City spiders may spin low-vibe webs

Spiders that spin webs on concrete and steel structures may be missing some good vibrations — and have a harder time catching their next meal.

Attaching silk strands to walls, metal poles and other human-made materials can dampen the usual background vibrations in spider webs, says Chung-Huey Wu of National Taiwan University in Taipei City. In lab tests, spiders in vibrationally quiet webs were not as quick as spiders in livelier webs to detect prey, Wu and Damian Elias of the University of California, Berkeley report in the April Animal Behaviour.

Biologists have investigated other impacts of the rumbles of human activity on wildlife, such as urban noise influencing bird songs or roaring machinery at gas wells changing which species are willing to nest in the area (SN: 8/27/11, p. 26). As far as he knows, Wu says, this is the first study to look at the effects of human environments on the vibrations in a spider web.

Wu did the project during a year he spent at the University of California, Berkeley, focusing on a commonspecies that has invaded North America, the European garden spider (Araneus diadematus). Like many spiders, the garden spider builds its web anew early each morning and eats the used web at night. The spider spends the day hanging in the web’s hub, monitoring for telltale twitches of a fly or other prey that has bumbled into a strand.

To get an idea of vibrations in a human-made landscape, Wu visited 50 webs in downtown and suburban Berkeley. He checked the basic background vibrations at 283 points where the webs attached to a surface, such as a parking sign, a Buddhist bookstore, a leaf or a twig. “What it looked like was a man with a tripod, wearing headphones, doing serious things staring at a tree,” Wu says.

The routine background hum of downtown traffic, Wu and Elias concluded, wasn’t likely to drown out the cues spiders need to detect prey. What might be more important, however, are materials in human-built landscapes. Overall, the metal, concrete, plastic and other human-made supports for webs were less sensitive to disturbances: The artificial materials didn’t vibrate with ups and downs as big or as variable as the leaves, twigs and other natural substrates.

To see whether low-vibe materials have consequences for web hunting, Wu and Elias mimicked various prey-capture scenarios in the lab. The researchers provided frames for spiders to build webs in and then shook the frames at low, medium and high background vibrations to mimic ambient, breezy, and extreme (a construction site) vibratory environments. Wu then checked to see how mild a twitching of prey the spiders could detect (and rush toward) against different shaky backgrounds. To imitate prey, Wu ordered a stereo woofer online and built a device that vibrated a rod against a spot in the web, starting with just the slightest hum and then cranking up to vigorous twitches.

The spiders proved most sensitive to supposed prey when the frame that held their webs vibrated at the intermediate, breezy level. Wu speculates that a moderately lively background keeps the spider more alert.

Wu isn’t ready yet to say whether the vibrations in human-built landscapes actually pose a hazard for web-building spiders. In human-made environments, prey may be so abundant that a less-than-optimal web doesn’t matter in the end.

Chad Johnson of Arizona State University in Tempe has noticed that the black widows he studies build smaller webs in town than they do in the desert. Webs take a lot of resources to make, he says. “This paper may be showing us that city spiders cannot afford to spend a lot on a web that is going to be lousy at transmitting vibrations.”