Our sleep is characterized by cycles of different brain activity. The best known of these is probably rapid eye movement or REM sleep, which is characterized by the loss of muscle control resulting in convulsions 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 beyond vertebrates can get a little tricky, however, as it’s not always clear what constitutes sleep, and many animals don’t have eyes that move in the same way that vertebrates do. (Flies, for example, have to move their entire heads to realign their eyes.) However, an international team of researchers identified a group of jumping spiders that can realign the inner 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 associated with muscle twitches.
Spiders, and jumping spiders in particular, may have more going on mentally than their tiny size and correspondingly tiny nervous system would suggest. But key to this new study was the discovery that sometimes all they seem to need is a nap. A year ago were some of the same team members Authors of a publication who reported sleep-like behavior in these spiders. At night they found some overhanging vegetation, attached a single thread to it so they could dangle, and then stayed there until the light returned in the morning. They seem to be asleep.
And that gives researchers a chance to avoid one of the bigger challenges in cross-species sleep studies. The eyes of jumping spiders contain structures called retinal tubes that can be moved to focus the spider’s vision on specific areas. These tubes are not visible in adult spiders due to the pigment in the spider’s cuticle. But newly hatched spiders take time to develop this pigment because they have translucent bodies that can be used to track the movements of the retinal tubes.
And so the researchers decided this was the perfect opportunity to see if spiders could have a REM-like phase of their night’s sleep. “The most salient indicator of REM sleep is eye movement during this phase,” they write. “However, motile eyes evolved in only a limited number of lineages — an adaptation particularly lacking in insects and most terrestrial arthropods — limiting comparisons between species.” For these jumping spiders, that limitation doesn’t apply.
So they turned off the lab lights, allowed the spiders to enter their sleep-like state, and then tracked every movement with an infrared camera.
Are rapid eye movements REM?
Just as you can see in a mammal, the spiders experienced periodic periods of rapid eye movements – although they did involve the movement of retinal tubes. Although these events varied somewhat from instance to instance and between individuals, they generally lasted a similar length of time and repeated with a similarly consistent period.
Perhaps more significantly, the movements of the retinal tube were often accompanied by twitching or curling of the spiders’ legs. Only about 40 percent of eye movement periods were associated with leg twitches, but every leg twitch that occurred during the sleep period was associated with eye movements.
It’s not clear that this behavior represents REM sleep, as it performs the same function as REM sleep in humans (something we’re still working on understanding). But physically, the hallmarks appear to be there, which has some significant implications. “That these characteristic REM sleep-like behaviors exist in a highly visual, long-diverging lineage further challenges our understanding of this sleep state,” the researchers note. This is especially true given that other researchers have published results of REM-like behavior in distantly related animals like squid.
But the spiders in question offer a definite way to test how deep the parallels run. People have suggested that REM eye movements are a product of the repetition of visual memories during sleep. In a laboratory setting, it is possible to subject these spiders to visual stimuli that force them to perform specific patterns of eye movement. After that, you can turn off the light and see if the same pattern repeats itself as you sleep.
PNAS2022. DOI: 10.1073/pnas.2204754119 (About DOIs).
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