Spiders show signs of REM-like activity, raising the question: Do they dream?
If you've ever seen a close-up photo of a jumping spider, you'll know they're the cute ones.
Many fit on the tip of a finger and sport fuzzy, colorful bodies and big eyes. They're quite amazing, said Daniela Roessler, a behavioral ecologist at the University of Konstanz in Germany.
"I think they're just special in a lot of ways," she said. "They have this incredible vision that is really not comparable with any other ... insects or arthropods ... and they've been shown to be really smart."
Jumping spiders tailor their hunting strategies to the species of their prey. They can leap great distances relative to their size, and they have elaborate mating dances.
They might even dream at night.
Roessler recently led a study that found jumping spiders experience something like rapid eye movement sleep when they rest at night.
REM sleep, also known as paradoxical sleep, is the phase of our sleep cycle where our brains show an increase in activity but our bodies remain immobilized — our eyes dart quickly, and we can experience really visual dreams.
The team filmed jumping spiders overnight and observed behaviors that mirror REM sleep in other species.
"All the legs would curl into the body and they would twitch," Roessler said. "We would see that always when this happened. We also detected quite significant and very obvious eye movements."
The study, which appears in the Proceedings of the National Academy of Sciences, takes advantage of the fact that baby jumping spiders have translucent exoskeletons. Jumping spiders can't move their eyes like humans, but the team observed their tube-shaped retinas moving through their clear bodies.
Octopuses have REM-like sleep. Do spiders too? We need to have a closer look at how universal REM sleep and REM-like behaviors might be across the animal kingdom. There are likely many things we can learn from this, since even in humans REM sleep is still a big mystery. 5/7 pic.twitter.com/ZXtlvSUFdj— Dr. Daniela Rößler 🌎 (@RoesslerDaniela) August 8, 2022
Roessler cautioned that it's too early to say whether the spiders are technically "asleep" during these rapid eye movements. She said they'll be testing whether the behavior meets the definition of sleep next.
But even these early findings are exciting to sleep scientists like Mark Blumberg at the University of Iowa.
"These behaviors themselves are just, you know, if I can say so, they're just beautiful behaviors," he said. "What we want to know at some fundamental level is why do these animals do this?"
Blumberg was not involved in the work, but he said studying these behaviors across the animal kingdom might someday tell us more about how REM sleep evolved. REM-like activity has been observed in almost all mammals and birds, as well as some insects and cephalopods.
Teresa Iglesias, a neuropathologist at the Okinawa Institute of Science and Technology, agreed. She has studied REM-like activity in cuttlefish.
"I think we're on the cusp of finally addressing the [questions] of: What is REM sleep and what is it for ... why did it evolve in the first place?" she said. "It's quite possible that maybe sleep and REM sleep evolved once a long time ago and everyone shares the same origin, but we're all manifesting it slightly differently."
As for the question on whether spiders dream, Roessler said it was too early to say. And even if they do, it wouldn't be like humans do.
"I mean, it's beautiful to think about it that way," she said, "that these spiders hang there and they have a visual scene of catching a fly or trying to get a mate. It's quite cute, but probably is going to be very different."
After all, Roessler pointed out that some spiders have very poor vision but are very attuned to vibrations. She wondered if they might "dream" in vibrations instead.
Something to ponder, perhaps, as we all drift off to sleep tonight.
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