Researchers have found that while they are hanging from a thread at night, jumping spiders show a behavior pattern known from sleep research: They move certain eye structures and sometimes twitch their limbs while doing so. In us and in many vertebrates, this is typical of the REM phase, also known as dream sleep. The observation raises the question of whether jumping spiders, and possibly other arthropods, also experience visual dreams during sleep, the scientists say.
From humans to worms, studies have shown that sleep-like states are common across the animal kingdom. In some species, parallels to the phases of sleep known to us were also found. In addition to deep sleep, we also go through the so-called REM sleep phases during the night. They are named after the characteristic rapid eye movements. REM sleep is also characterized by reduced sleep paralysis of the body – this occasionally leads to smaller movements. It is also during this stage of sleep that we dream most, which is reflected in brain activities that are similar to waking. While there are still many unanswered questions about the importance of this sleep phase, it seems to be related to learning processes in humans and animals.
It is already well known from some vertebrate species that their sleep also includes a REM phase and that they appear to dream during it. This is why dogs and cats sometimes move their paws while they sleep. But could there be something comparable in insects and arachnids? The scientists around Daniela Rößler from the University of Konstanz have now investigated this question using the example of the jumping spider species Evarcha arcuata. They report that their previous research has shown that these animals exhibit a nocturnal behavior pattern that mimics how we sleep: they hang themselves upside down from a thread, often remaining totally inactive for long periods of time. However, the researchers also observed that the spiders move their legs and other body structures in a stereotypical manner at certain stages.
Looking into the eyes of sleeping spiders
Rößler and her colleagues asked themselves whether this pattern might correspond to the behavior known from REM sleep. In order to follow the trail, they have now investigated whether the jumping spiders also show the second behavior typical of this phase: eye movements. As the scientists explain, jumping spiders do not change the direction of their gaze by changing the position of their eyes, but instead they can use muscle movements to adjust the direction of their retinal tubes. As part of their study, the researchers took a closer look at these structures of the spider’s eyes at night using infrared cameras. For this they used young specimens that have slightly transparent head structures due to their not yet so pronounced pigmentation.
It turned out that the spiders actually went through phases during the night in which their eye structures moved in a stereotyped manner for no apparent reason. The duration of these phases increased over the course of the night. The movement pattern also coincided with previously noted leg curling and twitching, the researchers report. Interestingly, however, the spiders did not show the special movements of the retinal tubes when cleaning themselves at night or adjusting the thread. Apparently, these were short periods of wakefulness during the night, which differ from the observed conditions, say the scientists.
Do the little ones have dream images in their minds?
“The combination of periodic limb twitching and eye movements during the sleep-like state, as well as the increase in the duration of the phases, corresponds to the central behavioral criteria of REM sleep in vertebrates, including humans. Our study thus provides clear indications of a comparable condition in a terrestrial invertebrate – an arthropod,” write Rößler and her colleagues. According to them, the fact that there are parallels between evolutionarily distant lines of development in the animal kingdom is an interesting new aspect in sleep research.
The exact meaning of the REM sleep-like state for the jumping spider remains unclear, but against the background of the known functions there is now an interesting possibility: perhaps the jumping spiders also experience visual dreams in the special sleep phases. There is evidence that eye movement patterns during REM sleep are directly related to imaginary scenes. According to the researchers, future studies of the jumping spiders could now provide further insight into the function of their mysterious sleeping behavior.
Source: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2204754119