By Andrea Hargraves 
During the day, jumping spiders hunt their prey, chase and jump like cats. When the lights go out, these pea-sized predators cling to—and perhaps their minds weave dreams.
When you grab your legs and move your eyes, the Evarcha arcuataWriting in the Proceedings of the National Academy of Sciences, researchers report that a type of jumping spider shows something reminiscent of rapid eye movement or REM sleep.
Rapid eye movement is the stage of sleep during which most human dreams occur. The study suggests that REM may be more common than is thought in all animals, which may help unravel the mysteries of its purpose and evolution.
“Monitoring REM sleep in something as far away from us as spiders is pretty cool,” said Lauren Sumner Rooney, a sensory biologist at the Leibniz Institute for Research in Biodiversity and Evolution, who was not involved in the new study.
Daniela Rossler, a behavioral ecologist at the University of Konstanz in Germany and one of the study’s authors, was surprised to find that jumping spiders sometimes hang upside down at night. Rossler began photographing spiders at rest and noticed other strange behavior.
“All of a sudden they were making crazy moves with their legs and starting to get confused. It instantly reminded me of a sleeping cat or dog, not to mention a dream,” Rossler said.
Jerky limb movements are a sign of REM sleep, a condition in which most of the body’s muscles are relaxed and the brain’s electrical activity mimics a waking state. Then there is the eye twitching, which gives Reem its name. But it is difficult to detect in animals with eyes that do not move, such as spiders.
However, part of the jumping spider’s eye moves. Acrobatic spiders have eight eyes in total, and behind the lenses of the two largest is a light-gathering retina that moves to scan their surroundings. The outer surface of these arthropods often obscures the banana-shaped tubes, except when spiders are babies and have transparent exoskeletons. So Roessler’s team looked for resting retinal twitching in young spiders less than 10 days old.
“It’s really smart,” said Paul Shaw, a neuroscientist at the University of Washington School of Medicine. He added that the researchers chose the appropriate animal for this question.
During the night, the researchers photographed the spiders with an infrared camera. In all 34 spiders, they saw episodes of encountered retinal and limb movements, usually lasting about 80 seconds and occurring every 15 to 20 minutes. The team recorded behaviors from spinning their spinnerets (silk-producing tubes) to shrinking all of their legs, making them look like dead spiders. But watching the spiders at rest for hours did not make Roessler sleepy. She said that each spider’s movements seemed unique. “I was always looking forward to my next REM.”
What the researchers saw is very similar to some features of REM sleep, said Sumner Rooney. Spasms, relaxed muscles, and eye movement: “They all look like mammals.”
Scientists have studied REM primarily in mammals. While it has been difficult to discern what would be considered rapid eye movement in other animals, studies have also found evidence for this in birds, cephalopods, and reptiles. With this hint of arthropods, REM sleep may be older or more comprehensive than scientists thought.
Roessler’s team is working to find out if the spiders are actually asleep. One way to prove sleep is to test whether stimulating a still spider is more difficult than stimulating a standing spider. If experiments indicate that spiders don’t just rest their eight eyes, researchers may get a better picture of spiders’ need for sleep by depriving them of it.
If sleep-deprived spiders sleep faster and spend more time in a REM-like state, this will provide more evidence that they are experiencing REM sleep.
They may even have some of the benefits associated with sleep and dreams in humans. “There is no reason to believe they are not dreaming, depending on how you define dreaming,” said Barrett Klein, an entomologist at the University of Wisconsin-La Crosse, who was not involved in the study but wrote a related report. Unpublished paper.
There is no reason to believe they don’t, depending on how you define the dream.
“I can imagine a replay of the memories that would allow them to solve potential problems,” Klein said. With brains complex for their size, jumping spiders plan their tracks. They are hunters who kill insects or other spiders, sometimes their size is quite large. They perform coordinated movements – jumping from sheet to sheet while tying them with a silk thread. Some even perform elaborate courtship dances.
“In my opinion, a dream about a jumping spider will include the most demanding, most fitness-related, and possibly the most dramatic moments in their lives,” Klein said.
Translated by Luis Roberto M. Gonsalves