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What happens when 50,000 spiders hunt together?

It turns out Charlotte may not be the only friendly spider.

Biologists once thought arachnids were aggressive, territorial loners. But this summer, two giant cobwebs spun by millions of tiny spiders drew throngs of visitors to a park in North Texas. And next month, The American Naturalist will publish breaking Canadian research that shows why some spider species live in large co-operative groups - banding together to spin webs, hunt prey and even care for one another's offspring.

The author of the paper, University of British Columbia biologist Leticia Avilés, started studying spiders on an undergraduate field course in her native Ecuador. Unlike the hostile creatures of Arachnophobia, she found them easy to work with and "great to answer lots of questions about the evolution of sociality."

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Prof. Avilés now splits her time between a Vancouver laboratory and the Cuyabeno Nature Reserve in the Amazon Basin, where she lives in a wooden hut and travels by canoe in search of communal colonies. Over the past decade, she has surveyed hundreds of kilometres of roads and waterways.

Her findings show that spider teamwork depends on the size of their dinner: They join together when faced with the larger prey of lowland rain forests, but go it alone at higher elevations, where insects are smaller.

"Being social and co-operating allows spiders to enter an ecological niche that's not available to solitary individuals," she says.

Social spiders live in self-contained nests that house up to 50,000 insects. Group living can enhance foraging success and provide better protection against strong rains and predators such as wasps, ants and praying mantises. But all this togetherness also leads to high rates of inbreeding, leaving colonies vulnerable to disease.

And there are exceptions to the rules that Prof. Avilés has observed. She has discovered one new social species living 1,800 metres above sea level - about 1,000 metres above where loners were thought to take over. Prey insects are smaller at this elevation, however, so the species lives in smaller groups than its lowland cousins.

Then there are those spiders in Texas. Unlike their social counterparts, these opportunistic critters were less co-operative than tolerant of activity among other species in a time of rare bounty. A particularly lush summer created fantastic feeding conditions that attracted a huge variety of spiders to the park's lakeshore - and in this tangle, each spider remained territorial to its own piece of the lucrative cobweb.

So what sets truly social spiders apart? Their collectivism offers a natural example of what computer scientists - studying social networking on that other Web - call "crowd-sourcing." This occurs when a large group without defined instructions achieves a task, say, an entry on Wikipedia, greater than the sum of its parts.

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The key to making such systems work is a lack of top-down hierarchy. Unlike social insects such as ants, bees and termites, social spiders don't live in caste-structured societies. "There's no queen spider," says Deborah Smith, an entomologist at the University of Kansas. Rather, they take an egalitarian approach to the division of labour, each spider contributing to keeping the web up to scratch.

Of course, collective-minded spiders, just like their human counterparts, can fall prey to freeloaders. And when some spiders feast on the colony's spoils without helping out, the whole social system can fall apart.

But the same genetic similarities that can put a colony of relatives at risk for illness, Prof. Avilés says, generally mitigates such cheating.

Meanwhile, thriving spider societies are challenging more than arachnophobic prejudices. Once shunned by evolutionary biologists, the concept of group selection - which suggests that Darwinian survival skills can occur among entire groups, not just on an individual level - is seeing a resurgence, thanks to the spiders.

Elie Dolgin studies biology at the University of Edinburgh.

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