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Do Spiders Dream of Eight-Legged Sheep?

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By day, jumping spiders hunt their prey, stalking and pouncing like cats. When the lights go down, these pea-sized predators hang around — and possibly their minds spin dreams.

As they twitch their legs and move their eyes, Evarcha arcuata, a species of jumping spiders, show something paying homage to rapid eye movement, or R.E.M., sleep, researchers report Monday within the Proceedings of the National Academy of Sciences. R.E.M. is the phase of sleep during which most human dreaming occurs. The study suggests that R.E.M. sleep could also be more common than realized across animals, which can help untangle the mysteries of its purpose and evolution.

To “take a look at R.E.M. sleep in something as distantly related to us as spiders is just utterly fascinating,” said Lauren Sumner-Rooney, a sensory biologist on the Leibniz Institute for Biodiversity and Evolution Research who wasn’t a part of the brand new study.

Daniela Roessler, a behavioral ecologist on the University of Konstanz in Germany and one in every of the study’s authors, was surprised when she noticed that jumping spiders sometimes dangle the wrong way up in the course of the night. Dr. Roessler began filming the resting arachnids and noticed other odd behaviors. “Rapidly, they might make these crazy movements with the legs and begin twitching. And it just jogged my memory immediately of a sleeping — to not say dreaming — cat or dog,” said Dr. Roessler.

Such jerky movements in limbs are a marker of R.E.M. sleep, a state by which a lot of the body’s muscles go slack and the brain’s electrical activity mimics being awake. After which there’s the darting eyes, from which R.E.M. gets its name. But that’s tricky to identify it in animals with eyes that don’t move, including spiders.

Nonetheless, a part of a jumping spider’s eye does move. The acrobatic arachnids have eight eyes in total, and behind the lenses of their two biggest eyes are light-catching retinas that move to scan the environment. The arthropods’ exterior typically obscures these banana-shaped tubes, except when the spiders are babies and have translucent exoskeletons. So Dr. Roessler’s team searched for flitting retinas during rest in spiderlings younger than 10 days old. “It’s really clever,” said Paul Shaw, a neuroscientist on the Washington University School of Medicine. The researchers selected the fitting animal for this query, he added.

In the course of the night, the researchers filmed the arachnids with an infrared camera. For all 34 spiders, they saw bouts of coinciding retinal and limb movements, typically lasting around 80 seconds and occurring every 15 to twenty minutes. The team logged behaviors from the shifting of silk-producing spinnerets to a scrunching of all legs that resembled a dead spider. But watching hours of resting spiders didn’t lull Dr. Roessler to sleep. Each spider’s movements looked unique, she said. “I used to be at all times looking forward to the following R.E.M.”

What the researchers saw overlapped closely with some hallmarks of R.E.M., said Dr. Sumner-Rooney. The twitches, relaxed muscles and eye movement: “All of them appear to be similar to they’re in mammals.”

Scientists have studied R.E.M. sleep mostly in mammals. While it has been difficult to discern what counts as R.E.M. in other animals, studies have also found evidence for it in birds, cephalopods and a reptile. With this hint in arthropods, R.E.M. sleep could also be more ancient or universal than scientists have assumed.

Dr. Roessler’s team is working to nail down whether the spiders are indeed sleeping. One option to exhibit sleep is to check whether it takes more to awaken a spider at rest, than one which is solely not moving. If experiments suggest the spiders aren’t just resting their eight eyes, the researchers can then get a greater picture of spiders’ need for sleep by depriving them of it. If sleep-deprived spiders go to sleep faster and spend more time in a R.E.M.-like state, then that will provide further evidence that they experience R.E.M. sleep.

They might even be getting a few of the advantages related to sleep and dreaming in humans. “There’s no reason to think that they don’t dream, depending on the way you define dreaming,” said Barrett Klein, an entomologist on the University of Wisconsin-La Crosse who wasn’t involved with the study but wrote a forthcoming perspective article accompanying it.

“I could imagine a replay of memories that allow them to work out possible problems,” said Dr. Klein. With complex brains for his or her size, jumping spiders have been shown to plan their routes. They’re hunters that take down insects or other spiders, sometimes as large as they’re. They execute coordinated moves — jumping from leaf to leaf while anchored on a silk strand. Some even perform elaborate courtship dances.

“A dream, in my mind, for a jumping spider would involve probably the most demanding, fitness relevant, possibly dramatic times of their lives,” Dr. Klein said.

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