During the day, jumping spiders hunt their prey, stalking and jumping around like cats. When the lights go out, these pea-sized predators hang on — and maybe their minds weave dreams.
As you contract your legs and move your eyes, the evarcha arcuataa species of jumping spider, shows something reminiscent of rapid eye movement, or REM, sleep, researchers reported Monday in the Proceedings of the National Academy of Sciences.
REM is the phase of sleep during which most human dreams occur. The study suggests that REM is perhaps more common than is thought in all animals, which could help unravel the mysteries of its purpose and evolution.
“Observing REM sleep in something as far away from us as spiders is absolutely fascinating,” 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 Roessler, 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. Roessler began filming the resting arachnids and noticed other strange behavior.
“All of a sudden they would make crazy movements with their legs and start squirming. And it immediately reminded me of a sleeping cat or dog, not to mention dreaming,” Roessler said.
These jerky limb movements are a sign of REM sleep, a state in which most of the body’s muscles are relaxed and the brain’s electrical activity mimics the waking state. And then there’s the eye twitching, which gives REM its name. But it’s hard to spot it in animals with eyes that don’t move, like spiders.
However, part of a jumping spider’s eye moves. Acrobatic arachnids have eight eyes in all, and behind the lenses of their two largest eyes are light-gathering retinas that move to scan their surroundings. The exterior of arthropods often obscures these banana-shaped tubes, except when spiders are babies and have translucent exoskeletons. So Roessler’s team looked for quivering retinas during rest in baby spiders under ten days old.
“It’s really smart,” said Paul Shaw, a neuroscientist at the University of Washington 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. In all 34 spiders, they saw episodes of coincidental retinal and limb movements, usually lasting around 80 seconds and occurring every 15 to 20 minutes. The team recorded behaviors from spinning spinnerets (silk-producing tubes) to shrinking all of their legs, which made them look like dead spiders. But watching hours of spiders at rest did not make Roessler sleepy. Each spider’s movements seemed unique, she said. “I was always looking forward to the next REM.”
What the researchers saw closely resembled some features of REM sleep, Sumner-Rooney said. The spasms, the relaxed muscles and the eye movement: “They all look like mammals.”
Scientists have studied REM primarily in mammals. While it has been difficult to discern what counts as REM in other animals, studies have also found evidence of it in birds, cephalopods and reptiles. With this hint from the arthropods, REM sleep may be older or more universal than scientists thought.
Roessler’s team is working to find out if the spiders are actually sleeping. One way to demonstrate sleep is to test whether it is more difficult to stimulate a resting spider than one that is simply still. If the experiments suggest that the spiders aren’t just resting their eight eyes, the researchers might get a better picture of the 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, this will offer more 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 they don’t dream, 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 has written a related, as-yet-unpublished paper.
There’s no reason to think they don’t dream, depending on how you define dreaming.
“I could imagine a replay of memories that would allow them to solve potential problems,” Klein said. With complex brains for their size, jumping spiders plan their routes. They are hunters that kill insects or other spiders, sometimes as big as they are. They perform coordinated movements – jumping from leaf to leaf while tied to a silk thread. Some even perform elaborate courtship dances.
“In my mind, a dream of a jumping spider would involve the most demanding, fitness-relevant and perhaps dramatic moments of their lives,” Klein said.
Translated by Luiz Roberto M. Gonçalves