Jumping spiders rapidly move their eyes and twitch during rest, suggesting they have visual dreams, never before observed in arachnids.
Research “in the field” typically means a journey to the remote Brazilian Amazon for Daniela Rößler, an ecologist the University of Konstanz. But during the coronavirus lockdowns of 2020, the best she could do was a patch of scrubby grass near her home in Trier, Germany.
Rößler (pronounced RUES-slur) quickly became enchanted by the field’s tiny jumping spiders. After nightfall, some jumping spiders, about the size of her pinky fingernail, retired to little silken pouches called “retreats.” She found others immobile, dangling upside-down from a single strand of silk with legs neatly curled—and occasionally moving.
“The way they twitched just made me think of dogs and cats dreaming,” Rößler says.
It wasn’t long before Rößler set up a nursery for baby spiders in her lab to observe their nightly dangles. Her new research, published August 8 in Proceedings of the National Academy of Science, reveals jumping spiders experience a sleep-like state with rapid eye movements similar to those observed in dreaming humans.
So-called REM sleep, also characterised 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 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 said. 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 repose.
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 that looks like can vary wildly. Some birds and marine mammals only sleep with half their brain 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.
- National Geographic