Spider Solidarity Forever

Science News, May 8, 1999 by Laura Helmuth

Most spiders have beastly social skills. They're aggressive, territorial loners that would just as soon eat a sibling as look at one.

Of the 35,000-odd spider species that have been described, however, a few dozen flout tradition. These social spiders live in groups. They cooperate while hunting and building their communal homes. They even care for their own--and sometimes each other's--young, whereas typical spiders lay their eggs and creep away.

Nineteenth-century biologists, including Charles Darwin when he voyaged to South America, discovered a few spider species that gathered in huge colonies. In the past 20 years, researchers have found more examples of gregarious spiders. Now, scientists are exploring the social webs that bind together these infamous individualists.

By looking at the social world from a spider's-eye view, biologists are gaining insights into the evolution of sociality, the costs and benefits of group living, and the ways that creatures relate to their kin.

Entomologists have long studied the social worlds of a variety of insects--ants, bees, and termites--that live in large, cooperative networks. Like the six-legged social species, many cooperating spiders hunt together and share food.

Although arachnid societies bear a superficial resemblance to these insect communities, they operate by markedly different reproductive rules. In insect groups, workers are sterile and only the queen lays eggs, whereas all spiders in a colony are able to reproduce.

In that regard, social spider species interact more like a herd of wildebeests than like a hive of bees, says George W. Uetz of the University of Cincinnati.

Deborah R. Smith of the University of Kansas at Lawrence compares social spiders to a pride of lions. "It's always interesting to see an organism one usually thinks of as asocial, predatory, and cannibalistic, forming large cooperative societies," says Smith.

The most social of the social spiders live in multigenerational colonies in the rain forests of South America. Anelosimus eximius, one of the best studied of these cooperative species, builds a hammock-shaped web suspended from the lush vegetation by long threads. Their mahogany bodies are about the size of pencil erasers. They band together in colonies of hundreds to tens of thousands of individuals, spinning their collective web above rivers and roads and where light filters in through the tree canopy.

Several generations of spiders live together in the community, and with constant repairs, the meter-long nest can last several years. Adult spiders care for the young, but they don't distinguish between their own progeny and those of others. They guard eggs against predators, move egg sacks to the web areas with the most comfortable temperatures, and feed hatchlings. When a colony grows too large, the nest starts to break up of its own weight, Smith says. The spiders split into two or three groups, or the young adult females crawl away on bridges of silk to spawn their own colonies.

Group living has its benefits, says Leticia Aviles of the University of Arizona in Tucson, who studies cooperative spiders in Ecuador. Working together, social spiders can capture prey as large as 10 times their size, whereas an individual spider is lucky to bag a bug twice as big as itself.

Cooperative spiders also save on the cost of silk. Frequent tropical rains pelt the sheet like webs. By working together, the cooperative spiders conserve on energy and protein as they repair the damage from a web-ripping storm.

As the colony expands, however, parasites are more likely to find it and infest its egg sacks. Field studies show that females living in an intermediate-size colony raise the largest numbers of offspring, Aviles reported in the September 1998 AMERICAN NATURALIST.

Cooperative behavior evolved in eight unrelated spider genera in different families, says Smith. She has gleaned clues to the evolution of this behavior by looking at some modern species that are related to social spiders. They have some social traits but haven't fully committed to group living.

In such species, the mothers care for the young well after they have hatched but do not establish colonies. Each generation of young goes off and makes its own single-family web. These species, Smith speculates, resemble forerunners to the fully social spiders. After some point in evolution, she says, "the babies just never leave home."

All this togetherness over many generations inevitably leads to inbreeding, normally considered an evolutionary no-no. "We had to ask, Were they really doing this?" says Smith.

They were indeed, according to recent research on the genetics of spider societies. Smith finds genetic variation between colonies of one cooperative species, but within one nest, the individuals are virtually identical.

When the living is easy, it's fine to be a clone, Smith says, but without genetic variation, an entire population could be wiped out by an epidemic. For instance, a mysterious spider plague swept through Panama in 1983, killing entire nests of cooperative spiders.