Are these spiders dreaming? [VIDEO]

(YouTube Screenshot / Rößler et al - PNAS)

A group of German zoologists have discovered an analogue of REM sleep in jumping spiders.

The researchers observed Evarcha arcuata moving their eye tubes and twitching their limbs and body in an indication they might be dreaming.

The sleep of many animals consists of alternating periods of non-REM and REM sleep.

During REM sleep, which makes up 20% to 25% of a person's total night's sleep, brain activity is increased, muscles ar-e relaxed, and eyeballs make rapid movements.

REM sleep was first described in humans about 70 years ago, but has since been found in other mammals, birds, reptiles, and even cephalopods.

Researchers hope that studying REM sleep in animals will provide more insight into its origins and functions.

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THE STUDY
Now, zoologist Daniela C. Rößler and her team decided to look for REM sleep in spiders.

They discovered that Evarcha arcuata jumper spiders hang upside down on a cobweb at night.

Since this characteristic posture is combined with complete inactivity, the researchers suggested that this is how the spiders sleep.

From time to time, resting spiders bend and twitch their limbs, their abdomen and their spinnerets. REM sleep appears in a similar way in vertebrates.

The spiders do not have eyeballs, which movements help to accurately determine that they are in the REM phase.

Instead, their two main eyes are long tubes with a retina at the posterior end.

To focus on a particular object, these spiders move the eye tubes with the help of specialized muscles (while the lens covering the eye from the outside remains motionless).

The team suggested that if the retina of jumping spiders moves during rest, then these spiders have an analogue of the REM sleep phase.

To test this idea, the researchers conducted lab experiments with young E. arcuata, whose integument is still transparent, and the movements of their eye tubes can be observed directly.

Resting spiderlings, aged from one to nine days, were filmed with an infrared video camera from seven in the evening to seven in the morning.

The authors recorded the movements of their eye tubes, as well as twitching of the limbs and body.

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THE RESULTS
It turned out that during the rest, the spiders do move their eye tubes from time to time.

The average duration of such phases was 77 seconds, and the intervals between them were about 17 minutes.

At the same time, during the night, the duration of the periods of movement of the eye tubes and the intervals between them gradually spinnerets.

During the eye tube movement phase, the juvenile spiders flexed their limbs and also twitched their limbs, abdomen, and spinnerets.

Interestingly, resting spiderlings flexed their limbs only during periods of ocular tube movement.

However in less than half of the cases, the ocular tube movements were accompanied by flexion of the limbs.

When the spiderlings were active, such as cleaning themselves or adjusting the web thread on which they hung, their eye tubes remained motionless.

The jumping spiders usually cleaned themselves after the phase of movements of the eye tubes; perhaps after they woke up from sleep.

At the next stage, the researchers repeated the experiment with three adult E. arcuata.

Due to the pigmentation of the integument, the researchers could not follow the movements of their eye tubes.

However, they found that, like spiderlings, adults periodically begin to bend or twitch their limbs, abdomen and spinnerets during rest.

The duration of such movements averaged 83 seconds and 91 seconds, respectively.

The intervals between these phases were approximately equal and averaged 28 minutes.

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ARACHNID DREAMS?
Sometimes E. arcuata spiders do not rest on a cobweb, but in a cobweb shelter or simply on the ground.

Rößler’s team found that in this case, too, they bend their legs from time to time.

And when the authors filmed adult spiders resting on the ground with an infrared camera, they were able to capture how their eye tubes move as their limbs flex.

The results of the study demonstrate that jumping spiders have an analogue of REM sleep.

Like vertebrates, during this phase they move their eyes (more precisely, the eye tubes) and twitch their limbs and body.

Even the duration of periods resemble REM sleep, and the intervals between them are about the same in jumping spiders as in rats and mice.

It is possible that during the fast phase, spiders even dream... and if they do, What do you think they would be dreaming of?

(Spiderdreams / Image generated by DALLEmini)

Source:

• Proceedings of the National Academy of Sciences: https://www.pnas.org/doi/full/10.1073/pnas.2204754119
• Scientific American: https://www.scientificamerican.com/article/spiders-seem-to-have-rem-like-sleep-and-may-even-dream1/
• Informationsdienst Wissenschaft: https://idw-online.de/de/news799574

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