Do spiders dream of arthropod sheep?

You ever watch a sleeping cat or dog, and see their little paddy-paws twitching and their legs all curled up? Cute, isn’t it? You can just imagine they’re dreaming.

Spiders do it, too.

Jumping spiders are special because, unlike most arthropods which have fixed eye positions — they have to turn their heads to change their field of view — they have telescope eyes which can swivel, so you can actually see their eyes move while they are presumably sleeping. This has led some investigators to suggest they have a kind of REM sleep.

Sleep and sleep-like states are present across the animal kingdom, with recent studies convincingly demonstrating sleep-like states in arthropods, nematodes, and even cnidarians. However, the existence of different sleep phases across taxa is as yet unclear. In particular, the study of rapid eye movement (REM) sleep is still largely centered on terrestrial vertebrates, particularly mammals and birds. The most salient indicator of REM sleep is the movement of eyes during this phase. Movable eyes, however, have evolved only in a limited number of lineages—an adaptation notably absent in insects and most terrestrial arthropods—restricting cross-species comparisons. Jumping spiders, however, possess movable retinal tubes to redirect gaze, and in newly emerged spiderlings, these movements can be directly observed through their temporarily translucent exoskeleton. Here, we report evidence for an REM sleep–like state in a terrestrial invertebrate: periodic bouts of retinal movements coupled with limb twitching and stereotyped leg curling behaviors during nocturnal resting in a jumping spider. Observed retinal movement bouts were consistent, including regular durations and intervals, with both increasing over the course of the night. That these characteristic REM sleep–like behaviors exist in a highly visual, long-diverged lineage further challenges our understanding of this sleep state. Comparisons across such long-diverged lineages likely hold important questions and answers about the visual brain as well as the origin, evolution, and function of REM sleep.

If this were happening in a cat or a dog we’d easily assume the behaviors were a reflection of whatever they were dreaming about. It would be nice to imagine this was just as true of spiders, but I’m going to be slightly skeptical of the idea that the behavior is indicative of “dreaming” or even analogous to REM sleep. The authors are cautious, too.

The complex visual and cognitive behaviors of salticids and their relatively small nervous system facilitate experimental tests of the role of visual experience in REM sleep–like retinal movements. Eye movement patterns during REM sleep have been hypothesized to be directly linked to the visual scene experienced while dreaming—begging the deeper question of whether jumping spiders may be experiencing visual dreams. This raises a unique opportunity to test this “scanning hypothesis” in jumping spiders, where retinal movements can be observed. Since visual input can be controlled in jumping spiders early on (unlike in humans), retinal responses to repeated visual stimuli presented during the day might partially reappear during REM sleep–like states.

The problem I have is that we don’t know all that is going on in the brains of humans during dreaming or REM sleep — so how can we compare that to what spiders experience? A deeper analysis of the activity of the nervous system would be needed to do a good comparison, and this paper is only looking at behavior.

An alternative to “dreaming”: spiders go through waves of alertness during sleep. At some levels, they are awake enough to monitor their environment, so those eye movements aren’t a consequence of dreams — that’s the spider doing sentry duty, scanning what’s around it to search for prey or threats. The movements tell us nothing about the inner life of a spider, only that it wants to eat or avoid being eaten. You know they don’t have eyelids, right? They aren’t as blind as we are while sleeping.

That doesn’t mean the behavior lacks any homology with vertebrates. Maybe our REM sleep is also a consequence of the evolution of rising and falling levels of alertness, and dreaming is just a side effect of brains randomly invoking patterned activity on top of all that.

If spiders dream, though, I do wonder what bloodthirsty scenarios are playing out in their heads. That may also be similar to vertebrate dreaming — I don’t think my cat dreams of frolicking in fields of dandelions, but of ripping the heads off smaller mammals. But that’s my psycho cat.


  1. birgerjohansson says

    And to paraphrase the title of the novel on which Blade Runner was based, “do spiders dream of electric arthropods”?

  2. René says

    What do spiders count when trying to get to sleep after a difficult spider’s working day?
    Curious minds wanna know.

  3. René says

    BTW. I can get really mad at the inconsistent use of hyphen, en-dash and em-dash in the junks of text PZ quotes. Then again, I am hopelessly professionally deformed by many years of editing scientists’ articles.

  4. says

    PKD was thinking counting sheep, dreaming of sheep, are androids sentient and do they dream like people do? But you knew that. It’s a strongly culturally dependent analogy. I think arthropod sheep is spot on. If we wanted to connect more directly to their world we might observe that ants herd aphids and harvest them for their sugar sweat.

    So if spiders had culture they might have their own stories about sleep and dreaming, and they would ask “do humans dream of mammal aphids?”

  5. René says

    @Stevo: Not an aficionado, just amused by the fuzz around it. In fact I find the amount of money going around in F1, tennis, soccer, … to put it bibblically an abomination.
    Always enjoying italicizing a full stop.

  6. hemidactylus says

    I would file this under Nagel’s “What Is It Like to Be a Bat?”. It’s a bit presumptuous of us to project our experiences upon animals lacking vertebrate brain features. I’m not dissing spider capacity, but they are so far removed from us the stuff between stimulus and response may be quite divergent.

  7. hemidactylus says

    I’m for Sergio Perez myself (so I hate Russell). Carlos Sainz has done amazing things though Ferrari has some nagging platform issues.

  8. unclestinky says

    And from the Scientific American article about this

    “To show the arachnids’ sleep is REM-like, the scientists will also need evidence that the spiders’ brains are active as they twitch and move their eyes, Rattenborg says. Measuring activity in a poppy-seed-sized brain will be challenging, but Rößler says there are potential ways to do it. For example, other scientists recently figured out how to insert an electrode into the brain of a different jumping spider species without deflating its pressurized body and killing it.”

    How did they manage that?

  9. nomdeplume says

    Hmmm, I think it’s dreaming in spiders. Another variable is that in an active hunter like a salticid dreams would involve movement and visual responses. In an orb spider it wouldn’t.

  10. blf says

    The mildly deranged penguin asserts spiders aren’t dreaming, not even sleeping as such, instead they are quietly waiting, very still, for the next student or other food to attach themselves to the spider’s web. The REM-like movements are not the spider doing anything, but the rest of the Universe quivering in expectant dread / anticipation who will be the spider’s next meal. That Universe-sized quivering and shaking jiggles the spider a bit, making it appear to be moving, rather like Brownian Motion only on a much larger scale.

  11. StevoR says

    @ ^ blf : I don’t really know and am just speculating and imagining here so coudl well be mistaken but webs may well well be kinda how (most?) spiders sense the world maybe just as we do largely through sight and dogs through smell? So I wonder if webs count as spider sensory organs and if they might dream in touch as we do in vision and just maybe dogs do in smell?

    @11. hemidactylus : I’m a Danny Ric fan whose second fave driver is Lewis Hamilton. Yeah, this has NOT been a good season for me so far.

    @PZ Myers : Do you think you would be able to do so non-lethally these days? Would it be worth trying again now with the new technology and techniques we’ve developed since?