During the day, jumping spiders hunt their prey, stealthily and pounced like cats. When the lights go out, these pea-sized predators hang out — and maybe their minds are spinning dreams.
As they tremble their legs and move their eyes, Evarcha arcuata, sort of jumping spiders, show something reminiscent of rapid eye movements, or REM, sleep, researchers report Monday in the Proceedings of the National Academy of Sciences. REM is the sleep phase in which most human dreams take place. The study suggests that REM sleep is more common than is realized in animals, which may help unravel the mysteries of its purpose and its evolution.
To “watch REM sleep in something distantly related to us as spiders is just extremely fascinating,” said Lauren Sumner-Rooney, a sensory biologist at the Leibniz Institute for Biodiversity and Evolution Research who was not part of it. of the new study.
Daniela Rößler, a behavioral ecologist at the University of Konstanz in Germany and one of the authors of the study, was surprised to find that jumping spiders sometimes dangle upside down at night. dr. Rößler began filming the resting arachnids and noticed other strange behavior. “All of a sudden they were making these crazy movements with the legs and they started shaking. And it immediately reminded me of a sleeping – not to say dreaming – cat or dog,” said Dr. Rößler.
Such jerky movements in limbs are a sign of REM sleep, a state in which most of the body’s muscles relax and the brain’s electrical activity mimics wakefulness. And then there are the flashing eyes, from which REM takes its name. But that is difficult to recognize in animals with non-moving eyes, including spiders.
However, part of a jumping spider’s eye does move. 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 the environment. The outside of the arthropods usually obscures these banana-shaped tubes, except when the spiders are babies and have translucent exoskeletons. So the team of Dr. Rößler to fluttering retinas during rest in spiders younger than 10 days old. “It’s very smart,” said Paul Shaw, a neuroscientist at Washington University School of Medicine. The researchers chose the right animal for this question, he added.
At night, the researchers filmed the arachnids with an infrared camera. For all 34 spiders, they saw attacks of coincident retinal and limb movements, usually lasting about 80 seconds and occurring every 15 to 20 minutes. The team recorded behaviors ranging from shifting silk-producing spinnerets to crunching all the legs that resembled a dead spider. But watching spiders rest for hours on end brought Dr. Rößler not asleep. Each spider’s movements looked unique, she said. “I always looked forward to the next REM”
What the researchers saw closely overlapped with some features of REM, said Dr. Sumner-Rooney. The twitching, relaxed muscles, and eye movements: “They all seem the same as in mammals.”
Scientists have studied REM sleep mainly in mammals. While it has been difficult to distinguish 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 older or more universal than scientists have assumed.
The team of dr. Rößler is trying to determine whether the spiders are indeed asleep. One way to demonstrate sleep is to test whether it takes more to wake a spider at rest than a spider that just isn’t moving. If experiments show that the spiders don’t just rest their eight eyes, the researchers can get a better idea of the spiders’ sleep needs by depriving them of that sleep. If sleepless spiders fall asleep faster and spend more time in a REM-like state, that would provide further evidence that they experience REM sleep.
They may even get some of the benefits associated with sleeping and dreaming in humans. “There’s no reason to think they don’t dream, 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 future perspective article at the.
“I can imagine a repetition of memories that will help them solve potential problems,” said Dr. Small. With complex brains for their size, jumping spiders have been shown to plan their routes. They are hunters that shoot insects or other spiders, sometimes as big as they are. They perform coordinated movements – jumping from leaf to leaf while anchored to a silk thread. Some even perform elaborate courtship dances.
“A dream, in my mind, for a jumping spider would entail the most demanding, fitness-relevant, perhaps dramatic times of their lives,” said Dr. Small.