These Spiders Look Like They're in REM Sleep... Which Could Mean They're Dreaming




A recent finding about leaping spiders may call into question some very significant human presumptions about arthropods' cognitive capacities.

It's possible that these tiny spiders are not only sleeping, but also dreaming, entering a sleep state that is fascinatingly similar to the rapid eye movement (REM) sleep seen in humans and other vertebrates, according to a study that looked at the movements of their eyes and bodies while they slept.

This may deepen our understanding of REM sleep, its effects on animal cognition, and other sleep-related phenomena.

The majority of sleep research to far has been centered on vertebrate animals. It has only lately been discovered that invertebrates, namely cephalopods like cuttlefish and octopuses, exhibit signs of REM sleep.

The topic of whether these creatures dream has been brought up. Which additional species of animals sleep during REM?

Maybe finding the answers to these queries will help us to understand even more puzzling issues, like how REM sleep came to be. What function does it fulfill, if any?

A group of scientists from Harvard University, led by behavioral and evolutionary ecologist Daniela Roessler, recently discovered that the jumping spider species Evarcha arcuata appeared to sleep.

The tiny spiders would dangle themselves on a single thread at sundown and remain immobile there throughout the night.

Or, more accurately, not quite still. Periods of increased activity were visible in the adult spiders Roessler and her colleagues observed; these spiders' tiny legs, spinnerets, and abdomens would twitch, or their legs would curl up in what appeared to be a defensive posture.

The scientists noticed that the movements resembled the twitches that cats and dogs experience during REM sleep. But it was hard to pinpoint exactly what the spiders were doing, at least with adult spiders.

However, the restrictions do not apply to young members of the species. Their bodies, which are still developing and expanding, are translucent because they lack pigment. This implies that during this time of nighttime stillness, it is feasible to observe and record what is happening within their bodies.

the spiderlings' retinal canals in particular. Large, black, limpid eyes of jumping spiders are set in their little heads and do not move, but their retinal tubes can move to change the spiders' field of vision while they do their crucial spider business.

Rapid eye movement is one way to identify REM sleep. Therefore, it may be possible to determine if what is occurring to these spiders indeed resembles REM sleep by directly seeing the retinal tubes of E. arcuata spiderlings.

34 E. arcuata spiderlings were seen on camera for four-hour stretches while they went about their nighttime business. In order to recognize the spiders' retinal movement, they also trained a neural network. The produced movies were then thoroughly examined.

Their recordings not only caught the sleeping spiders' retinal activity, but those movements also perfectly matched the twitches and curls of the spinnerets and legs.

In fact, although leg curling wasn't recorded in every bout of retinal movement, every incidence of leg curling that was observed was connected to one.

The spiderlings sometimes stretched or groomed themselves. The researchers observed that these occurrences would follow REM-like states but were unrelated to retinal movement per se. This, according to the experts, suggests brief awake intervals.

The retinal movement bouts were seen to occur at regular intervals when the spiders remained immobile and at lengths that were equivalent to REM sleep in other species, as they do in other animals. This meets all the criteria, according to the researchers.

In their publication, they stated that their findings "give direct evidence for a REM sleep-like state in a terrestrial invertebrate, an arthropod, with clear similarities to REM sleep in terrestrial vertebrates."

"The key behavioral requirements of REM sleep found in vertebrates, including humans, are met by the periodic limb twitches and eye movements during this sleep-like state as well as the rise in duration of REM sleep-like episodes."

This discovery is intriguing because it reveals that jumping spiders are tiny arthropods with astoundingly sharp vision.

For their size, they have two enormous eyeballs situated on the front of their faces, and six smaller eyes are arranged in an array around their heads to provide a wide field of view. Even more research raises the possibility that their vision is tetrachromatic.

The visual, "movie-like" experience of dreaming is thought to be strongly related to human REM eye movement patterns.

Therefore, a spider's sleep might involve dreams involving vision or could have another purpose related to vision. Other spiders who rely less on vision and more, perhaps, on vibration to detect the environment may do it in a way that resembles REM sleep.

More information about other sleeping animals may become available via further study, which may also provide fresh perspectives on the meaning of dreams and sleep.

While sleep is common across animals, the researchers said that it has yet to be proven if REM-like sleep is as widespread and how these sleep phases may be reflected in less visually complex species.

On the other hand, eye movement during REM sleep could be a special trait of visual brains, with this convergent evolution pointing to some important vision-specific role.


The research has been published in PNAS.


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