A small Australian spider spends its nights sleeping, with movements that remarkably resemble our own sleep.
The species studied is called Portia fimbriata. It is a jumping spider that hunts other spiders in particular. To rest, it sometimes hangs upside down from a strand of silk. Biologists from the University of Bonn decided to film its nights to understand what happens when it appears to be asleep.
A jumping spider Portia labiata photographed in India. It is a species closely related to Portia fimbriata, which was studied for its sleep.
Photo: sunnyjosef, via Wikimedia Commons, CC BY 4.0.
Their cameras revealed two very different periods. During one, the spider remained almost motionless. During the other, its legs twitched and its eyes moved rapidly. This behavior resembles a phase of human sleep called REM sleep, during which we dream.
But is a motionless spider necessarily asleep? To find out, the researchers made small sounds and observed its reaction. When it was resting, it took longer to react than when it was awake. Its behavior therefore corresponded to actual sleep, rather than simply a moment of calm.
The team conducted a second experiment: preventing some spiders from sleeping normally. After these interruptions, they spent more time sleeping. They were therefore trying to make up for lost sleep, as we may do after a bad night.
The researchers thus identified two sleep states and showed that these spiders' bodies genuinely regulate their need for rest. This is an important discovery, because we know much more about sleep in mammals and birds than in small invertebrates.
Does this mean that spiders dream? Not yet. The movements of their legs and eyes resemble those observed during our REM sleep, but no one knows what a spider experiences during this phase. The researchers did not measure its brain activity either.
This study raises another question: what is sleep for in animals whose brains are so different from ours? By observing more species, biologists hope to discover whether these resting phases already existed very early in the evolution of animals.