Colonial Spiders Get Better Web Sites Not By Fighting But By Rising Early, Researchers Discover

Researchers at the University of Cincinnati and Cornell University have discovered how large female spiders in colonies are able to claim enough territory to rebuild their daily webs in the face of competition from other large spiders and smaller ones. The researchers assumed they would observe belligerent behavior every day as the spiders fought for space. Not so.

Instead they found that large spiders in colonies use the time-honored capitalist technique of getting to market first.

"I had anticipated that the spiders would wake up every morning and fight with each other to get to build in the safer core of the colony. Actually, nothing like that happens," says Linda S. Rayor, Cornell instructor in entomology. "Instead the larger spiders compete for open spaces within the colony by building their webs earlier in the morning than other spiders. So the spiders pre-empt the space before anyone else can get there."

The study, "Age-related sequential web building in the colonial spider Metepeira incrassata (Araneidae): an adaptive spacing strategy," by Rayor and George W. Uetz, a University of Cincinnati professor of biology, appears in the May 2000 issue of the scientific journal Animal Behaviour. Rayor worked on this study while she was a postdoctoral researcher in Uetz's laboratory. The research was funded by the National Science Foundation.

Rayor explains this research has general applicability to animal behavior. "We wondered how they wrestle for advantageous sites without actually beating the tar out of each other," she says. "It is also just really interesting natural history."

The scientists wanted to learn how social animals space themselves in groups by examining the behavior of hundreds of colonial spiders in Mexico. "In this case I was studying a spider that lives in groups of hundreds to thousands of individuals. But they need to have private space where each spider can build its own web within the colony," says Rayor. "This is a situation that many social animals -- from colonial seabirds to marine invertebrates -- need to deal with."

Colonial spiders are orb-weavers, which means that only one spider can efficiently operate a single web and catch prey. Thus, tolerance rules -- to a point. The researchers learned that large spiders compete with each other nonbelligerently for big spaces in the colony. In some insect species, the social hierarchy is fixed: Bee colonies, for example, have queens, workers and soldiers. "But spider societies are very different, and they are very egalitarian," says Uetz, who has worked with these colon web-building spiders for over 20 years. The colonial orb-weaver Metepeira incrassata provides an excellent alternative model system for understanding the benefits and costs of group living. "This study has allowed us to tease apart the mechanisms that structure colonies," he says.

Female spiders with egg sacs have special needs, as they attach their egg sacs to lines near the orb web. Once the sacs are in place, the females cannot move them. Thus the females have to stay put and guard the eggs, or they will be parasitized. That's why large females with eggs build much earlier than those without eggs, say the researchers, to make certain that they have enough space to build a large orb web.

Not that the world of the colonial spider is always courteous: Large spiders were observed by Rayor and Uetz to bully the smaller ones. Small spiders are likely to be threatened by larger ones, particularly if they start to build their webs while larger spiders are still building.

"If a smaller individual has the chutzpah to start building at the same time as larger spider, the big ones threaten it by shaking the silk lines, so that it ends up scared and refrains from building its web for a much longer time," says Rayor. But once the larger spider completes its web and settles into it, the threats to smaller spiders subside.

"Inevitably some industrious, littler guys always start earlier. But when they do they get disrupted, and the overall web takes a lot longer to build than if they had just waited until the big gals were done with their webs. The result of delayed web building is less food caught over the day," says Rayor. "The key here is that medium and smaller spiders don't have much of a chance to win if they decide to take on a large individual, even if they are residents on their own webs. So the smaller individuals tend to back off without a physical confrontation when the big spiders are building their webs."

Interestingly when the researchers removed the big spiders from the colonies, the little ones finished their webs over an hour earlier because they were not being hassled.