In a spider's world, visuals and vibrations reign. These arthropods keep an eye out – or eight, in the case of jumping spiders – for predators and prey. They also sense how close another animal is from how its movement vibrates the floor, wall, or web where the spider rests.
But arachnids might have another trick up their hairy sleeves: sound.
Scientists thought spiders were only able to pick up on noises within just a few feet of their bodies. But new research suggests one species of jumping spider, Phidippus audax, can actually hear sounds from as far away as 10 feet.
"These animals that we've lived with for the entire existence of humanity, there's now this new thing that we didn't know that they could do," the study's first author Paul Shamble, now a biologist at Harvard University, tells The Christian Science Monitor in a phone interview. "Now when you see one of these creatures in your house or in your garden, just know that they can hear you."
"We haven't rewritten any textbooks or anything like that," Dr. Shamble says. "But it seems like the underlying assumptions that a lot of biologists were using for guessing at how well these animals could hear don't apply."
Spiders don't have ears that pick up on changes in air pressure like mammals' ears, so how can the invertebrates even hear at all? Scientists have found that spiders are able to pick up on air movement associated with audible signals using specialized sensory hairs all over their bodies.
Scientists have long known that spiders can "hear" in this way, but they thought that the arachnids could only pick up on sounds made within about three feet. "This doesn't really seem to be true," Shamble says after studying the jumping spiders. "It seems that they can actually hear over much larger ranges" using these special sensory hairs.
Nathan Morehouse, a biologist at the University of Pittsburgh who was not part of the study, wonders how this newfound long-distance hearing ability might be combined with the surface vibrations scientists already knew the spiders could detect.
"It’s clear to me that these animals can detect relatively loud sounds from longer distances than we might typically expect," Dr. Morehouse tells the Monitor. But, he says, spiders are incredibly sensitive to vibrations through whatever they're standing on, be it a hard surface or even their web.
"Loud sounds will travel through air, but may also create vibrations in things like leaves on the ground," Morehouse explains. "So in natural environments, these animals might respond to both airborne and vibrational aspects of loud sounds in their environment."
Because spiders are so sensitive to vibrations, Shamble says the team did their best to eliminate that explanation. The lab at Cornell University, where the experiments were conducted, focuses on hearing in invertebrates so they already had special equipment to minimize vibrations, but the team also used a laser vibrometer to measure if any of the equipment was shaking and then adjusted the experimental setup.
And University of Cincinnati biologist George Uetz, whose own research has focused on spiders sensing airborne sound, points out in an email to the Monitor that the team "tested elongate sensory hairs in salticids [jumping spiders] and connected them to perception of airborne sound, evidenced by neural activity in the specific auditory brain region."
As such, Dr. Uetz says, these researchers "have added yet another answer to the question of spider 'hearing' - BOTH proposed mechanisms are correct. So, not only do some species 'hear' by perceiving airborne sound through substrate surfaces like leaves or webs, but others 'hear' through sensory hairs that detect air movement."
The new results are described in a paper published Thursday in the journal Current Biology.
A surprise discovery
Shamble and his colleagues stumbled upon the jumping spider's long-distance hearing when studying the eight-eyed sight of the animal.
"Our group was working on making recordings of the brains of jumping spiders," says Shamble, who was a graduate student at Cornell at the time. "This was something that hadn't been done before and we came up with a new technique to do it."
The experiment was set up so that a "pop" sound came out of the equipment whenever a neuron in the jumping spider's brain fired.
One day Gil Menda, one of Shamble's colleagues and an author on the new paper, had just set up the equipment when he scooted his chair back and it squeaked against the floor. Then he heard a "pop." Intrigued, he tried again. Another "pop."
That's when he called Shamble over and the pair began to discuss what they knew about spider hearing – that they could only hear noises close to their bodies.
To demonstrate, Shamble began clapping. He clapped right next to the spider and, "pop," a neuron fired.
"To make my point, I said, but you'll see that as I back up and keep clapping, it'll stop responding in just tens of centimeters, or maybe a meter," Shamble recalls. But a few minutes later he was standing three times that distance from the spider, clapping, and still there was that popping response.
"Everything we knew shouldn't have made it possible," he says. So the team designed an experiment and, sure enough, the spider was able to pick up sounds of 65 decibels from about three meters (or about 9.8 feet) away.
Why the spidey sense of sound?
One of the main predators for jumping spiders, and other spiders, are parasitic wasps. Shamble and his colleagues suggest that the spiders may be able to pick up on the sound of the predators' wingbeats, which matches the frequencies the animals can pick up on, according to the team's research.
Furthermore, "we found that when we played sounds to the spiders, they would freeze, and that's a common anti-predator response," Shamble says. Although the spiders, which are carnivorous predators themselves, could also use their hearing to locate prey.
With eight eyes and a 360-degree view of the world, jumping spiders are difficult to sneak up on, Morehouse explains in a phone interview with the Monitor. The animal has eyes on the back of its head, the front of its head, and the top of its head.
But the eyes on the top of a jumping spider's head are actually the weakest, Morehouse says. So predators might be able to surprise it from above more easily.
"If I were a jumping spider, I'd want to have some sort of backup system," Morehouse says. And perhaps that's why these jumping spiders evolved this long-distance hearing, he muses.
Or perhaps the hearing could have something to do with the animal's courtship dances, as Morehouse's own research suggests the males use vibrational songs to attract females.
Many questions remain, Shamble says. In addition to figuring out what function this long-distance hearing serves, scientists want to figure out if other types of spiders can do it too.
There are about 5,000 species of jumping spider but this new paper focuses on just one species, Phidippus audax, or bold jumping spiders. The tiny spiders range in size from one quarter of an inch to half an inch long and have powerful hind legs used to propel quick leaps and pounce on prey. Bold jumping spiders are common across North America, and are frequently spotted in fields, open woodlands, backyards, and gardens.
"This study is going to motivate a bunch of research into the sonic world of these animals which hasn't really been done before," Morehouse says.