One clue that dogs, cats, and other animals are dreaming is that their legs sometimes twitch as if they're running. Now, scientists have recorded video (below) of jumping spiders doing the same thing, suggesting that they're in a state resembling REM sleep and likely dreaming. University of Konstanz behavioral ecologist Daniela Rößler made the discovery during the pandemic when her laboratory was closed due to COVID-19 restrictions. In order to continue her research, she caught local spiders and trained an inexpensive night-vision camera on their container. From Scientific American:
There is abundant evidence for REM sleep or an analogous state in many mammals and birds. Scientists have also found something similar in two reptile species and hints of a state like it in zebra fish. Both octopuses and cuttlefish appear to have an REM phase, complete with eye movements, arm twitches, and rapid skin color and texture changes that resemble displays they perform when awake. Beyond those animals, there are glimmers but not much evidence of REM sleep in invertebrates, including insects and arachnids.
"It wouldn't surprise me at all if [jumping spiders] have dreams," says behavioral ecologist Lisa Taylor, who studies the spiders at the University of Florida and was not involved in the new research. "They live in such a rich sensory world, and we know they have amazing cognitive capabilities and memory."[…]
The jumping spiders' visible eye tubes might offer a way to test the theory that rapid eye movements are related to visual dream sequences and if those scenes are replays of things the arachnids witnessed while awake, Rößler says. It may be possible to play spiderlings a video of a simple scene of, say, a cricket hopping while tracking and measuring their eye movements and then see if those movements are re-created during sleep.