When night falls over the dense tropical rain forest of El Yunque in northeastern Puerto Rico, the air reverberates with animal vocalizations. “It can be deafening,” says Peter Narins, a vertebrate physiologist at the University of California, Los Angeles. “There are all kinds of things calling.”
But when he sticks his geophone in the ground to record the rumbles beneath the soil, he hears a very different sound track—one that’s much quieter, but far from silent.
It was by eavesdropping on El Yunque’s subterranean world that, in the early 1980s, Narins discovered that the rain forest’s white-lipped frogs could communicate by sending vibrations through the ground.
When white-lipped frogs call, they make a series of audible chirps about four times per second by inflating and deflating a vocal sac under their jaws. But because they often call with their rear ends buried in the mud and their heads and forelimbs just above it, when the vocal sac expands to produce a chirp, it hits the ground and produces a vibration.
The frequency of that seismic vibration was so well attuned to the sensitivity of certain sensory cells of the white-lipped frog that Narins had a hunch that the males were drawing some kind of information from the vibe. Having observed that the male frogs often paused their chirps when another male called, he wondered whether thumps alone might have the same effect. With a makeshift “thumper” that he built using typewriter parts, Narins vibrated the ground and found that the call patterns of nearby frogs would, indeed, change in response to thumps alone. “If you don’t put the thump in the ground, it won’t change the call pattern.”
Thirty years have passed since Narins first described seismic communication in the Caribbean white-lipped frog—the first time this type of communication was described in a vertebrate—yet it is still unclear what the frogs are using it for. “We think that males are communicating among themselves,” Narins says. “They’re spacing themselves out.”
While the field of seismic communication is still in its infancy, it has experienced a renaissance in the past decade, as more examples, particularly in smaller invertebrates, are being described and characterized. And the picture that is emerging suggests that many species across the animal kingdom rely on the tactile sensing of vibration as a crucial way of communicating and extracting information from the outside world.
“It’s not surprising that the majority of things that are small—things like spiders and insects—use seismic signals very often,” says Damian Elias, a behavioral ecologist at the University of California, Berkeley. “This is actually one of the most predominant ways of communicating across the animal kingdom.”
Elias studies seismic signaling in jumping spiders—a wildly diverse group of arachnids better known for their colorful and elaborate visual displays. Over the past decade, he has found that their vibratory signals are also incredibly complex and are generated by a variety of mechanisms that include drumming body parts against the ground, rubbing body parts together, and vibrating organs. “I would argue that they are among some of the most spectacular acoustic behaviors known in the animal kingdom,” Elias says.
The songs that male jumping spiders produce are not only beautiful, but they serve a vital purpose. When Elias glued males’ body parts together with wax to prevent them from making vibrations, females were less likely to mate with them, and more likely to eat them.
The same is true for other ground-dwelling spiders, such as wolf spiders. “This is one of the predominant modalities that they use in courtship,” says Eileen Hebets of the University of Nebraska-Lincoln. “If the seismic signal is absent, a male has almost no chance of successfully mating with a female.”
In fact, Elias believes eventually seismic communication will be described in the majority of insects. “It’s way easier for a small animal to vibrate the ground than to vibrate air,” he explains. It’s also a lot harder for a small animal to be heard above the din at a place like El Yunque. “Trying to have a conversation in that high level of ?background noise is a challenge for any animal,” says Narins, who currently studies seismic sensing in small mammals such as golden moles. In the ground, animals “have a communication channel they can take advantage of, which is quiet relative to the airborne channel.”
And such seismic conversations are not limited to the small creatures of the world. In the late 1990s, Caitlin O’Connell-Rodwell, a researcher at the Stanford University School of Medicine, suggested that African elephants could also communicate through seismic waves. While observing elephants in Namibia, she noticed that sometimes the animals would freeze, lean forward, keep their ears flat against their sides, and lift one of their front feet off the ground. “A whole bunch of them will be doing this mid-stride,” she says, “and it was always in conjunction [with] or just prior to the arrival of another group of elephants or some kind of seismic event, such as a vehicle approaching.”
O’Connell-Rodwell suspected that the elephants’ low-frequency calls were travelling through the ground as seismic waves. After a decade of characterizing the elephants’ low-frequency vocalizations, she was able to show that when she played vibratory antipredator calls to a nearby group of elephants, they would lean forward, freeze, bunch up into their family groups, and leave the area.
Now O’Connell-Rodwell is using a captive female elephant from the Oakland Zoo as a model to study the physiological and molecular basis of their vibrotactile sense, and compare it to that of humans and other mammals. Already, she has found that humans and elephants have the capacity to detect vibrations, although she suspects elephants may be more sensitive to them.
“There’ve been some interesting studies showing that people that are profoundly deaf or have hearing impediments actually use the auditory cortex to process vibrotactile signals,” O’Connell-Rodwell says. “So it’s not that we don’t use it, it’s just that we don’t think about it.” (See “The Pliable Brain”).