Jumping spiders have REM-like twitches when they sleep, suggesting dreams may be much more widespread in the animal kingdom than previously realized.
Daniela Rößler, a behavioral ecologist, collected local jumping spiders and housed them in clear plastic cages on her windowsill, intending to evaluate their reactions to 3-D-printed predatory spider models. When she returned home from supper one night, she discovered something odd. “They were all hanging from the lids of their boxes,” Rößler, a postdoctoral researcher at Germany’s Konstanz University, explains. She’d never seen leaping spiders dangling in midair on silk threads before. “I had no notion what was going on,” Rößler recalls. “I assumed they were dead.”
The leaping spiders were merely sleeping, and Rößler had uncovered an alternate sleeping behavior of the species Evarcha arcuata, which had previously been known to make silk resting tunnels in curled-up dead leaves. The true surprise came when she decided to follow them around all night. She purchased a low-cost night-vision camera, duct-taped some magnifying lenses to it (E. arcuata is normally six millimeters long), and pointed it at a sleeping female spider. Rößler was astounded by what she saw on the recording.
The spider mostly simply hung there. But suddenly her legs began to quiver, as did her belly and even her silk-producing spinnerets. Her legs folded up against her sternum at times. These strange motions only emerged during distinct bouts that lasted a little more than a minute and happened regularly throughout the night, according to every spider Rößler documented. “They were simply twitching wildly in a way that looked a lot like when dogs or cats sleep and have their brief REM periods,” she adds.
Rapid eye movement (REM) sleep is distinguished by a condition of partial or near-total muscular paralysis combined with an active, awakelike cognitive state, which is why it is frequently referred to as “paradoxical sleep.” This mood has long been associated with dreaming in humans. Rößler and her colleagues questioned if the twitching spiders were going through a REM period of sleep and potentially dreaming. “We were thinking, ‘OK, that’d be wild,'” she recalls. “Let’s figure it out,” she thought, and instantly revised her spider study plans.
There is plenty of evidence that many animals and birds experience REM sleep or a similar condition. Scientists have discovered something similar in two reptile species, as well as evidence of a comparable condition in zebra fish. Both octopuses and cuttlefish appear to have a REM phase, replete with eye movements, arm twitches, and fast skin color and texture changes that mimic awake displays. There are glimmers but not much evidence of REM sleep in invertebrates, including insects and arachnids, beyond those species.
“It wouldn’t surprise me in the least if [jumping spiders] experience dreams,” says Lisa Taylor, a behavioral ecologist at the University of Florida who was not involved in the current study. “They live in such a sensory world, and we know they have incredible cognitive and memory ability.”
Because of certain elements of their ocular anatomy, jumping spiders provided a unique chance to broaden the domain of dreaming creatures. Though most spiders can’t move their eyes even when they’re awake, jumping spiders have lengthy tubes that move their retinas around beneath their enormous primary eyes. Furthermore, for the first few days of life, the spiders’ exoskeleton is transparent, allowing those eye tubes to be seen within their heads.
Jumping spiders caught sleeping suspended on a silk thread exhibit muscular twitches and eye movements comparable to those observed in people during the REM sleep period linked with dreaming. Scientists discovered the eye movements by recording the sleep of young spiders with translucent exoskeletons, which revealed movement of the tubes within their heads that govern the position of their retinas. Daniela C. Rößler is the photographer.
Rößler discovered that when she videotaped 34 sleeping spiderlings, their twitches were accompanied by obvious eye-tube motions that did not occur during other stages of sleep. “It’s lovely. I mean, it’s insane. “It instantly makes a sleep researcher think of rapid eye movement sleep,” says Barrett Klein, an entomologist at the University of Wisconsin-La Crosse who studies bee sleep but was not directly involved in Rößler’s work. “And to have the first clue that such a thing might be studied, or that it’s even significant in arthropods, is amazing to me.”
Klein adds that it is too soon to tell for certain if the spiders are experiencing something similar to REM sleep in humans. The researchers must first establish that the spiders are asleep during this period by demonstrating that they are less attentive to their surroundings, he explains.
Rößler and her “dream team” of co-authors have already begun the testing process. She also points out that the leg curling is a particularly remarkable element of the spiders’ REM-like phase because it is generally found exclusively in dead spiders. Spiders employ muscular pressure to keep their legs stretched, and the curling might be caused by the muscle paralysis that characterizes REM sleep. The team’s preliminary findings were published in the Proceedings of the National Academy of Sciences USA on Monday.
According to Niels Rattenborg, who researches bird sleep at the Max Planck Institute for Biological Intelligence in Seewiesen, Germany, and was not involved in the current study, the films alone seem quite compelling. “If I had to wager money, and this is not very scientific, but I will guess that they are sleepy,” he adds, partly because the motions do not appear deliberate like those of waking spiders.
To demonstrate that the arachnids’ sleep is REM-like, the researchers will also require evidence that the spiders’ brains are active when they twitch and move their eyes, according to Rattenborg. Measuring activity in a poppyseed-sized brain will be difficult, but Rößler believes there are viable solutions. Other scientists, for example, have discovered a way to implant an electrode into the brain of a different jumping spider species without inflating its inflated body and killing it.
According to Rattenborg, studying REM sleep in a variety of animals, including spiders, might help us better understand how it works and why it occurs. Some believe that the typical eye movements of REM sleep in humans represent visual events that occur during dreaming. This offers the intriguing prospect that other animals in a REM-like condition have dreams as well. Scientists cannot, of course, ask animals about their fantasies, but analyzing brain activity may one day provide an answer. According to neuroethologist Teresa Iglesias, who researches cephalopod sleep at the Okinawa Institute of Science and Technology in Japan and was not involved in the jumping spider research, “we are still discovering which patterns of brain activity are connected with dreaming in humans.”
“So it’s extremely early to think that we’ll be able to recognize dreaming in other species in a physiological sense,” she adds.
The philosophical ramifications of nonhuman creatures dreaming are potentially enormous, according to David Pea-Guzmán, a philosopher at San Francisco State University and author of the forthcoming book When Animals Dream: The Hidden World of Animal Consciousness. Dreaming provides an entrance point into concerns of awareness in other species, he says: “It’s impossible to think that even a simple dream is conceivable without anything like an ego or a “I” perceiving it.” “It may mean that we start talking about spiders having something like a minimum self,” adds Pea-Guzmán, who was not engaged in the spider research.
According to Rößler, the leaping spiders’ visible eye tubes may provide an opportunity to test the notion that fast eye movements are associated with visual dream sequences and if such images are repetitions of what the arachnids experienced while awake. It would be conceivable to show spiderlings a video of a basic scenario, like as a cricket jumping, while tracking and analyzing their eye movements, and then check if similar motions are replicated during sleep.
Rößler also wants to explore for REM sleep in other spider species, pointing out that it may appear very different in creatures that rely on senses other than vision, such as spiders that detect prey by vibrating their webs. “Perhaps orb weavers dream in vibrations,” she speculates. “I believe the story is that REM is just as common in the animal realm as sleep, but we haven’t looked.”
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