Web decorating with garbage

Researchers propose decaying food remains in spider webs confuse would-be predators

Picked-over carcasses as home decor attract the wrong kind of attention, even among spiders, researchers say. But the decorations may work well as decoys.

Cyclosa mulmeinensis spiders tie old bits of prey onto their webs. Many spiders add extra features, mostly of silk, to their webs, and biologists are still debating what functions the apparent decorations serve.

About 950 hours of video of C. mulmeinensis spider webs in natural conditions show that webs generously decorated with food bits draw extra visits from predatory paper wasps, report I-Min Tso and Ling Tseng of Tunghai University in Taichung, Taiwan.

After analyzing decorations in light of what’s known about wasp vision, Tso and Tseng argue that the wasps probably have trouble telling the wads of trash from the spider. Attackers may indeed visit the web more often but end up pouncing on a carcass decoy. In the end, that ploy could improve spider survival, the researchers propose in an upcoming Animal Behaviour.

“I like the approach and consequently the novel explanation,” says Dror Hawlena of Yale University. Hawlena is investigating a similar idea in lizards with a colorful tail that can be discarded when a predator bites it. The color slightly increases the chance that predators will spot the lizard to begin with but dramatically increases the chances that an attack will target the expendable tail. “In other words, you may pay a little tax to get better benefits,” Hawlena says.

Even though Hawlena welcomes the idea, he says he would want to see more data before ruling out alternative explanations for carcass decorations.

Biologists have come up with plenty of ideas. Decorations might camouflage the web so predators ignore it or might fascinate little insects, luring in prey.

Tso and his colleagues tested the camouflage and attractant ideas in another carcass decorator of the same genus and in 2005 reported that neither seemed to work. The new test switched species and more than tripled the amount of spider video in hopes of observing more web activity.

With lots of new video, researchers observed 34 wasp attacks. Twelve wasps hit webs with only one carcass or no garbage at all. Twenty-two wasps zinged in to the more decorated nests. Decorations do attract extra predators, the researchers concluded.

Reporting spider mortality wasn’t possible, Tso explains. He didn’t catch a killing on camera though spiders did sometimes disappear. What he can say is that wasps went after the spider itself in nine out of the 12 attacks on sparsely decorated webs and only seven out of the 22 attacks on more garbage-filled ones.

The spiders also tie garbage around their pinkish egg cases, and the team’s analysis suggests that, in this case, the carcasses may make the cases harder for predators to spot.

Daiqin Li of the National University of Singapore has studied the same spider species in his country. After watching almost 200 hours of videos, he and a colleague reported in the June 15 Journal of Experimental Biology that they did not see appreciable differences in predator attack rates between sparsely and heavily decorated webs. Yet the more carcass-enhanced ones caught more prey. Predator behavior and sensory systems could differ depending on habitat, he suggests.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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