Do spiders dream of eight-legged sheep?

During the day, jumping spiders hunt their prey, stalking and striking like cats. When the lights go down, these pea-sized predators hang out—and maybe their minds are spinning dreams.

As they twitch their legs and move their eyes, Evarcha arcuata, a species of jumping spiders, show something reminiscent of rapid eye movement, or REM, sleep, researchers report Monday in the Proceedings of the National Academy of Sciences. REM is the stage of sleep in which most human dreams occur. The study suggests that REM sleep may be more common than realized across animals, which may help solve the mysteries surrounding its purpose and evolution.

“Looking at REM sleep in something as distantly related to us as spiders is just absolutely fascinating,” said Lauren Sumner-Rooney, a sensory biologist at the Leibniz Institute for Biodiversity and Evolution Research who was not part of the new study .

Daniela Rößler, a behavioral ecologist at the University of Konstanz in Germany and one of the study’s authors, was surprised when she noticed that jumping spiders sometimes dangle upside down during the night. Dr. Rößler began filming the resting arachnids and noticed other strange behavior. “All of a sudden they would make these crazy movements with their legs and start twitching. And it just immediately reminded me of a sleeping – not to mention dreaming – cat or dog,” said Dr. Rößler.

Such twitching movements in the limbs are a marker of REM sleep, a state in which most of the body’s muscles become relaxed and the brain’s electrical activity mimics being awake. And then there are the darting eyes, from which REM gets its name. But it’s hard to detect in animals with eyes that don’t move, including spiders.

However, part of a jumping spider’s eye moves. The acrobatic arachnids have a total of eight eyes, and behind the lenses of their two largest eyes are light-catching retinas that move to scan their environment. Arthropods’ exteriors usually hide these banana-shaped tubes, except when the spiders are babies and have transparent exoskeletons. So Dr. Rößler’s team looked for fluttering retinas during rest in spiders younger than 10 days old. “It’s very clever,” said Paul Shaw, a neuroscientist at the Washington University School of Medicine. The researchers chose the right animal for this question, he added.

During the night, the researchers filmed the arachnids with an infrared camera. For all 34 spiders, they saw bouts of coincident retinal and limb movements, which typically lasted about 80 seconds and occurred every 15 to 20 minutes. The team recorded behavior from moving silk-producing spinnerets to a crushing of all legs that resembled a dead spider. But watching hours of resting spiders did not put Dr. Rößler to sleep. Each spider’s movements looked unique, she said. “I always looked forward to the next REM”

What the researchers saw overlapped closely with some characteristics of REM, Dr. Sumner-Rooney said. The twitches, relaxed muscles and eye movements: “All appear to be the same as in mammals.”

Researchers have studied REM sleep mostly in mammals. While it has been difficult to see what counts as REM in other animals, studies have also found evidence for it in birds, squid and a reptile. With this hint in arthropods, REM sleep may be more ancient or universal than scientists have thought.

Dr. Rößler’s team is working to find out if the spiders actually sleep. One way to demonstrate sleep is to test whether it takes more to wake a spider at rest than one that is simply not moving. If experiments suggest that the spiders don’t just rest their eight eyes, researchers can get a better picture of spiders’ need for sleep by depriving them of it. If sleep-deprived spiders fall asleep faster and spend more time in a REM-like state, that would provide further evidence that they are experiencing REM sleep.

They may even gain some of the benefits associated with sleep and dreams in humans. “There’s no reason to think they’re not dreaming, depending on how you define dreams,” said Barrett Klein, an entomologist at the University of Wisconsin-La Crosse who was not involved in the study but wrote a forthcoming perspective paper accompanying it .

“I could imagine a replay of memories that would allow them to figure out possible problems,” Dr. Klein said. With complex brains for their size, jumping spiders have been shown to plan their routes. They are hunters that take down insects or other spiders, sometimes as large as they are. They perform coordinated movements – jumping from leaf to leaf while anchored on a silken string. Some even perform elaborate courtship dances.

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

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