Things That Go Thump

Vibrational messages that we're just beginning to get

What in the world? The wails bleating out of the phone sound more like science fiction than science. Yet, on the other end of the line, entomologist Rex Cocroft swears that he’s playing a recording he made on this very planet. A whale with a very bad cold?

Elephants, which make low-pitched airborne sounds, are now under study to see if they pick up vibrations through the ground, as well. O’Connell-Radwell

A mother thornbug guards her young, ready to defend them whenever they vibrate a chorus of ch’s in unison. Cocroft

Leafhoppers fill the grass of a suburban lawn with a chorus of vibrations. Hunt

Nope. It’s a phone-friendly version of a treehopper insect’s call.

These particular wavering moos come from a Calloconophora pinguis nymph about the size of a rice grain. Cocroft decodes the youngster’s pulsed “mwooahhah . . . mwooahhah” as a signal that the little treehopper has found a good spot to insert its needlelike mouth parts and suck up sap.

Tell Cocroft that you’ve never heard anything like this, and he’s not surprised. That’s exactly his point.

This treehopper’s communiqué reaches its siblings not so much through vibrating air as through quivers in the plant where they live. Cocroft has recorded those vibrations and translated them into airborne sound waves for the benefit of vibrational dunces like Homo sapiens.

Unless something cataclysmically goes bump in the night, people just don’t get the message. That deafness to the shakes and shivers rippling through soil, grass blades, and twigs around us has delayed the study of a whole world of communication, claim Cocroft and other pioneers in seismic decoding. Even though we may not vibrate to one another, it’s starting to seem as if plenty of other creatures do.

At a symposium on vibrational communication in Chicago in January, speakers discussed seismic sayings of insects, frogs, burrowing mammals, and even elephants.

“There are thousands and thousands of vibrations out there,” Cocroft says. Now, all we have to do is figure out how to sense them.

Early descriptions

Some of the earliest descriptions of these vibrations came from biologist Frej Ossiannilson early in the 20th century. He would pop leafhoppers one-by-one onto a bit of a plant, fit the assemblage into a test tube, and hold it close to his ear. He detected enough tiny sounds to argue that this insect group should no longer be classified as mute. Trained as a violinist, Ossiannilson transcribed the leafhopper quivers onto pages and pages of musical scores, which he included in a 1949 treatise on the insects.

His work left unanswered the question of how the leafhoppers themselves perceive the sounds. Among subsequent experiments, some of the most elegant came from Toshihide Ichikawa and his colleagues during the 1970s. In one study, they placed a male leafhopper on one plant and a female on another. When the researchers nudged the two plants close enough to touch, the leafhoppers began a call-and-response duet.

Then, the researchers edged the plants apart, and the pair’s timing strayed out of synch, as if they could no longer hear each other. Solid-borne vibrations seemed more important than any airborne one.

Now that entomologists know what to check for, such courtship buzzings offer great opportunities to look for patterns in male-female communication, according to Randy Hunt at Indiana University Southwest in New Albany. At the symposium, he described his latest, not-yet-published work on a leafhopper called Graminella nigrifrons. The males advertise their charms by taking turns in a vibrational opus, somewhat like frogs croaking around a pond. To indicate interest, a female sends back a thump during one of the pauses in a male’s elaborate call. All the males then scramble to find her.

In his lab, Hunt is investigating why the males alternate their calls. He overlaps the calling of one male with a recorded male-female interchange, played through the plants where they perch. The calling male doesn’t respond to the recorded female’s thump, which doesn’t fall in synch with his song. The silent males, however, immediately begin their search. This suggests that in a cacophony of calling, males would miss their chance.

Disease alarms

Vibrations aren’t just for mating. Ongoing research on the dampwood termite Zootermopsis angusticollis, for example, has revealed vibrations that seem to be a novel disease alarm. Researchers released spores from a disease-causing fungus, and termites directly bombarded by the spores pounded out vibrations into the walls of the nest. Nestmates not yet exposed moved away from the source, reports Rebeca Rosengaus of Boston University.

Communication within insect groups intrigues Cocroft. The Costa Rican thornbugs that he works with, Umbonia crassicornis, grow up with a doting mom, who stands guard over her hundred or so young.

Cocroft found that when a predator looms, the young coordinate a series of pulsed vibrations which he translates for human ears as “ch” sounds. During this chorus, Mom clambers toward the attacker and rears her hind legs up, ready to kick. Cocroft’s experiments show that she responds to a coordinated chorus but not to random ch’s.

Mom talks, too. Cocroft found that when the danger has passed, she gives a long series of vibrations that he translates as clucks. The young then quiet down.

Insect nymphs may use vibrations to communicate among themselves, too, at least in the tropical treehopper C. pinguis. A cluster of young treehoppers moves to a new feeding ground several times before turning into adults and leaving the plant where they hatched. Cocroft found that a nymph going out prospecting begins sending vibrational moos when it settles into a new feeding spot. Other nymphs arrive, probe, and start mooing, too. Eventually, the whole cluster migrates.

“As far as we know, this is the first time anyone’s demonstrated a food call in insects outside of the truly social species,” Cocroft says.

Reptile rumbles

“We’re getting beyond just describing the phenomenon” in work on insect vibrations, says Cocroft. For other animals, though, researchers are still at work on finding the message.

Just 4 years ago, Cocroft helped document another vibrational first–a reptile rumble. Entomologist Ken Barnett was handling a veiled chameleon, Chamaeleo calyptratus, when he noticed a buzzing that seemed to come from just in front of the animal’s legs. Barnett enlisted Cocroft to help attach a vibration detector to a branch, on which they placed the chameleon. The chameleon, branch, and detector all just sat there mutely.

