Do spiders dream of catching flies? Maybe! A team of researchers led by Dr. Daniela C. Roesler from the University of Konstanz has revealed that jumping spiders, like mammals, go through different sleep phases, including REM-like sleep, which Never before seen in arachnids.
This exciting discovery sheds new light on the enduring mystery of sleep in the animal kingdom. We know from human studies that our most vivid dreams occur during the rapid eye movement (REM) phase of sleep, and these findings also raise the possibility that spiders may also have visual dreams.
REM sleep is characterized by eye movements as well as relaxation of the major weight-bearing muscles. To the observer, one of the most obvious visual indicators of REM sleep is eye movement, or twitching, during this phase. However, this observation requires the subject to belong to a species with movable eyes, an adaptation absent in insects and most terrestrial arthropods.
Most spiders have two major eyes that see detail and color, as well as six smaller eyes. There are more than 6,400 species of jumping spiders (salticids) in the world and they are famous for their excellent eyesight, which is far greater than that of other species of spiders. This is because of the movable retinal tubes at the back of their main eyes, which allow them to redirect their gaze.
“Jumping spiders are incredibly visual, the fact that they have moving parts of their eyes allowed us to see it in the first place,” explains lead author Roesler.
Juvenile jumping spiders have another feature that researchers have used to their advantage; They are translucent. Young spiderlings take about 10 days to develop pigment, during which time they have a see-through exoskeleton. Add to that the fact that their head is almost completely filled with eyeballs, and jumping spiders come close to being a perfect subject. By looking directly through the spider, researchers have been able to study retinal movement directly while the spiders are asleep.
“Looking to collect a different species for my predator identification project at the time – too late in the year for the zebra jumping spider – I collected these Evercha Instead of spiders”, Roesler explains. “We found they were suspended (they hung out at night in plastic boxes) and I got interested in this from a practical point of view – what a smart way to stay safe at night. What keeps you hanging like this all night? So, I started filming them at night,” Rosler said.
Using an infrared camera, the team observed nocturnal resting behaviors in 34 newly hatched jumping spiders (avercha arcuata) The spiderwebs were motionless throughout the night, hanging upside down from a filament of silk, their legs turned inward.
Periodic bouts of retinal movement were observed, accompanied by limb twitching and leg-curling behavior. This was followed by a sequence of clear brushing moments, indicating brief awakenings followed by REM sleep-like states.
They found that tips of retinal movement consisted of regular periods and intervals, both increasing during the night. This is in line with known REM sleep-like behavior from other species.
Scientists are fairly confident that all animals sleep, although this looks different for different species. However, one thing most agree on is that REM sleep is potentially important for the consolidation of memories and honing important survival skills.
“We still need to do experiments to demonstrate that they are, in fact, asleep, both by stimulation experiments (show that they respond in this state to external stimuli with little, delay or no response at all) and Also by sleep rebound experiments, to see if sleep pressure increases when we deprive them of sleep”, Rosler says.
The authors also note that sleep, and REM sleep, in particular, have mostly been studied under laboratory conditions, which limits our understanding of sleep in the natural world.
Looking to the future, Roesler explains, “Given that we have some of the first evidence that something like REM sleep may be present in terrestrial invertebrates, opens up a lot of new research. Is it present in other arthropods as well? Insects? Just because eyes aren’t moving, doesn’t mean a state like REM can’t exist.”
“Most importantly, we have found a system in which we will be able to study sleep, and states such as REM sleep, in the field. How much and how ‘well’ these animals sleep in nature, compared to the laboratory, is a Maybe an entirely different story – and it could have an effect on the way these REM-like states work.”