Hurricanes Make For More Aggressive Spiders

Anelosimus studiosus, Loxahatchee National Wildlife Refuge, Boynton Beach, Florida. This spider lives in colonies, which become more aggressive after recent hurricanes. Judy Gallagher via Wikimedia commons. CC-by-2.0

You've survived hurricane and flood, what more can the storm throw at you? Aggressive spiders, apparently, whole colonies of them. Spider colonies filled with hot-tempered spiders thrive after tropical cyclones, while their docile counterparts take a hit. If you need us, we'll be somewhere without hurricanes. Or colonies of aggressive spiders.

Dr Jonathan Pruitt of the University of California, Santa Barbara is studying the ecological effects of tropical cyclones. Not just the obvious ones like fallen trees opening up forest canopy to sunlight, but more subtle legacies like natural selection within animal populations. As noted in Nature Ecology & Evolution, this is challenging to test. Studying the aftermath of hurricanes is easy, but to see what changes they have wrought you need a baseline of conditions beforehand. This either means sampling a lot of places, or predicting where a tempest will strike.

Pruitt used weather warnings to select likely hurricane tracks two to three days before landfall. He then tested colonies of Anelosimus studiosus, a spider for whom group-living means sharing digs with a few hundred other female spiders, not moving in with you and your housemates. A. studiosus hangs large webs from branches over water bodies, catching river-dwelling insects in their strands.

Every species varies in how it balances the need to find food while exposed to danger, with some individualsĀ taking a safety-first approach while others risk all for a meal. Rather than the usual spectrum, however, A. studiosus goes to extremes, either being highly aggressive or very docile, making it much easier to study and analyze.

Although A. studiosus colonies include a mix of personality types, some are dominated by one sort or the other, since it is an inherited characteristic. Colonies with highly aggressive populations don't just take more risks, they're more likely to descend into infighting when times are bad, and also to cannibalize visiting males or their own eggs.

Shortly before three 2018 hurricanes hit, Pruitt and co-authors vibrated bits of paper near three colonies and measured aggressiveness by how many spiders attacked it. Two days later the same colonies were assessed to see if they had survived. Follow-up studies looked at the number of eggs laid and populations in winter. Another 211 colonies outside hurricane zones were tested as controls.

At hurricane sites, the higher the average level of aggression in a colony, the better it recovered from the storm. On the other hand, among colonies that did not experience a 2018 hurricane, more docile populations slightly outdid the aggressive ones.

The authors admit they don't know why aggressive colonies flourish in post-hurricane conditions. Unsurprisingly however, since natural selection is a powerful thing, counties that have been hit by more hurricanes over the last century had more aggressive spiders. As if they hadn't suffered enough.

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