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Why Jumping Spiders Spend All Night Hanging Out — Literally

Jun 1, 2021
Originally published on June 2, 2021 1:42 am

Jumping spiders, which use their four pairs of big eyes to spot prey so that they can pounce, can spend a lot of the night just hanging around—literally.

The gorilla jumping spider, Evarcha arcuata, frequently hangs by a single thread at night, suspended in mid-air for hours. Researchers suspect these visually-oriented spiders may cope with darkness by switching to a strategy that lets them use vibrations as a warning signal of danger.

"Maybe they use this silk as a kind of an alarm system or as a way of getting out of reach for predators," says Harvard University researcher Daniela Roessler, who notes that this new finding shows how science seems to know very little about the night-time resting habits of tiny critters, even common ones.

Roessler had been experimenting with jumping spiders when the pandemic hit, so she went back home to Germany and collected some of the local gorilla jumping spiders. That's when she happened to notice that these spiders had a strange nocturnal ritual.

The first time she saw it was one night after coming home from dinner. "I switched on the light," Roessler recalls, "and I looked on the windowsill and was like, 'Oh God, what happened to these animals? Are they dead?' All of them were hanging from the ceiling of the little plastic boxes that I put them in."

A jumping spider both makes small nesting webs and hangs by a thread - not behavior spider researchers expected from the creature.
Daniela C. Rößler

Jumping spiders don't weave webs to capture prey, but they are known to build silken retreats for resting in rolled up leaves and other protected spots. But this was something very different. So Roessler went back out to the spot where she collected these spiders, this time in the dark.

"We found a lot of them. So basically, we found three times the number of individuals during the nights compared to what we found during the day because it's just so easy to spot them when they hang from the vegetation like this," she says.

The hanging spiders were easy for the researchers to find—but perhaps not as easy for crawling nocturnal predators, like ants. And when Roessler would gently touch she silk thread, the jumping spiders would all almost always drop to the ground and scurry away.

Interestingly, though, if she touched the plant they were hanging from, "none of the females would drop down. They would all just climb back up," says Roessler, who explains that only about half of the males would climb up, and the others would drop in this situation. She calls this "really weird...but it was very consistent. We had a large number of spiders. So that is definitely something to look at further in the future."

Still, she notes that the spiders she had in boxes at home would still sometimes make a silken retreat to hide in. "And that meant that they were switching between different resting strategies," says Roessler. "Something informs them about how to make this choice, where to spend the night, how to spend the night."

She's still trying to figure out what's going on, and says there hasn't been much past research looking at resting behaviors among invertebrates in the wild, in part because it's a challenging thing to study.

"Most of the people that have looked at sleep in nature tend to be focused on mammals and birds," agrees Paul Shaw of Washington University School of Medicine in St. Louis, who adds that most researchers have long thought that a complex brain was required for sleep.

In fact, a couple of decades ago, when two groups of researchers established that fruit flies actually go to sleep, the idea met with a lot of resistance. "It took a few years for us to be able to win over the skeptics, and now it's fairly well accepted," says Shaw.

In his view, these jumping spiders are clearly sleeping. "I guarantee it, one hundred percent," he says.

Generally, the fact that animals rest or sleep even though it makes them less alert to danger and more vulnerable to predators has been seen as evidence that sleep must serve some essential biological function.

For jumping spiders, however, "what I thought was really interesting was that they were taking about this suspension as being something that helps them avoid predation," says Amita Sehgal, a sleep researcher at the Perelman School of Medicine at the University of Pennsylvania.

Increasingly, she says, scientists are examining sleep in invertebrates ranging from worms to jellyfish to hydras. "I think it's really caught on, this idea that if sleep is such an important state, it is probably not restricted to just a few species and is more widespread among the animal kingdom," says Sehgal.

Roessler says that since she published her findings, others have reported seeing this nocturnal suspension behavior in jumping spiders in California, New Zealand, and India. "We think that it is a very widespread phenomenon," she says, adding that there's still a lot to learn about these common, charismatic spiders and why they do the things they do.

"We kind of always feel like the things in the backyard, we already know so much about," says Roessler, "but it's crazily really not true."

Copyright 2021 NPR. To see more, visit https://www.npr.org.

MARY LOUISE KELLY, HOST:

One of the most common spiders you might see around your home is a jumping spider. They don't make webs. Instead, jumping spiders have four pairs of big eyes that help them spot prey and pounce. NPR's Nell Greenfieldboyce reports scientists recently realized that at night, in the dark, these spiders do something unexpected.

NELL GREENFIELDBOYCE, BYLINE: Daniela Roessler had been doing experiments with jumping spiders at Harvard University. She wanted to understand whether and how jumping spiders recognize predators that might try to eat them. But then the pandemic hit, so she went home to Germany. And there, in a dry, grassy field, she went out and collected a different species of jumping spider.

DANIELA ROESSLER: They have really cute, stripy faces. I think in English they're actually called gorilla jumping spider because the males have these huge, big, black forearms, which makes them look a little bit funny.

GREENFIELDBOYCE: The spiders were in plastic containers on her windowsill. One night, she came home after being out for dinner.

ROESSLER: And I switched on the light. And I looked on the windowsill and was like, oh, God, what happened to these animals? Are they dead?

GREENFIELDBOYCE: The spiders were hanging from the ceilings of their plastic boxes - just hanging by a single thread, suspended in midair, motionless. Roessler thought, what is this? It was very different than the little silken retreats that jumping spiders are known to build for resting in, say, rolled-up leaves. She went back out to the spot where she collected these spiders, this time at night.

ROESSLER: We found a lot of them. So basically, we found three times the number of individuals during the nights compared to what we found during the day because it's just so easy to spot them when they hang from the vegetation like this.

GREENFIELDBOYCE: It turns out the spiders spend hours at night hanging upside down. And if she gently touched the silk thread, almost all spiders would immediately drop to the ground.

ROESSLER: Maybe they use their silk as a - kind of an alarm system or as a way of, like, getting out of reach for predators.

GREENFIELDBOYCE: Like ants or other spiders - still, the spiders she had in boxes would sometimes make a little silk retreat for themselves.

ROESSLER: And that meant that they were switching between different resting strategies. Something informs them about how to make this choice, where to spend the night, how to spend the night.

GREENFIELDBOYCE: She's still trying to figure out what's going on but says this just goes to show that science knows very little about the nighttime resting habits of very tiny critters, even common ones. Amita Sehgal studies sleep at the University of Pennsylvania.

AMITA SEHGAL: What I thought was really interesting was that they were talking about this suspension as being something that helps them avoid predation. We kind of talk about sleep as something that makes them more susceptible.

GREENFIELDBOYCE: Because a sleeping animal isn't alert to danger - the sleep field has long focused on mammals and birds. The few invertebrates that are studied, like the fruit fly or worms, generally get examined in the lab, not out in the wild where predators lurk.

Nell Greenfieldboyce, NPR News.

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