Do spiders dream of catching flies? Maybe! A team of researchers, led by Dr Daniela C. Rößler from the University of Konstanz, have uncovered tantalising evidence that like mammals, jumping spiders go through different sleep stages, including REM-like sleep, which has never before been observed 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) stage of sleep, and these findings also raise the possibility that spiders may also have visual dreams.
REM sleep is characterised by eponymous eye movements, as well as the relaxation of major weight-bearing muscles. To the observer, one of the most obvious visual indicators of REM sleep is the movement, or twitching, of eyes during this phase. This observation, however, requires the subject to belong to a species with moveable eyes, an adaptation absent in insects and most terrestrial arthropods.
Most spiders have two principal eyes that see detail and colour, as well as six smaller eyes. There are over 6,400 species of jumping spiders (salticids) in the world and they are renowned for their excellent vision, which far exceeds that of other species of spiders. Part of the reason for this is down to the moveable retinal tubes at the back of their principal eyes, which allow them to redirect their gaze.
"Jumping spiders are incredibly visual, the fact that parts of their eyes are movable allowed us to observe this in the first place," lead author Rößler explains.
Juvenile jumping spiders have a further attribute that the researchers have used to their advantage; they’re translucent. It takes around 10 days for the young spiderlings to develop pigment, during which time they have a see-through exoskeleton. Add to that the fact that their head is almost entirely filled with eyeball, and jumping spiders come close to being a perfect subject. Researchers have been able to directly study retinal movement while the spiders are asleep, by looking straight through the spider itself.
"Looking to collect a different species for my predator recognition project at the time – it was too late in the year for the zebra jumping spiders – I collected these Evarcha spiders instead", Rößler explains. "We found they rest suspended (they were dangling in plastic boxes at night) and I became interested in this from a behavioural perspective – what a smart way to be safe at night. What are they doing hanging all night like that? So, I started filming them at night," Rößler said.
Using an infrared camera, the team observed the nocturnal resting behaviours in 34 newly hatched jumping spiders (Evarcha arcuata). The spiderlings were immobile throughout the night, hanging upside-down from a strand of silk, with their legs curled inwards.
Periodic bouts of retinal movements were observed, coupled with limb twitching and leg-curling behaviours. This was followed by apparent cleaning sequences of brushing moments, which imply brief awakenings after the REM sleep-like states.
They found that the bouts of retinal movement were consistent, including regular durations and intervals, with both increasing over the course of the night. This is consistent with known REM sleep-like behaviours from other species.
More like this
Scientists are fairly confident that all animals sleep, although this looks different for different species. One thing that most agree on, however, 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 in fact are asleep, both by arousal experiments (show they respond less, delayed or not at all to external stimuli while in this state) and also by sleep rebound experiments, to see if the sleep pressure increases when we deprive them of sleep", Rößler adds.
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 towards the future, Rößler explains, "Given that we have some first evidence that something like REM sleep may exist in a terrestrial invertebrate, opens up tonnes of new research. Is it also present in other arthropods? Insects? Just because eyes are not movable, may not mean that a state similar to REM cannot be present."
"Most importantly, we found a system in which we will be able to study sleep, and REM sleep-like states, in the field. How much, and how 'well' these animals sleep in nature compared to the lab, may be a completely different story – and this may also have implications on the function of these REM-like states."