Our sleep is characterized by cycles of different brain activity. The most famous of these is probably rapid eye movements, or REM sleep, which is characterized by loss of muscle control leading to twitching and paralysis, along with the eye movements of the same name. REM sleep is widespread in vertebrates and occurs in many mammals and birds; similar periods have also been observed in lizards.
Figuring out what’s going on outside of vertebrates can be a bit challenging, though, because it’s not always clear what sleep entails, and many animals don’t have eyes that move in the same way that vertebrates do. (Flies, for example, have to move their entire head to reorient their eyes.) But an international team of researchers identified a group of jumping spiders that can reorient internal parts of their eyes during what appears to be sleep.
And according to this team, the spiders experience all the hallmarks of REM sleep, with periods of rapid eye movements accompanied by muscle twitching.
Spiders napping
Spiders, and jumping spiders in particular, may have more going on mentally than might be assumed based on their small size and associated small nervous system. But the key to this new study was the discovery that, apparently, sometimes they just need a nap. A year ago, some of the same team members were the authors of a publication reporting sleep-like behavior in these spiders. At night they would find some overhanging vegetation, attach a single wire to it so they could dangle from it, and then stay there until the morning light returns. It seems they are sleeping.
And that gives the researchers a chance to avoid one of the bigger challenges in interspecies sleep studies. Jumping spiders’ eyes contain structures called retinal tubes that can be moved to direct the spider’s vision to specific locations. These tubes are not visible in adult spiders because of the pigment in the spider’s cuticle. But newly hatched spiders take some time to develop that pigment, because they have translucent bodies that allow it to track the movements of the retinal tubes.
And so the researchers decided this was the perfect opportunity to see if spiders have an REM-like phase in their nighttime sleep. “The most notable indicator of REM sleep is eye movement during this phase,” they write. “Movable eyes, however, have evolved only in a limited number of genera — an adaptation notably absent in insects and most terrestrial arthropods — limiting comparisons between species.” This restriction does not apply to these jumping spiders.
So they turned off the lab lights, let the spiders go to sleep, and then tracked their every move using an infrared camera.
Are Rapid Eye Movements REM?
Just as you might see in a mammal, the spiders experienced periodic periods of rapid eye movements, albeit with the movement of retinal tubes. While these events varied somewhat from instance to instance and between individuals, they generally lasted the same amount of time and repeated with a period of time that was equally consistent.
Perhaps more importantly, the retinal tube movements were often associated with twitching or curling of the spiders’ legs. Only about 40 percent of the eye movements were associated with leg twitching, but any leg twitching that occurred during the sleep period was associated with eye movements.
It’s not clear that this behavior represents REM, as it performs the same function as REM sleep in humans (something we’re still trying to understand). But physically, the features seem to be there, which has some important implications. “That these characteristic REM sleep-like behaviors exist in a highly visual, long divergent line further challenges our understanding of this sleep state,” the researchers note. This is especially true as other researchers have published findings on REM-like behavior in distantly related animals such as squid.
But the spiders at stake here provide a clear opportunity to test how deep the parallels go. People have proposed that the eye movements of REM are a product of replaying visual memories during sleep. In a laboratory setting, it is possible to expose these spiders to visual stimuli that force them to perform specific patterns of eye movement. After that, you can turn off the lights and see if the same pattern repeats during sleep.
PNAS2022. DOI: 10.1073/pnas.2204754119 (About DOIs).
