The sudden sound of chair squeaks in a research lab tipped off researchers to a new world of eavesdroppers.
Unlike people and many other animals, spiders lack eardrums. But that doesn’t keep them from hearing sounds moving through the air. These animals pick up on vibrations humming through solid objects, like web silk and leaves. To do this, they rely on super-sensitive hairs on their legs.
Biologists had assumed, however, that spiders would only “hear” such nearby sounds — ones from no more than a few centimeters (an inch or so) away.
They were wrong.
Nerve cells in the brain of a tiny jumping spider can pick up airborne sounds from at least three meters (almost 10 feet) away, reports Ronald Hoy. A neurobiologist, Hoy works at Cornell University in Ithaca, N.Y.
The spider that his group studies is known as Phidippus audax (FID-dih-pus AW-dax). Hoy’s team had inserted tiny electrodes into the spiders’ brains. When somebody accidentally scraped the floor while moving a chair, the spider responded. Or at least its brain did. The sound provoked a burst of activity in the brain’s nerve cells. Called neurons, those cells were located in an area known to work on processing sound.
This surprised the scientists. Indeed, it prompted them to probe that sound connection further.
Those experiments showed that spiders could hear even relatively quiet sounds — similar to human conversation — and from several meters (yards) away!
The researchers also tested the spidey hearing in a different way. They broadcast a low droning noise. It resembled the wing sound of an approaching predatory wasp. In an instant, the spiders hunkered. They also ceased all movement. Clearly, they heard the sound and were on the alert.
The team reported its findings online October 13 in Current Biology.
A need for steady hands
Jumping spiders have brains about the size of poppy seeds. So inserting an electronic probe to record activity within the tiny part related to sound was not easy. Hoy credits the probing’s success to Cornell coauthor Gil Menda. He has rock-steady hands.
First, Menda says, “I close my eyes.” Then he listens his way along, one slight nudge of the probe at a time. He’s trying to find the sound-processing cells. He knows he’s on the right track if faint popping sounds come out of the monitor. It’s set to translate the nerve activity into sounds that people can hear. As the probe gets closer to the right place, the popping gets louder.
When Menda first realized a spider’s brain reacted to a chair squeak, he and study coauthor Paul Shamble started clapping their hands. (Shamble is now at Harvard University.) Then they backed away from the spider and clapped again. The claps didn’t seem as if they should have made the furniture shake, creating vibrations that the spiders feet would pick up. Yet the spider’s brain did register the sound — even when the scientists had backed even further away, clapping from the hallway.
It was possible, though, that such clapping or other test sounds might confuse any interpretation of the tests by slightly vibrating the test equipment holding the spiders. In that case, the spiders might have “heard” through their feet, not by sound waves ruffling their leg hairs.
To test that possibility, the researchers placed the spiders on a table protected from vibrations. And still the spiders heard the sounds — this time, clearly, through the air.
Then the scientists broadcast various other airborne sounds. And neuron tests showed the spiders didn’t seem to hear all of them equally. The spiders were really sensitive to low-pitched sounds.
Jumping spiders are sensitive to predator’s vibe
Then the researchers took this special setup to the lab of coauthor Ronald Miles. He’s at the State University of New York at Binghamton. They placed their spiders into a chamber that blocks out nearly all outside noise. “It’s really eerie,” Hoy says of this quiet.
Whether a sound is high or low is referred to as its pitch. And that pitch depends on the length of the sound’s waves. Scientists measure the length of waves in hertz (or cycles per second). Jumping spiders picked up rumbly tones in a narrow range. They seemed to hear best those sounds between 70 and 200 hertz, Hoy and his colleagues report. People hear best between 500 hertz and 1 kilohertz. (One kilohertz is equal to 1,000 hertz.) But people can detect tones across a far broader range than that — from 50 hertz to 15 kilohertz.
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Hoy and his colleagues suspect spiders hear low rumbles, such as a wasp’s drone, much as they do the vibrations of a web: with specialized leg hairs. And their experiments support that idea. They found that making a hair twitch could cause a sound-responsive neuron in the spiders’ brains to fire.
Hoy says the benefits of hearing an approaching wasp might have favored the evolution of what these spiders hear. Indeed, they have good reason to be alert to these flying predators. Wasps carry off jumping spiders and immobilize them with venom, Hoy says. A mother wasp then tucks an unmoving — but still alive —spider into each cell of her nest. Eventually, a wasp egg will hatch in the nest and feed on fresh spider flesh.
Wasps are major predators of many kinds of spiders, notes Ximena Nelson. An expert in jumping spiders, she works at the University of Canterbury in Christchurch, New Zealand. If detecting the wing drones of wasps was important in the evolution of the spiders’ hearing, then other species of these eight-legged animals might do long-distance eavesdropping, too.