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Page last updated at 12:52 GMT, Monday, 6 July 2009 13:52 UK
Spider builds life-sized decoys
Matt Walker
Editor, Earth News

C. mulmeinensis with decoys of itself
An adult C. mulmeinensis alongside decoy prey pellets (L) and decoy egg sacs (R)

There is a species of spider that builds models of itself, which it uses as decoys to distract predators.

The spider may be the first example of an animal building a life-size replica of its own body.

So believe the scientists who made the discovery, which is published in the journal Animal Behaviour.

The arachnid's behaviour also offers one explanation for why many spiders like to decorate their webs with strange-looking ornaments.

Many animals try to divert the attentions of predators by becoming masters of disguise.

Some try to avoid being seen altogether by using camouflage to blend in against a background, such as the peppered moth evolving motley wings that blend into tree bark, or stick insects that look like sticks.

Others evolve more conspicuous ornaments designed to distract a predator, such as butterflies that grow large eyespots or lizards that quickly move colourful tails, which they detach from their bodies if grabbed.

This latter strategy has puzzled biologists, because attracting predators in the first place is usually a bad idea.

One hypothesis is that animals which grow conspicuous ornaments benefit overall, because directing a predator to attack an expendable part of the body, such as the lizard's tail, outweighs the costs of attracting the attention of the predator in the first place.

Under attack

But animals do not tend to actually build life-like replica models of themselves to act as decoys.

However, that is exactly what a species of orb spider called Cyclosa mulmeinensis does, biologists Ling Tseng and I-Min Tso of Tunghai University in Taichung, Taiwan, have discovered.

This and other related spiders in the same genus decorate their webs with material such as detritus, plant parts, prey remains or egg sacs.

Because such detritus is often of a similar colour to the spider, researchers suspected it might help camouflage the arachnid.

Our study seems to be the first to empirically demonstrate the function of animal-made decoys
Spider specialist I-Min Tso

Initially Tseng and Tso decided to test the idea by videoing another related species called Cyclosa confusa living in the wild. They measured how often predatory wasps attacked the spiders in webs decorated with detritus compared with those in undecorated webs.

"We predicted that spiders with prey carcass decorations on webs should receive fewer wasp attacks because spiders should be well camouflaged by such objects," says Tso. "To our surprise, spiders on decorated webs received far more attacks than those on undecorated webs."

That confirmed that the decorations attracted predators rather than acting as camouflage.

However, Tseng and Tso suspected that these decorations might also redirect enough attacks to make them worthwhile.

Life-like decoys

So they tested the idea on another species Cyclosa mulmeinensis living on Orchid Island off the southeast coast of Taiwan. This species decorates its web with both the remains of dead insect prey and egg sacs.

Intriguingly, the spiders made prey pellets and egg sacs that were the same size as its own body.

The researchers also found that these decorations appeared to wasps to be the same colour, and reflect light in the same way, as the spider's body.

In short, the spider made decorations that were of the same size, shape and appearance as itself.

"Our results show that this vulnerable spider protects itself from predator attacks by constructing decoys that increase the conspicuousness of the web, and resemble its own appearance in size and colour," the researchers write in Animal Behaviour.

Female C. mulmeinensis
A female C. mulmeinensis

"When both spiders and web decorations are present on the same web, they look like a string of nearly identical oval objects to the predators."

"I don't know of any animal that actively builds a decoy of itself. Our study seems to be the first to empirically demonstrate the function of animal-made decoys," says Tso.

The decoys worked, too. More often than not, a wasp would attack a decoy rather than the spider, thinking it to be a tasty meal.

But all wasp strikes on spiders living on undecorated webs were directed straight at the spider.

"Decorations built by Cyclosa spiders function as a conspicuous anti-predator device instead of a camouflaging device. The benefit of successful escape from predator attack seems to outweigh the cost of increased detection," says Tso.

Web decorations

Scientists have been trying to answer the question of why many species decorate their webs for more than 100 years.

Tso suspects that there is no single answer.

"I think that the functions of web decorations might be very diverse and differ from taxa to taxa. Different spiders seem to decorate their webs for different reasons," he says.

For example, spiders often decorate their webs with silk ornaments, which might strengthen the web, act as a warning signal to predators, to deter large animals from accidentally walking into the web, destroying it, or to act as a visual signal to attract prey.

Others, including Cyclosa species, may use non-silk decorations primarily as anti-predator devices.

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