A cross spider, Araneus diadematus, hangs in her web in Orrington. Photo courtesy of Joel Crabtree
A few years ago, I asked an arachnologist who studies cognition if spiders sleep, and if so, do they dream. She replied it was an interesting question, but she didn’t know of any specific studies on the subject.
It has been noticed for a long time that many spiders at least behave as if they’re sleeping. They don’t have eyelids, so physiologically they’re not doing exactly the same thing as humans or cats. But many do seem to have distinct rest periods. Some, such as Evarcha arcuata, a jumping spider native to Europe, have resting routines in which they suspend themselves from a line of silk and dangle there quietly, as if asleep. Evarcha arcuata typically hangs out upside down in this resting state during the night and gets up in the morning to resume its business in the waking world.
Some researchers based in Germany noticed that since young jumping spiders are translucent, it should be possible to observe what’s happening inside the spiders’ bodies during this resting period. So they set up an experiment that included video cameras to watch.
What they discovered is that not only do spiders pretty surely sleep, but, perchance, they dream.
Sleep research shows that humans and other animals have alternating quiet and active spells while asleep. In the quiet phases, the sleeper is still and breathing regularly. The active phases are characterized by different kinds of physical activity. These include eye movements beneath the eyelids (whose most distinct manifestations are known as REM, or rapid eye movement); sleep paralysis (also described as muscle atonia, or loss of muscle tone); respiratory irregularities; and twitching of limbs or other body parts.
Using measuring technology, it’s been shown that during active sleep, the brain gives off signals similar or identical to signals it gives off when awake. The natural (some would say obvious) inference is that during active sleep, the sleeper is having dreams. But conclusively correlating actual dreaming to these different bodily manifestations is essentially impossible because there is no objective way to observe someone else’s dream.
The upshot of this is that after decades of study, no scientist has gotten much of anywhere explaining what a dream is, or what it does, or why it occurs. There are theories. But not to put too fine a point on it, no scientist knows.
So anyway, these arachnologists set up their equipment to watch the translucent bodies of the sleeping E. arcuata spiderlings. They found out that the spiders go through the same kinds of active and quiet sleep phases as other animals. In active phases, their legs may curl, and spinnerets, abdomen and legs may twitch. Curling and twitching are associated with muscle atonia, which in spiders refers a drop in the hydraulic pressure that powers the limbs. Also during active phases, the retinae in the spiders’ eyes move around. Jumping spiders cannot manipulate the lenses in their eyes as humans do, but they have very good short-range vision and can adjust it by moving the retinal tube. In the videos, you can see the tubes jumping around inside the sleeping spiderling’s head.
It looked a lot like the spiders were asleep and having dreams.
In a subsequent study that has not yet been officially published, another team of researchers found that the cross spider, Araneus diadematus, which we have here in Maine, also showed signs of REM sleeplike phases. The researchers used different techniques to gently prod the spiders, who spend most of their time in their webs, to see if it took them longer to respond when sleeping than when awake.
Overall, it did. In the process, they discovered that the cross spiders sometimes appeared to take naps during the day, and that sometimes the spiders had an “increased arousal threshold,” meaning it took them a long time to wake up, as if they were groggily finding their way up out of deep sleep.
It has long been known that mammals and birds exhibit distinct evidence of REM sleep. It seems like a good bet, based on the twitching and whimpering dogs and cats do while asleep, that they’re dreaming. Sleeplike behavior has been documented in some jellyfish, snails, roundworms, bees, crayfish, scorpions and other animals. There’s evidence suggesting that lizards, cuttlefish and octopuses dream.
Beyond describing what happens to the body while it’s asleep, the scientists admit they can’t say much of anything else confidently. They have no consensus idea or theory of what a dream is. Even though it is obviously something. There’s the rub.
A few hours after my grandson, Silas, was born, I was holding him in my arms and watching him sleep. Newborns sleep up to 18 hours a day. It’s exhausting coming from another world into this one, where nothing makes sense.
They spend about half of their sleep in an active, REM phase. I could see that underneath his eyelids, Silas’s new little eyeballs were darting intensely back and forth. What could he possibly be seeing in his dream? He was only 12 hours old, had barely opened his eyes if he’d opened them at all, and had no visual material to make dreams on. And yet, something was clearly happening in his mind that involved his eyes. What was the material of his dream, and where did it come from?
Native cultures take it more or less for granted that dreams are access events to other worlds. They tend to interpret dream encounters with animals as communications, on the experiential evidence that humans and animals all inhabit the same dream worlds. My cat, Brian, spends as much time going about his business in that world as do newborn babies.
It does not surprise me at all to find out spiders dream. The arachnologists are way behind many indigenous cultures on this.
Dana Wilde lives in Troy. You can contact him at dwilde.naturalist@gmail.com. His book “A Backyard Book of Spiders in Maine” is available from North Country Press. Backyard Naturalist appears the second and fourth Thursdays each month.
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