The right kind of "purr" makes a female wolf spider go weak at the joints.
Biologists have known for awhile that wolf spiders (Gladicosa gulosa) can make sounds that humans can hear, explains Laura Geggel for Live Science. However, wolf spiders don't have ears themselves — at least in the traditional sense. Instead, the sounds are part of an elaborate communication system that male spiders use to woo females.
Male spiders actually produce vibrations, which hit surrounding dried leaves and cause them to vibrate. The vibrating leave produces a low "purring" sound audible to humans, and that sound travels. If it hits leaves near a female spider, causing them to vibrate, she can pick up on the vibrations.
For this to work, male and female spiders need to be on a good surface that can vibrate. Dead leaves, in particular, are ideal. Leaves serve as a sort of telephone line or radio wave through which the spiders call females, and they're essential to the wold spider communication system, as researchers reported May 20 at the Acoustic Society of America's annual meeting in Pittsburgh.
Instead of using an organ to produce a sound, like crickets or katydids, the spiders vibrate inanimate objects around them. "They're courting on dead leaves, and that leaf itself is what's resulting in the airborne sound," Alexander Sweger, a biology grad student at the University of Cincinnati, told Live Science. It's a bit of a roundabout way to flirt, but it could help researchers discern why some organisms communicate through sound, while others use vibrations.
Other wolf spiders are known to produce vibrations to communicate, but those vibrations don't come with audible sounds. Purring wolf spiders produce both. That made them an intriguing group for Sweger and his advisor George Uetz to study.
The spiders have specialized arm-like appendages called pedipalps, one on each side of the mouth. One has a rough tip, while the other is shaped for scraping. They rub the two limbs together to generate vibrations that hit nearby leaves.
To the human ear, the sound of the vibrating leaf sounds like a low purr, quieter than a cricket:
That the sound of the vibrating leaf travels to other leaves where females stand. When the sound waves hit those leaves, they vibrate, and the female picks up on the vibrations.
In the lab, Sweger and Uetz recorded male spiders making the vibrations and sounds on different surfaces: paper, which can vibrate, and granite, which can't vibrate. Using a special device, they were also able to convert the vibrations to audible sound, so here's what the direct vibrations themselves would sound like to us if we could hear them:
They found that the surface is key to the male's "purring" game. Males could only make the vibrations when standing on paper, and females could only receive the signal when standing on paper. When researchers just played the vibration sound, only females responded. That suggests this might be a communication style that's reserved for talking to females, but they don't know yet.
The study leaves a few key questions unanswered. Whether or not female spiders could hear other noises, like threatening bird calls, by this method also remains unclear. Because this leaf telephone system is a bit of a departure from typical spider communication, further examination of the species could lend insight into the evolution of communication through sound and vibration.