Do spiders dream of invertebrate sheep?

Similar to the active sleep of dreaming humans, jumping spiders (Evarcha arcuate) might have a rapid eye movement (REM) sleep-like state as well – according to new research published in PNAS.

An international team of researchers studied the retinal movements of baby jumping spiders as they slept and found they coincided with body movements associated with REM sleep in other animals.  

“This report provides direct evidence for a REM sleep-like state in a terrestrial invertebrate – an arthropod – with clear parallels to REM sleep in terrestrial vertebrates,” the authors say. “The combination of periodic limb twitches and eye movements during this sleep-like state, as well as the increase of duration of REM sleep–like bouts, meets core behavioural criteria of REM sleep observed in vertebrates, including humans.

“Eye movement patterns during REM sleep have been hypothesised to be directly linked to the visual scene experienced while dreaming – begging the deeper question of whether jumping spiders may be experiencing visual dreams,” they add.

And if spiders do dream – as a certain iconic 1968 science fiction novel asks – are they dreaming of invertebrate sheep?

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Read more: REM sleep the key to dream imagery

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In many animals sleep comes in alternating periods of two distinct states – one where there isn’t movement called non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.

As you might expect, during REM stages of sleep, rapid, jerky eye movements occur. But it’s also characterised by sleep paralysis that suppresses most body movements but allows smaller-scale muscle twitches in the limbs, as well as brain waves that appear similar to the activity when awake.

Importantly, it’s during the REM sleep that we dream.

Though the best indicator of REM sleep is eye movements, moveable eyes have only evolved in a limited number of lineages across the animal kingdom. This makes it hard to compare REM sleep across species, and scientists still don’t know the exact evolutionary origin and function of it.

The team had previously found that jumping spiders suspend themselves upside down and remain inactive on a line of silk throughout the night, suggesting that they might be sleeping. During this period, the spiders exhibit phases of activity including leg curling and twitching, which the researchers had hypothesised may be expressions of a REM sleep-like state.

But while jumping spiders can’t move their eye lenses, they can move their retinal tubes to adjust their gaze. And since baby spiders’ exoskeletons lack pigmentation, researchers could use infrared (IR) cameras to look right through them and observe them during the night.

The authors recorded and analysed nocturnal IR footage of 34 juvenile jumping spiders and found that the spiderlings exhibited consistent periods of retinal movements at regular intervals. Not only that, but these periods and their duration increased over the course of the night.

Importantly, they also coincided with leg curling (broad leg contractions toward the sternum) and limb twitching – similar to the movements seen in other animals during REM sleep.