So-called REM sleep, also characterized by muscle relaxation and changes in electrical activity in the brain, is thought to be important in memory consolidation and could play a role in developing important survival skills. Confirming REM-like sleep in jumping spiders could alter our understanding of when and how it evolved—to date, REM-like sleep has only been identified in animals with backbones (reptiles, birds, fish, and most mammals).
“Spider intelligence”—and spider dreams—“[are] likely to be in most ways completely different from our own,” says Nate Morehouse, an associate professor at the University of Cincinnati who studies vision and decision-making in jumping spiders and wasn’t involved in the study. “I can’t wait to find out what this new study has opened up for all of us to understand them, on their own terms.”
eye of the spider
You can’t perform a brain scan on a spider like you can on humans or other larger animals, nor can you ask them how they slept, but for baby spiders, you can see into their heads. In their first ten days of life, jumping spiders, also called salticids, haven’t developed pigment on the exoskeleton covering their tiny noggins, a space almost entirely devoted to their eyeballs.
They’re basically “walking retinas,” Morehouse says.
Six smaller eyes provide a 360-degree, monochromatic view of the world that’s very sensitive to motion, while the principal eyes—the “big, round, cute” eyes—provide high-resolution vision similar to a house cat’s in acuity, Morehouse explains . Though their eyeballs are fixed and can’t rotate in sockets like ours, boomerang-shaped retinas move around the back of the principal eyes, shifting the spider’s field of vision.
In her lab, Rößler set about trying to record dozing spiderlings to learn about their sleep habits, using a magnifying glass and a night vision camera. She focused on the spiders’ eye and body movements, which provide clues about what’s happening as they rest.
She soon found they experience periods of rapid retinal movement, which increased in duration and frequency throughout the night, lasting about 77 seconds and happening approximately every 20 minutes. It was during these REM-like periods that Rößler observed uncoordinated body movements—their abdomens wiggled, their legs curled or uncurled.
The spiderlings’ spinnerets, organs on the tip of their abdomen responsible for creating silk, would periodically “go nuts,” Rößler says. Like the rhythmic foot twitches of a sleeping puppy, the spiders seemed to be “practicing” one of their waking behaviors.
While jumping spiders don’t make webs, “they constantly set little silk anchors wherever they go,” she explains. “They never walk around without leaving a silk trace so in case they jump, they always have a backup line, like a bungee [cord].”
Morehouse says that one of the leading theories about REM sleep is that it allows animals to hone essential survival skills.
“Occasionally, there are things happening that I can only explain with the theory of them having a nightmare,” Rößler says. They’ll be peacefully dangling, legs curled in neatly, when suddenly “all the legs get extended at the same time, like aah!”
There were also periods of coordinated movement when the spiders stopped to stretch, adjust the silk line they were dangling from, or clean themselves. Judging by the lack of retinal movement, it appears the spiders were just briefly rousing to get comfortable before returning to their rest.
To sleep, perchance to dream?
Rößler emphasizes that we have yet to prove this period of inactivity in spiders can technically be considered sleep. For that, several boxes need to be checked—including demonstrating the spiders are less arousable, or slower to respond to stimuli, and need “rebound sleep” if they’re deprived.
From her observations outside, “they seem to really be able to distinguish what is an actual disturbance” and what isn’t, Rößler says.
If, for example, there’s “vibration on the vegetation or on the silk—they react immediately,” she says. But when it’s windy, they swing about in the breeze “and just don’t care.”
Scientists are confident that all animals sleep, though what looks like can vary wildly. Some birds and marine mammals only sleep with half their brains at a time, while hibernating animals can sleep almost continuously for weeks or months. Defining “dreaming” is even more challenging—but the periods of REM-like rest do imply the animals are having visual dreams.
Jumping at the chance to learn
Other jumping spider researchers described Rößler’s study as incredibly exciting.
“It was a clever idea with relatively simple methods that gave a really profound result,” says Alex Winsor, a PhD candidate at the University of Massachusetts Amherst, who researches vision in jumping spiders. He and his advisor, Beth Jakob, who’s studied jumping spiders for decades, say they’re eager to reach out to Rößler with ideas for follow-up studies.
“We’re interested in whether they do respond to visual stimuli” during this sleep-like state, Jakob said—they don’t have eyelids, after all. Winsor is already developing a system to monitor brain activity in jumping spiders, which could provide even more evidence that they’re dreaming.
“I’m using a single tungsten electrode—a really thin wire” placed outside of the head to detect electrical activity, Winsor says. The team also plans to combine this with a setup the team previously used to monitor retinal movements in spiders watching tiny TVs.