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Hurricanes give aggressive spiders a leg up on their docile kin

For Anelosimus studiosus spiders, the storm survival checklist apparently includes a combative personality.

ByKatherine J. WuNOVA NextNOVA Next
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If you live in hurricane territory, your might want to steer clear of the local Anelosiumus studiosus spiders. Image Credit: Judy Gallagher, flickr

In October of 2018, Hurricane Michael was kicking into high gear. But while Florida residents busily stockpiled non-perishables and shuttered their homes, Jonathan Pruitt was steering his rented Nissan Titan straight into the path of the storm.

Armed with antifreeze, shovels, and a rock solid auto insurance policy, the University of California, Santa Barbara evolutionary ecologist had one clear mission: to figure out whether aggressive Anelosimus studiosus spiders might have a leg up—or eight—on their more bashful brethren in the wake of a tropical cyclone.

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It was a weird area of exploration—and a difficult one. After all, natural disasters don’t exactly play by the rules of experimental design. But Pruitt’s spider storm-chasing would eventually pay off: As he and his colleagues report today in the journal Nature Ecology & Evolution, arachnids with an attitude seem better equipped to navigate a post-hurricane world.

It’s still unclear exactly how aggression behooves storm-battered spiders. But as climate change continues to alter the frequency and intensity of tropical cyclones, the study’s findings highlight the unexpected ways in which environment impacts the behavior—and ultimately, the wellbeing—of species of all kinds.

“This study points to yet another one of the indirect ways in which climate change will affect species persistence,” says Marie-Claire Chelini, a behavioral ecologist at Duke University who was not involved in the study.

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In Anelosimus studiosus colonies, “females are the movers and shakers,” says study author Jonathan Pruitt. “Males just wander around trying to find virgin females to inseminate. Then they die.” Image Credit: Courtesy of Jonathan Pruitt, University of California, Santa Barbara

Some children get their eyes from their mothers; others acquire their father’s winning smile. A. studiosus offspring, however, inherit a more unusual heirloom: their parents’ personalities. In this species, spiders segregate into two camps—docile and aggressive—and the trait appears to pass from generation to generation. And, because A. studiosus often lives in large, female-dominated colonies, groups of these spiders can collectively skew toward one disposition or the other.

Years of research have shown that more combative spiders are better at most spidery things—building webs, catching prey, defending their colonies against invaders. Conversely, their docile relatives are shy, sluggish couch potatoes, difficult to rouse from their silken perches even when tempted with a freshly snared insect. “I’m often shocked that the docile ones accomplish anything,” Pruitt says.

But hostility, too, comes at a cost: For all their gumption, aggressive spiders are loose cannons, prone to infighting and even slaughtering their young. Despite being pushovers in most other respects, docile spiders are the colonies’ parents and peacekeepers, every bit as necessary as their hot-blooded groupmates.

This delicate balance made Pruitt wonder how aggressive colonies would fare in the aftermath of a natural disaster like a tropical cyclone. Food scarcity might favor go-getters that excel at scrounging up meals—or it could trigger violence, causing more bellicose groups to self-destruct.

To tease out the two possibilities, Pruitt and his dog Winifred Sanderson packed their bags and headed to the Southeast. Each time a hurricane loomed, the pair would leave their inland post to pick out spider colonies on and off the storm’s predicted path.

First, Pruitt would count the spiders. Then he’d administer a personality test, touching the tip of an electric toothbrush to a square of paper fixed to the center of a cobweb. The shudder of silk—which mimics thrashing prey—sends aggressive spiders into a frenzy, Pruitt says. Their docile counterparts, however, are far slower to react, sometimes ignoring the vibrations entirely.

After marking dozens of colonies with identifying tape, Pruitt would hunker down inland and wait for the storm to take its course. Two days after the cyclone cleared, he and Winifred Sanderson returned to tabulate survivors.

Collectively, three of 2018’s tropical cyclones—Subtropical Storm Alberto, Hurricane Florence, and Hurricane Michael—felled docile and aggressive spiders indiscriminately, with just 75 percent of colonies making it through.

But when the researchers checked in on the spiders in the months following, they discovered that many of the docile colonies in cyclone-disrupted zones had begun to falter. Only a handful of egg sacs clung to their nests; fewer of their spiderlings lived to see the start of winter. More aggressive colonies in these spots, however, were reproducing just fine.

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An Anelosimus studiosus web, home to dozens of female spiders that segregate into two personality types. Image Credit: Courtesy of Jonathan Pruitt, University of California, Santa Barbara

The study’s results didn’t offer an explanation for why storms favor the belligerent, but considering aggressive spiders’ aptitude for hunting and territory defense, it’s not hard to speculate, Pruitt says. When the going gets tough, the tough get going—and in the case of spiders, aggression may beget resilience.

Another set of data seemed to back this up: When Pruitt and his team surveyed a larger swath of the American Southeast, they found that the more tropical cyclones a region had suffered in the past century, the more hot to trot its spiders were.

“We often think about evolution occurring bit by bit, on a slower pace than a cyclone can initiate,” says Kasey Fowler-Finn, an evolutionary biologist at St. Louis University who was not involved in the study. “But the fact that big weather events like cyclones can cause shifts in selection that can be picked up over the course of 100 years is pretty exciting.”

That doesn’t necessarily mean the future will be dominated by angry, cyclone-ravaged spiders. In areas the cyclones had bypassed, it was the meeker spiders who won out—possibly because pacifist colonies could afford to invest their energy in parenting instead of warfare, points out Trinity Walls, who studies spider behavior at the University of California, Berkeley but was not involved in the study. “There’s costs and benefits to both ends of the spectrum of personality,” she says.

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Study author Jonathan Pruitt prepares to touch a vibrator to a piece of paper positioned at the center of an Anelosimus studiosus cobweb. The fluttering paper mimics the thrashing of snared prey. (In the study, an Oral B toothbrush performed a similar function.) Image Credit: Courtesy of Jonathan Pruitt, University of California, Santa Barbara

Still, the findings underscore the myriad ways in which extreme events affect animals, all the way down to the minutiae of personality, Chelini says. Harsh environments have a way of weeding out individuals and populations that fail to go the extra mile for food or mates. Impediments to those evolutionary chores might be physical or behavioral, but they’re impediments all the same.

Now more than ever, these relationships are especially important to understand, Chelini says. Under the influence of climate change, tropical cyclones are expected to become more common and severe in certain parts of the world. “Human activity is making the climate change much faster than species can adapt and evolve,” she says. “We will lose biodiversity through not just the direct, better understood ways [like habitat loss], but also to all of these less direct mechanisms that we are just beginning to understand.”

Climate change won’t have uniform effects on all species, however. And for Pruitt’s eight-legged study subjects, there could still be a silver lining. Spiders have been scuttling the Earth for at least a few hundred million years, he says. Regardless of what’s in store for the rest of us, arachnids will probably be around for generations to come.

“Spiders are incredibly tolerant little cysts of life,” he says. “Even if Earth changes, they’ll find a way to navigate it.”

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