Some spiders may enter REM sleep - and maybe even dream, study says

With their big eyes, furry legs and exotic colors, jumping spiders are described as some of the most adorable arachnids – but members of its more than 5,000 species are amazing in other ways, too.

They have spectacular vision, can make complex decisions and are capable of forming memories. Some do hypnotic mating dances like tropical birds. And many can leap relatively far distances.

A new study, from German, Italian and U.S. researchers, says that a species of jumping spider may have another fascinating trait: the ability to have rapid eye movement sleep, a phase of rest characterized by twitching limbs, high brain activity, and eyes that race in different directions. Scientists say humans have their most vivid dreams during REM sleep.

“There was no way ever in my life I would have thought that (jumping spiders) could have something like REM sleep,” Daniela Roessler, a behavioral ecologist at the University of Konstanz in Germany and the lead author of the study, told the Washington Post.

One night in September 2020, Roessler came home and noticed that some jumping spiders she had collected and placed in boxes on her windowsill were hanging upside down from their silk lines, very much like little Christmas tree ornaments.

“I was like, we don’t know what they’re doing, but they’re hanging really neatly, super exposed, not in a silk retreat, so let’s figure it out and let’s just film them throughout the night,” Roessler recalled. “So that’s what we did.”

What she and her fellow researchers saw amazed them. Using an infrared camera, they observed the young Evarcha arcuata jumping spiders experience bouts of limb twitching as they hung upside down. Because the spiders had translucent exteriors, Roessler also recorded the spiders’ interior “retinal tubes” – a part of the eye that allows the arachnids to shift their gaze – shake rapidly during the state of apparent inactivity. It did not happen when the spiders were active.

“These twitches seemed so classical, and they immediately reminded me of a dog dreaming,” Roessler said.

And it’s possible the spiders dream, too, she said.

“Whether that means that they’re visually experiencing this similar to how we experience visual dreams is a completely different story,” Roessler said, musing that the spiders may “dream in vibrations.”

Roessler said she and her team are not ready to answer those questions. First, they must prove that the spiders actually enter into REM sleep. The study cautiously characterizes the behavior as “REM sleep-like,” since the researchers haven’t yet shown that the spiders engage in sleep as it is typically defined by scientists. Roessler said she plans to test for those criteria – namely, lowered responsiveness and regulated periods of rest – in the future.

But experts in the field, including those who did not participate in the study, are excited by its findings.

“I’d be really surprised if this isn’t sleep,” Barrett Klein, an entomologist and sleep biologist at the University of Wisconsin at La Crosse, told the Post. “I’d be surprised if they don’t show lowered responsivity during these REM-like states.”

Little is known about REM sleep in mammals, birds or other creatures thought to exhibit similar behavior, like octopuses, experts said. Klein called REM sleep a “paradox” because an animal’s muscles become largely paralyzed during the sleep phase, while the brain appears to light up as though awake. He also called it a “black box” because it’s unclear why humans and other animals go into the paradoxical sleep state, and it has been debated since REM sleep was discovered in humans some seven decades ago.

Some studies suggest there are “some psychological or emotional aspects that are specifically orchestrated during REM,” Klein said. “Memory consolidation and a type of learning seems to be specifically benefited by REM sleep.”

And some of humans’ most vivid and bizarre dreams take place during REM sleep, said John Lesku, a zoology professor at La Trobe University in Australia who studies sleep in animals. So it’s not out of the question that animals in which REM-like sleep has been observed – like dogs, cuttlefish and maybe jumping spiders – might be dreaming, he told the Post, although it’s hard to say for certain because dreams, even in humans, are impossible to prove.

Two people might tell each other they dream, he said. “But strictly speaking, I don’t know that you dream, and so it becomes even harder when you’re talking about nonhuman animals for which you have no ability to ask what they are doing.”

But if he can assume his cat has dreams, Lesku said, “I’m willing to suggest that maybe the spider does as well.”

Before the study was published last week, not a lot of attention was paid to whether spiders slept, experts said. “The assumption is more like they just take little rests during the day or … whenever they’re active,” Roessler said. “But I don’t think there was such a clear idea if they actually sleep during some extended period of time.”

The idea that a jumping spider might go into REM sleep is fascinating to Lisa Taylor, a research scientist at the University of Florida whose research focuses on jumping spider behavior. She said jumping spiders have remarkably complex sensory systems – eight eyes, sensory hairs on their legs, the ability to feel vibrations through surfaces, as well as senses of smell and sound, and varying degrees of color vision.

“So it’s a really noisy world,” Taylor said, adding that “one of the big challenges that animals face is to make sense of all this information and to somehow decide what to let in and what to process and what to do with it all.”

If jumping spiders do enter REM sleep, they might be consolidating memories or behavior patterns, as some jumping spiders have sophisticated cognitive abilities and make complicated decisions, Taylor said.

“They’re not little robots that go out and attack anything that they see,” she said. “There’s a lot going on in their brains as they’re making decisions about whether to attack one thing over another.”

“So whether something that happens at night plays a role in that is particularly interesting,” she added.