During the 2018 hurricane season, when a big one was bearing down on the Gulf or Atlantic coasts and most people were trying to get out of the storm’s path, Jonathan Pruitt was fighting to catch the last flight in. Once on the ground he rented a Nissan Titan or a Ford F150—something with at least eight inches of ground clearance—and drove to wherever he expected the hurricane to hit. Then he went hunting.
Megan Molteni covers DNA technologies, medicine, and genetic privacy for WIRED.
In the days and hours before landfall, he scoured the area for trees whose branches dangled over a waterway. Inspecting those limbs up close, he tried to catch sight of a clutch of dead leaves cloaked in a ghostly silken shroud. When he found them, he dropped a shred of paper into each web and, using a modified electric toothbrush, vibrated the paper so it flitted back and forth as though it were alive. Then he’d count the number of spiders that came swarming out to seize it.
So what makes a man leave his comfortably landlocked home in Canada to pulsate spider colonies as a Cat 5 hurricane barrels straight toward him? The answer, of course, is science.
Spiders are usually solitary creatures. Of all the world’s 40,000 or so arachnid species, only two dozen of them live in groups, working together to catch food and raise their translucent spiderling children. Anelosimus studiosus is one of these rare breeds, living in colonies of up to a few hundred females. They make their homes in the US states that happen catch the brunt of the storms kicked out by the Atlantic Ocean every summer and fall, and Pruitt has spent most of his career studying them. An evolutionary ecologist by training (his lab at McMaster University in Ontario investigates how animal societies form and collapse), he’s best known to his peers as an expert in spider personalities—the Myers-Briggs of the arachnology world, if you will.
Years ago, he discovered that A. studiosus have two distinct personality types: They’re either bold and aggressive or shy and docile. And each spider inherits that personality trait from its parents. The bold ones surge out like a marauding war party to meet whatever gets caught in their web. The shy ones are happy to let the trap do the work on its own. If a few meals get away, so what? At least nobody got hurt.
The relative makeup of either bellicose or peaceable individuals determines the collective aggressiveness of a colony, which is also transmitted down through generations from parent to daughter colony. Most of the time it doesn’t matter much. But when resources are scarce, aggressive colonies tend to do better, sacrificing a few individuals so everyone else can eat. What Pruitt wanted to know was could hurricanes—which rip leaves from trees, blow insects away, and cause rivers to overflow their banks—make these spiders more aggressive?
Yeah, it looks like they can. And not just for a season, but maybe forever. That’s the conclusion Pruitt and his coauthors reached in their latest study, published today in Nature Ecology & Evolution. Last year, Pruitt went through this exercise three times—before subtropical storm Alberto, hurricane Florence, and hurricane Michael. Before each storm he tested the aggressiveness of dozens of colonies in the path of the cyclone, as well as control sites clear of the storm’s outermost windfields—altogether about 240 samples. Then he holed up in a hotel for 48 hours, riding out the worst of each storm, before venturing back out to see how each colony fared.
Navigating flooded roads and downed power lines, Pruitt often relied on chainsaw-wielding locals to cut a path for him. “They’d begin dismantling fallen trees before FEMA ever showed up, literally carving a way back to my sites,” he says. “Some I couldn’t get back to because of flooding. Those we figure have been submerged and swept away.”
But the ones that did survive, he tallied up. And to those sites he returned two more times, months later, to count how many eggs had been produced and how many spiderlings hatched. What he discovered was that colonies that had been more aggressive pre-cyclone had more babies, and more of those juvenile spiders ended up surviving, in the aftermath of each storm. To see if this was a longer-term trend, Pruitt’s team compared 13 study sites from Louisiana up to North Carolina with a century’s worth of cyclone strike data. They found that the most aggressive colonies were located in places with the highest historic exposure to cyclones.
“There’s clearly some kind of selection going on here for the aggressive individuals in response to these extreme weather events, and that’s what’s really fascinating,” says George Uetz, a spider biologist at the University of Cincinnati. Though not involved in the study, he was the one who, many years ago, invented the method Pruitt used for stimulating a spider’s web. Except that back then electric toothbrushes hadn’t yet been invented, so he used a vibrator instead. “I got a lot of teasing for that,” says Uetz.
In the wolf spiders he studies, he’s observed hints of a similar phenomenon; after a tornado smashed through an Ohio nature preserve, males born in the blowdown zones for generations after had smaller, less attractive claw tufts, making it harder for them to find mates. But he’s never seen anything like a storm changing the long-term behavior of a whole species.
“On the one hand, it’s not surprising that natural disasters have an impact on the survival of animals,” he says. “But to see their impact on selection, how they actually drive the direction of evolution in a species is pretty rare.”
The studies might be few and far between, but Pruitt suspects the phenomenon is more widespread. That’s why he spent the last seven weeks driving 22,000 miles through the bayous and lowlands of the Gulf coast, working 14-hour days setting sticky card traps and gathering thousands of vials of antifreeze-suspended insects with just his Australian shepherd-Border collie mix, Winifred Sanderson, to keep him company.
Even as someone who grew up in a hurricane-smacked “hick portion” of central Florida, Pruitt wasn’t always prepared for what he came across. More than once, while driving through miles of sugar cane stands, he says he has come across armed poachers loading freshly trapped reptiles into terrarium-lined truck beds. “I tried to just look as Canadian as possible,” he says. “Like, ‘Hi,‘I’m just here collecting INSECTS!’”
His ultimate goal is to see what sorts of species do well in the aftermath of hurricanes. Do such storms allow invasive species like red fire ants to get an edge over native animals? Or do they fare worse because they haven’t evolved in a hurricane-pummeled environment? What about species of mosquitoes that carry human diseases versus those that don’t? “The question is how important are these extreme events in maintaining trait diversity,” says Pruitt. “Clearly something very special is happening.”
Understanding exactly how species are evolving in response to disturbances from extreme weather is more urgent now than ever. As the planet continues to warm, scientists project that hurricanes will intensify, heatwaves will get hotter, and droughts will grow longer. You probably know from Ian Malcolm’s mansplaining of chaos theory in Jurassic Park that the flap of a butterfly’s wing in Brazil can kick off a tornado in Texas or a hurricane in North Carolina. Now we know—at least for spiders—the relationship works both ways.