Our sleep is marked by different brain activity. Rapid eye movement, or REM sleep, which is characterized by loss of muscle control and its eponymous eye movements, is the most well-known of these. REM sleep can be found in many mammals and birds.
Many animals don't have eyes that move in the same way as those of vertebrates so it's difficult to know what sleep is. It is necessary forlies to move their entire head to orient their eyes. A group of jumping spiders are able to reorient internal portions of their eyes during sleep.
According to this team, the spiders experience all the hallmarks of REM sleep, with periods of rapid eye movements.
Spiders, and specifically jumping spiders, may have more going on mentally than is thought based on their small nervous system. The key to this new study was the finding that they sometimes need a nap. Some of the team members were authors of a publication that reported sleep-like behavior in these spiders. At night, they'd find some overhanging vegetation, attach a single thread to it, and then stay there until the sun comes up. They are sleeping.
One of the biggest challenges in cross-species sleep studies is avoiding infections. There are structures in the eyes of jumping spiders that can be moved to direct their vision. The tubes are not visible in adult spiders due to their color. Newly hatched spiders have translucent bodies that allow their movements to be tracked.
AdvertisementThe researchers wanted to see if spiders had an REM-like phase to their sleep. Eye movement is the most important indicator of REM sleep. "Movable eyes have evolved only in a limited number of lineages, which makes it hard to compare them to other arthropods." There is no restriction for these jumping spiders.
They shut the lab lights off, let the spiders sleep in their sleep-like state, and then tracked their movements using an IR camera.
The spider's eye movement was similar to that of a mammal. The events varied from individual to individual, but they all lasted the same amount of time.
The twitching or curling of the spider's legs was associated with the Retinal tube movements. Every leg twitching that happened during the night was associated with eye movement, but only 40 percent of the periods of eye movement were associated with leg twitching.
We don't know if this behavior is a representation of REM because it does the same thing as REM sleep in humans. The hallmarks seem to be there, which has some significance. The researchers note that the REM sleep-like behaviors exist in a highly visual, long-diverged lineage. This is true since other researchers have found REM-like behavior in distantly related animals.
There is a chance of testing how deep the parallels go. The eye movements of REM are said to be a result of replaying visual memories. In a lab environment, these spiders can be exposed to visual stimuli that force them to perform certain eye movements. You can shut the lights off and see if the pattern stays the same during sleep.
Pnas in 2022. The DOI is 10.1073/pnas. 2204754119.
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