Species: Gladicosa gulosa
Habitat: Leaf litter of beech-maple forests in the east of the US and Canada
With a sound like a wad of money being quietly thumbed, it's hardly a Handel aria.
But for the purring wolf spider, it's music. For the first time, a spider has been recorded producing what appear to be audible courtship signals over and above pure vibrations.
"It's very quiet, but it's what you would hear if you were in the room with a courting spider," says George Uetz of the University of Cincinnati in Ohio, who discovered the purring wolf spider with colleague Alexander Sweger. "The sound is at a level that's audible by human hearing at about a metre away."
Many spider species rely heavily on vibrations to send signals
Thrum away
But the purring wolf spider is different. While using their pedipalps – the appendages next to their mouthparts – to vibrate dead leaves, they also create an audible thrumming sound. Click here to listen to the spider.
The discovery is a puzzle, as spiders aren't known to have anything equating to "ears".
To check that the "thrum" isn't a bog-standard vibration signal, Uetz and Sweger used a vibration detector called a laser Doppler vibrometer to detect the drumming of the spider and convert it into an audible "sound" output.
Then they used a standard microphone to pick up the audible sound produced at the same time. Comparing the two showed that the vibration and the thrumming sound were two separate emanations.
The pair also found that males produce the sound and only females respond to a played recording of the purr, suggesting that the purr is used in courtship.
"We have yet to see any evidence of any male-to-male communication in this species," says Uetz.
Sensitive spiders
The males only produce the sounds if they are standing on something that will vibrate, like a leaf, and females only respond when perched on a similar surface.
Uetz and Sweger think that the signal reaches the female by travelling as sound in air, which causes the leaves a female is standing on to vibrate.
"We think that's how she 'hears' the sound," says Sweger. "Spiders have very sensitive structures all over their bodies for detecting vibration, even at low levels, so we're working on the hypothesis that they detect a surface vibration induced by the airborne sound."
The purring spiders may help us better understand how communication by sound first evolved. "Animal acoustic signals may originally have evolved from vibration, and our findings suggest a possible mechanism," says Sweger.
The pair reported their findings today at the annual meeting of the Acoustical Society of America in Pittsburgh, Pennsylvania.
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