Extreme Weather Making Some Spiders Aggressive

Researchers have found that extreme weather events like Hurricanes Florence and Michael have an effect on spiders along the North Carolina coast — the storms appear to be making some of them more aggressive.

McMaster University scientists observed what they call evolutionary responses after studying over 200 colonies of spiders.  After a tropical cyclone, some groups started making coordinated efforts to hunt prey, which is important when so many flying insects — a major food source for these arachnids — die or are displaced during major storms.

WFDD's Eddie Garcia spoke with biologist Jonathan Pruitt about the research.

Interview Highlights

On the findings of the study:

I looked at colony differences in aggressiveness. So in this species there are some colonies that behave very aggressively towards prey and towards each other and towards intruders, and then there are docile societies that are nicer to each other but that are also more tolerant of intruders — that can be costly. So I found that there was no association between colony behavior and whether or not a colony lived through the storm itself. Basically that was all random. But provided that a colony managed to live through the storm and not get displaced or drowned, colonies that were more aggressive produced more egg cases and also had more of their spiderlings, i.e. little baby spiders. 

On what "aggressive" means:

I measure aggressiveness by basically putting a faux prey item inside of the colony's web and then I vibrate that faux prey item and then measure how long does it take for the colony to respond to the prey and attack it. And then what proportion of colony members come out to attack. So with docile societies they attack prey pretty slowly and it might only be one individual sort of bumbling out to go check it out.

But in an aggressive society, they form a little war party. They attack swiftly, and they attack en masse to subdue prey. It's highly correlated with a lot of other aspects of their aggression. So societies that respond swiftly and collectively towards prey are also more likely to fight with each other over access to that prey. They're more likely to cannibalize their mates. They're more likely to cannibalize each other if they get too hot, and they're also really good at defending themselves from other species of spider that try to intrude and steal their food. 

On why the spiders are becoming aggressive:

Last year it was just a bunch of question marks. But this year I've decided to try to address some of those question marks by going out and evaluating the whole insect community before and after storms, and then at sites that vary in their history with these storms. And I found that immediately following tropical cyclones the abundance of flying insects goes down and that persists at least for a year or two after the storm goes through. And it makes sense because, you know at least with storms like Hurricane Michael, they have really high wind velocities that could damage a lot of these flying insects and suppress their numbers for some time afterward. And these spiders are trap-building predators. They set up a little silken filter that basically filters some subset of these flying insects out of the sky and so if those insects aren't abundant then that will choke off the resources to these spiders. And the reason that helps aggressive societies is because aggressive societies are really good at subduing whatever prey hit their filter. And so I think that's why it pays to be aggressive under these post-cyclonic conditions. 

Editor's Note: This post has been updated from an earlier version to include interview highlights.