Then, the researchers eased a female chameleon onto the same branch. The male changed color. Attentively, he began edging toward the new arrival, and the detector came to life. It picked up 137 bouts of vibration from the male in little more than an hour.

The possibility of seismic communication in amphibians has interested Edwin R. Lewis of the University of California, Berkeley for decades.

He and Peter Narins, now at the University of California, Davis, set up equipment to tap the ground near some chirping white-lipped frogs in Puerto Rico and record their responses. The monitors picked up a surprise. “Instead of us thumping at the frog, the frog was thumping at us,” Lewis says.

In one of a series of vigils with the frogs, the researchers caught a rare glimpse of one vocalizing while half-submerged in water. The frog’s pouch exploded outward at the beginning of the sound and launched deep ripples. Lewis concluded that the thumps come from males who wriggle down into moist soil so their pouches give the ground a mighty whack as they start to call.

The researchers published their proposal that the frogs were communicating seismically–the first known instance in an amphibian–in 1985. At the recent Chicago symposium, Lewis described the challenges of taking the next step, figuring out whether the frogs really do pay attention to thumps. He’s had to devise some way to play soil-borne thumps without sending airborne signals, too, and he’s working on figuring out how to tell whether a frog is really responding. “It’s been a long haul,” he says.

Seismic communication may play a bigger part for animals that don’t gather for after-dark chirping. At the symposium, Narins and Lewis reviewed their work with Jennifer Jarvis in South Africa in the 1990s on the cape mole rat, which spends almost all its life tunneling underground, eating plants that it reels in by grabbing the roots.

During most of the year, an adult tunnels alone in its elaborate burrow, complete with kitchen and bathroom alcoves. Observations have revealed that when a solitary male seeks a mate underground, he drums his foot against the inside of his burrow. If he hears feminine drumming in response, he tunnels toward the sound until he breaks through the wall into the female’s burrow. After the pair mates, the male returns to his own tunnel and seals off the connection. A female tends her young until they’re old enough to tunnel away on their own.

To figure out how the mole rats detect the prenuptial thumping, the researchers dug their own tunnel and placed sensing equipment in it. When the researchers thumped in the real burrow nearby, microphones in the experimental tunnel failed to pick up airborne sounds loud enough for a mole rat to hear. However, sensors for vibrations in the ground registered clear signals. Getting the message over the distance of several meters between the animals’ tunnels would be more likely with a solid- rather than an airborne message, the scientists conclude.

Narins says he and a new researcher in his lab, Matthew Mason, are reopening Narins’ earlier work on some other mammalian seismologists, the golden moles of Namibia. “These guys are really impressive,” Narins says. Small enough to fit in a human hand, they venture above ground to forage perhaps 5 kilometers across the desert sands in a single night. Yet, they have no eyes capable of forming images.

Despite blindness, the moles hunt for live prey. In the desert where the moles live, 99 percent of life centers around hummocks of grass, and other researchers had assumed that the blind moles just scurried in random directions until they lucked into a teeming mound of grass.

Narins, Lewis, and their collaborators, however, thought the mole tracks looked too straight for rambling. Standing on grassy mounds, the researches calculated the chance that a random venture could locate nearby hummocks. “It was vanishingly small,” Narins says.

To demonstrate how a blind hunter might make a nonrandom approach, the researchers sank vibration detectors into the sand. From 15 to 20 meters away from a hummock, the detectors picked up what Narins describes as a low roar. “When the wind blows, the grass vibrates,” he says. As the detectors were moved closer to the mound, the ground vibrations increased to reveal the scratches and tickings of many busy creatures.

The scientists now plan to investigate whether the animals respond to ground vibrations.

The notion of vibration tracking fits older lore about golden mole behavior. A rambling mole stops every now and then, much as any hunter might pause to listen. But instead of cocking its head into the wind, the mole plunges headfirst into the sand. Sometimes a mole dives all the way beneath the sand and wriggles forward–one Namibian name for the creature translates as “sand swimmer.”

Elephant signals

The largest candidate for groundborne vibrational communications is the elephant. Scientists realized in the 1980s that airborne elephant signals rumble deep into registers around 20 Hertz. People can’t detect them, but elephants can. When an elephant’s vocal cords or stomping feet thunder such a low-frequency sound into the air, slower vibrations propagate through the ground, says Caitlin O’Connell-Rodwell of Stanford University. Could the elephants be detecting these vibrations over long distances?

Picking up ground throbs would fit elephants’ lifestyle, she points out in the December 2000 Journal of the Acoustical Society of America. Airborne signals from an elephant would travel up to 10 km. According to computer models of short-range measurements, the seismic waves from elephant vocalizations could remain detectable for some 16 km, and those from foot stompings, more than twice that distance. Since elephants scatter widely and travel over vast terrain, “these seismic signals could be useful for long-distance communication,” she concludes.

Of course, figuring out whether elephants actually pay attention to ground vibrations is proving a tricky problem. O’Connell-Rodwell and her colleagues looked for elephants’ gestures of attention, such as freezing in place or lifting a foot, not very precise signs. Elephants can react in complicated ways, though. O’Connell-Rodwell recalls one of the elephant handlers watching an experiment and exclaiming that the elephants “were just pretending they didn’t hear.”

Now, O’Connell-Rodwell plans to work with elephants trained to tap a target with their trunks if they detect certain signs. “We haven’t proven that elephants use seismic communication,” she says, “but maybe we’re just on the edge.”

From the infinitesimal to the elephantine, animals are perplexing vibration researchers with similar problems. Although the vibrating world may be new to people, other creatures have been buzzing and rumbling at each other for millions of years, mused several symposium participants. Difficult as the research is, maybe it’s about time for Homo sapiens to figure out what’s shakin’.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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