatives of cicadas suck fluids from tree leaves and stems using piercing mouthparts. A crest on their heads makes them look like little thorns. In the breeding season, male treehoppers whine and click, and females reply with lower grunts. This duet plays out entirely through tremors sent through leaves and stems. This diversity arose when ancestral species expanded their ranges to new host plants. Colonist treehopper species not only encountered new food when they switched to novel hosts, but their sonic environment changed. Treehoppers on wafer ash, another small woodland tree species, call much higher, about 350 hertz. The two varieties of the treehopper are the same size, and they stick with their song type even when plucked from one tree species and put on another. Each tree species has its own sonic qualities, transmitting some sounds better than others.