Spiders Seem to Have REM-like Sleep and May Even Dream

After being barred from her lab by the Pandemic restrictions, behavioral ecologist Daniela Rler caught local jumping spiders and kept them in clear plastic boxes on her windowsill to experiment with 3-D printed models of predatory spiders. She noticed something strange when she returned from dinner. Rler is a research fellow at the University of Konstanz in Germany. She hadn't seen jumping spiders on silk lines before. Rler didn't know what happened. I thought they weren't alive.

Rler discovered an alternate sleeping habit of the species Evarcha arcsuata, which had been known to build silk sleeping dens in curled up dead leaves. She spied on them all night. She got a cheap night-vision camera and used it to aim at a spider. She was amazed by what Rler saw.

The spider was mostly hanging there. She began to twitch her legs and her abdomen as well. She had her legs in the middle of her chest. The odd movements only appeared during bouts that lasted a little more than a minute and occurred periodically throughout the night. She says that they were twitching in a way that looked a lot like when a dog or cat dreams.

REM sleep involves a state of partial or near-total muscle paralysis coupled with an active, awakelike brain state, which is why it is sometimes called paradoxical sleep. This state has been associated with dreams in humans. Rler and her colleagues were wondering if the twitching spiders could be having dreams. She says that they were okay with that. She changed her research plans after she thought, "Let's figure it out"

Many mammals and birds have evidence for REM sleep. There is a similarity in two reptile species and in zebra fish. The eye movements, arm twitches, and rapid skin color and texture changes that resemble displays they perform when awake are what the octopuses and cuttlefish look like. There is not much evidence of REM sleep in arthropods.

Lisa Taylor is a behavioral ecologist at the University of Florida and was not involved in the new research. They live in a world that is rich in sensory information.

The jumping spiders offer a unique opportunity to expand the realm of dreaming animals, in part because of their eye parts. Most spiders can't move their eyes while awake, but jumping spiders have long tubes that move their eyes around behind their big eyes. For the first several days of life, the spiders have translucent eyes that can be seen inside their heads.

The jumping spiders that were found sleeping on a silk line experienced muscle twitches and eye movements similar to REM sleep in humans. The movement of the tubes that control the position of their retinas inside their heads was revealed by scientists when they recorded the sleep of baby spiders. Rler is credited with the credit.

When Rler recorded 34 spiderlings, she found that their twitches were accompanied by eye tube movements that did not happen during other phases of sleep. I like it. I don't think it's right. It immediately makes a sleep researcher think about rapid eye movement sleep. It is exciting to have the first indication that it is relevant in arthropods.

It's too early to say for sure if the spiders are experiencing REM sleep like humans. The researchers need to show that the spiders are not as responsive to their environment when they are asleep.

Rler and her team of co- authors have begun the tests. She points out that the leg curling pose is only seen in dead spiders, which is why it is so striking. Spiders use muscles to keep their legs extended, and the curling could be a result of muscle weakness. The findings of the team were published on Monday.

The videos alone are convincing, says a researcher who is not involved in the new study. He says he will predict that they are asleep because the movements don't look like the ones the spiders make when awake.

The scientists will need evidence that the spiders' brains are active as they twitch and move their eyes to show that they sleep like REM. Rler says that it will be difficult to measure activity in a small brain. Other scientists have figured out a way to insert an electrode into a spider's brain without deflating it and killing it.

REM sleep can help us understand how it works and why it exists. Some people think that REM sleep's eye movements reflect scenes that are playing out in dreams. There is a chance that other animals have REM-like states as well. Measuring brain activity might one day be used to ask animals about their dreams. Teresa Iglesias is a neuroscience researcher at the Okinawa Institute of Science and Technology in Japan and was not involved in the spider research. She says it is very early to think we will be able to identify dreaming in a different way.

David Pea-Guzmn is a philosopher at San Francisco State University and author of When Animals Dream: The Hidden World of Animal Consciousness. It's difficult to imagine a dream without an ego or an "I" feeling it, he says. Pea-Guzmn, who was not involved with the spider research, said that if spiders dream, it might mean that they have a minimal self.

Rler says the jumping spiders might be able to test the theory that rapid eye movements are related to visual dream sequence and if those scenes are replays of things the arachnids witnessed while awake. Spiderlings may be able to watch a video of a cricket hopping and see if the movements are re-created during sleep.

Rler wants to look for REM sleep in other spider species and thinks that it might look different in animals that rely more on senses other than vision. She thinks that orb weavers might dream. I think the story is that REM is just as universal in the animal kingdom as sleep is.