By day, jumping spiders hunt their prey, stalking and pouncing like cats. When the lights go down, these pea-sized predators hang out — and maybe their minds spin 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 phase of sleep during which most human dreaming occurs. The study suggests that REM sleep may be more common than realized across animals, which may help unravel the mysteries of its purpose and evolution.
To “look at REM sleep in something as distantly related to us as spiders is just utterly fascinating,” said Lauren Sumner-Rooney, a sensory biologist at the Leibniz Institute for Biodiversity and Evolution Research who wasn’t 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 started filming the resting arachnids and noticed other odd behaviors. “All of a sudden, they would make these crazy movements with their legs and start twitching. And it just reminded me immediately of a sleeping — not to say dreaming — cat or dog,” said Dr. Rößler.
Such jerky movements in limbs are a marker of REM sleep, a state in which most of the body’s muscles go slack and the brain’s electrical activity mimics being awake. And then there’s the darting eyes, from which REM gets its name. But that’s tricky to spot it in animals with eyes that don’t move, including spiders.
However, 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. Rößler’s team looked for flitting retinas during rest in spiderlings younger than 10 days old. “It’s really 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 coinciding retinal and limb movements, typically lasting around 80 seconds and occurring every 15 to 20 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. Rößler to sleep. Each spider’s movements looked unique, she said. “I was always looking forward to the next REM”
What the researchers saw overlapped closely with some hallmarks of REM, said Dr. Sumner-Rooney. The twitches, relaxed muscles and eye movement: “All of them seem to be the same as they are in mammals.”
Scientists have studied REM sleep mostly in mammals. While it has been difficult to discern what counts as REM in other animals, studies have also found evidence for it in birds, cephalopods and a reptile. With this hint in arthropods, REM sleep may be more ancient or universal than scientists have assumed.
Dr. Rößler’s team is working to nail down whether the spiders are indeed sleeping. One way to demonstrate sleep is to test whether it takes more to rouse a spider to rest, than one that is simply not moving. If experiments suggest the spiders aren’t just resting their eight eyes, the researchers can then 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, then that would provide further evidence that they experience REM sleep.
They may even be getting some of the benefits associated with sleep and dreaming in humans. “There’s no reason to think that they don’t dream, depending on how you define dreaming,” said Barrett Klein, an entomologist at 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 their 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 are. 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 the most demanding, fitness relevant, maybe dramatic times of their lives,” Dr. Klein said.