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Spider 'shenanigans'

By Sean McLachlan
Arizona Daily Wildcat
February 19, 1999
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Arizona Daily Wildcat

Photo Courtesy of Leticia Avilˇs Anelosimus eximius, a type of social spider, is found in Ecuador.

Most people like to relax on their summer vacations, but one UA professor prefers to spend her time canoeing up South American jungle rivers searching for spiders.

She isn't looking for just any spider, but a few rare species that live in vast webs with dozens, hundreds, even thousands of arachnids.

Leticia Avilés, assistant professor of Ecology and Evolutionary Biology Department, has spent 15 years studying these "social spiders."

She said the spiders are different than social insects such as ants, which have a "queen" and a strict division of tasks within the community, such as "warrior" and "worker".

"In the case of the spiders, there is no division of labor like that," she said. "They are much more egalitarian."

Unlike most spiders, which spin their own webs, social spiders gang up on their prey by constructing giant, communal webs. The unfortunate catch then becomes food for all the spiders in the community.

Where It's At

Avilés and Powers will appear on "The Desert Speaks" KUAT channel 6 at 8 p.m. on Thursday. They will be discussing their research on the subsocial spiders of Arizona.
Mates are selected from within the web, and all the females cooperate to take care of the young.

"There is no regular mixing between colonies at any stage," Avilés said "That is one of the things that makes these spiders interesting."

There are also "subsocial" spiders which live communally in their mother's web until they are old enough to leave and spin a home of their own.

Two species of subsocial spiders live in Arizona. While they have many similarities to their more communal cousins, the Arizona arachnids do not live together long enough to build the immense communities of true social spiders.

Avilés said she is intrigued with how these communities came to be.

All of the social spider species are found in tropical areas, either on the inside or the edge of jungles. Avilés believes that heavy rains in the tropical regions break up the webs, requiring the spiders to cooperate in rebuilding their homes. All of these species spin relatively thick strands of silk, making the task even more onerous for a single spider.

"There is a large investment in silk to make a nest," Avilés said, adding that cooperation also allows the spiders to catch bigger prey.

Avilés and others who have studied social spiders have become embroiled in an ongoing debate on the level that natural selection operates.

It had been assumed since Darwin that natural selection occurred on an individual level. In the 1960's, scientists began to discuss if natural selection could happen on the level of population.

Doubters of this theory said there would have to be a very limited set of circumstances for this type of natural selection to occur. There would have to be many groups of the same species in the same ecozone, a high turnover in the population and each group would have to be founded by one or very few individuals to make them genetically similar.

There would also be little or no mixing between populations. This would lead to isolated colonies that are highly uniform in genetic makeup.

If one spider had a bad trait, most or all of the others in its web would have the same trait. Good and bad traits would be shared among whole populations rather than just a few individuals.

The spiders match these criteria exactly, Avilés said. Several populations can be found within a square mile, spiders have a relatively high rate of reproduction, and new colonies are formed when a parent colony gets too large and splits in two.

"There is less selection acting on individuals and more on the group," said Kim Powers, a ecology and evolutionary biology graduate student who is writing her dissertation on social spiders.

This does not disprove the theory that most selection happens on an individual level, she said. Personal fitness is still considered the main determinate of whether a particular creature will survive.

"It is the exception that proves the rule," she said.