Dreaming spiders might be the thing of some arachnophobe’s nightmares – but in the case of the bug-eyed jumping spider, we think they might make an exception. New (and, frankly, adorable) research utilized the transparent exoskeletons of juvenile jumping spiders to get a look at their eyes during sleep and discovered evidence for rapid eye movement (REM) sleep while the spiderlings were snoozing.
Wondering what a sleeping spider looks like? Us too, so we asked the lead author of the new paper published to PNAS, Dr Daniela Rößler of the Max Planck Institute of Animal Behavior what exactly a sleeping jumping spider looks like.
“A usual night starts with the spider finding a good spot to drop down from,” explained Rößler, who said she never tires of watching jumping spider slumber. “They will build little silk anchors in a zig-zag fashion, and then drop down attached by a silk thread which they hold with one of the back legs.”
“They often first start cleaning themselves (brushing their legs against each other, cleaning their eyes etc.), sometimes they even have a little snack while suspended. Eventually, they become motionless. Then, in quite regular intervals we can observe what looks like active phases but with uncoordinated movements, these can be either periods with noticeable twitches of the abdomen, spinnerets or single legs, or more extreme with some or all legs curled up resembling a dead spider.”
Curled is the resting position for spiders’ legs, as was recently demonstrated in an… interesting… study that built on the hydraulics of these limbs to make dead spider necrobot grabbers. Rößler and colleagues wanted to get an idea of what was going on during these sleep-like stages when the spiders’ legs would curl and twitch, so turned to the translucent body plans of young jumping spiders for insights.
Jumping spiders can’t move their eye lenses like humans can, but they do have retinal tubes which can move to adjust their gaze. It was this movement the researchers were able to look out for in nighttime footage of 34 juvenile jumping spiders as they snoozed on camera.
The footage revealed that the spiders’ retinal tubes moved at regular intervals which increased over the course of a napping session, and that these periods were associated with leg twitches (two things observed in REM for another animals, too). Looking at just the leg movements of sleeping adult jumping spiders, the researchers noticed they seemed to reflect similar intervals to the eye-twitching phases for the young spiderlings.
The findings are, to Rößler and colleagues' knowledge, the first direct evidence for REM sleep in a terrestrial invertebrate.
As for how many peepers were getting in on the REM fun, the observable retinal movement appeared to only occur in the spiders’ biggest pair of eyes as no movement was detected in the other three pairs. However, Rößler explained that while the secondary eyes do have some muscles, they might just be moving too subtly to be detected by the study’s methodology, and they hope to explore this further in future research.
As for the big question, do spiders dream? There’s not yet definitive evidence that spider sleep involves short films about arachnids building webs on the moon, but Rößler feels quietly confident that dreams may be occurring.
“Personally, after watching hundreds of spiders, there is no doubt in my mind that they experience dreams, just like no one would doubt that when watching a cat or a dog dream,” she explained. “Whether we will be able to scientifically prove that this is the case… we will have to see.”
Which ties in nicely with the team’s goals for the future. The next step?
“Prove that they dream. ;),” said Rößler. “But also take sleep research to the field to better understand the ecological, evolutionary and functional aspects behind sleep, which is still such a poorly understood and yet possibly the most universal behavior in the animal kingdom.”