These spiders can purr

Wolves howl to let others know that they’re around — and maybe even that they are looking for a mate. But not the wolf spider known as Gladicosa gulosa. It makes a kind of a purr. It’s quite a trick for guys of this species. And that’s because it’s not clear that the target of their attentions can actually hear a purr. A female may just feel the effects of that sound as vibrations in her feet. But even that may not happen unless both he and she are standing on the right surface.

Most animal species use sounds to communicate. In fact, Cornell University has created a digital library of more than 200,000 such animal sounds. But for spiders, sound is not a big part of their lives. In fact, they have no ears or other specialized sound-sensing organs.

So it came as a big surprise to Alexander Sweger when he discovered one species of wolf spider communicates using sound.

Sweger is a behavioral ecologist at the University of Cincinnati in Ohio. He is doing research toward a PhD. In the lab, he works surrounded by wolf spiders. Among these is one species that for almost a century has been known as the purring spider. Biologists suspected this particular type of wolf spider might be using that purring sound to signal its interest in finding a mate. But no one had ever confirmed this, Sweger says.

So he decided to investigate.

Sounds create two types of waves. The first is a short-lived wave. It shifts air molecules around, which is something that can be detected across only a very short distance. This wave is followed by a second, longer-lasting one that causes very local changes in air pressure, explains Sweger.

Most animals, including people, can detect the second wave — usually with their ears. Most spiders can’t. But purring spiders, Sweger and George Uetz now report, can harness leaves and other things in their environment to broadcast and detect vibrations due to sound. The University of Cincinnati scientists described their findings May 21 in Pittsburgh, Pa., at the Acoustical Society of America annual meeting.

How the spider purrs

At mating time, male wolf spiders try to catch a female’s attention by creating “persuasive” vibrations, Sweger says. They strum one structure on their body against another — somewhat as a cricket does — to impress the gals. Getting the message right can be a matter of life and death to the guy who’s doing the wooing. If the female isn’t totally convinced that he’s the “one,” it could be worse than just being rejected, Sweger explains. “She could eat him.” About one out of every five male wolf spiders will be eaten by the female he had been wooing. But the guys who prove suitably persuasive will get to mate — and live to tell the tale.

Purring spiders “are using the same vibratory tactics as every other wolf spider in North America. More or less,” Sweger says. “They’re using the same structures. And they’re making vibrations.”

But the scientists showed that compared to the wooing vibrations made by other wolf spiders, those by Gladicosa gulosa are far stronger.

Sweger discovered something else as well. When a purring spider was on a surface that is good at conducting vibrations, such as leaves, an audible sound was produced.

If a person is within a meter of the courting spiders, they can actually hear the sound. “It’s very soft, but when we’re out in the field, you can hear them,” Sweger says. The sound, he explains, is a bit like a “little strumming chirp” or a “soft rattle or purr.” (You can judge for yourself.)

Wooing with sound

So why bother with an audible sound when a male needs only to convey some persuasive vibrations to a spidey gal? That’s been the real puzzle. And Sweger’s experiments now offer one likely answer: that the sound is just an accident.

The courtship vibrations by purring spiders — at least when leaves or paper are involved — create an audible sound so loud that it can broadcast a guy’s message to a distant gal. But she apparently only “hears” it if she’s also standing on something that can rattle, such as a leaf.

Sweger’s learned this in the lab.

His team let a male purring spider make those wooing “calls.” The scientists then played a sound recording of the guy’s purr through the air. Males in another cage ignored these calls. So did female spiders standing on something solid, such as granite. But if the female was atop a surface that could vibrate, like a piece of paper, then she began moving around. It signaled she had picked up the guy’s message. And it suggests she had to “hear” the audible call as a leaf’s vibrations under her feet before she got the message that a potential mate was out there.

When both spiders are standing on the right kind of surface, a male can broadcast his message over a relatively long distance (a meter or more) for a female to “hear.” At least, Sweger says, based on the new data, “that’s our working hypothesis.”

“This is highly interesting,” says Beth Mortimer. She’s a biologist who studies spiders at the University of Oxford in England, and was not involved in the study. The Cincinnati team’s data suggest “spiders can use materials as a sound detector,” she says. So they, “in a way, are using certain objects [here leaves] as a kind of ear drum, which then transmits vibrations to the spider’s legs.” Although they lack ears, spiders are superb at sensing vibrations, she notes. “This is another great example of the surprising ingenuity of spiders,” she concludes.