How Sharing Leads to Death

If you’re a spider, that is.

Guillermo Granja / Reuters

The colonial spiders of Ecuador spin homes for themselves the size of houses, wrapping cobwebs around the jungle trees where they live in groups of tens of thousands. The diminutive creatures raise young together, eat together, and die together. An apparently thriving colony can disappear almost instantly, with few or no survivors, for reasons that are still mysterious.

Biologists who study these creatures and their cousins have spun many theories about what the risk factors for collapse are, and it seems that the larger the colonies are, the greater the danger of collapse. A recent paper from researchers at University of British Columbia investigates whether a particular behavior common in large colonies—sharing prey among many colony members—might contribute to their downfall.

Ruth Sharpe, a UBC graduate student, finds her subjects by cruising along roads in Ecuador looking for clouds of bridal-veil webbing in the trees. Sharpe knew that large, spread-out colonies tend to catch larger prey than smaller ones. With a large catch, like a big wasp, it would be more difficult for a few spiders to monopolize it the way they might a small fly. A more likely scenario would be divvying it up amongst all comers. But that creates the possibility of there not being enough food to keep any of the spiders fit enough to reproduce. With small prey that can be kept amongst a few spiders, those individuals would get enough to keep the colony going, even if others eventually starve. Bad news, though, if one big wasp means only a mouthful for each member.

Does catching large prey rather than small ones correlate with more even—and hence potentially more dangerous—sharing? Sharpe set out to answer the question by plucking nearly 400 spiders from various nests and setting them up in mini-colonies in the lab, with 10 spiders per plastic box. Each spider got a dot of paint in a different color, so that Sharpe and her field assistant could tell them apart. Then, they began the task of feeding the colonies—a surprisingly difficult chore, as catching insects requires a great deal more legwork for humans than spiders. “We spent a lot of time,” Sharpe recalls, “running around with nets desperately trying to catch these big wasps.”

After capturing their prey, the researchers recorded their weights and released into each colony's box either one large insect or four small ones totaling the same mass. Every hour or so for two days, they checked the spiders' progress and recorded who was feeding, using the color dots to ID them. After the experiment was over, the spiders were released to their original nests. (None the worse for their sojourn in the lab, Sharpe notes: “We saw painted spiders in the nests for a time afterwards.”)

Running the numbers, they found that large prey were indeed divided more equally than the small. Spiders that weren't in the best of condition were more likely to be able to get a share when the prey was large than when it was small, suggesting that the less-fit benefit from the capture of large insects. This fits with the idea that larger prey could mean food for everyone, but in smaller amounts. “Large colonies may be sharing prey too evenly,” Sharpe says—if everyone is chronically underfed, there will be no one healthy enough to have offspring.

That may help explain the conundrum of large-colony collapse, though there are probably many factors in play. In a different type of colonial spider, Jonathan Pruitt of UC Santa Barbara has found that having the right balance of personalities among the members of a nest can contribute to survival. His species has two clear groups—the docile, who don't fight as much and tend the nest, and the aggressive, who are fiercer hunters. When a colony is small, and has lots of resources, having more docile members is beneficial. There's no need for many aggressive members. When the colony is larger and resources are scarce, having more aggressive members comes into its own. If a colony doesn't have the right balance of personalities for its size and scenario, it has a much higher chance of collapse. And intriguingly, the spiders can somehow sense this and adjust the make-up of the next generation of their offspring so that it has the right ratio, with those with the over-represented trait laying fewer eggs than the sisters.

The colonies of bees, ants, and other social insects do have their own woes--witness colony collapse disorder in bees, whose causes are mysterious but may include parasites or disease. But they seem to be different from what happens with spiders. “We think that what may be predisposing the spider colonies to go extinct when the colonies are very large is due to things internal to the colony," explains Sharpe, meaning that “even if there [were] favorable environmental conditions around the large colony, it may still go extinct. However, with large colonies of other insects we don't think this happens.”

Researchers will probably be searching for solid answers about spider societies' sudden disappearances for some time. But the dynamics of their rise and fall are a fascinating glimpse into how social creatures manage the difficult tasks of living intertwined lives.