Scientists have found evidence of patterns similar to a dream-like state in spiders, an advance that sheds more light on the origin, development and function of the sleep cycle.
The study was published Monday in the journal PNASobserved jumping spider babies using special cameras and found that the spiders’ limbs twitched, the retinas of their eyes moved and their legs curled when they rested at night.
Researchers, including those from the University of Konstanz in Germany, described this pattern as a “REM sleep-like state”.
REM, or rapid eye movement, is a stage of sleep closely related to dreaming, where parts of the human brain have been shown to show signs of activity.
While previous studies have shown evidence of sleep and sleep-like states across the animal kingdom, including in some insects, worms and even marine creatures like sharks, researchers said the existence of different phases of sleep across animal groups has remained unclear.
In particular, the study of REM sleep remains “largely centered on” terrestrial vertebrates, particularly mammals and birds.
Researchers said comparing REM sleep across species has been particularly difficult since movable eyes are only present in a limited number of animal lineages — notably absent in insects and other closely related groups.
However, jumping spiders offer a means of studying this sleep phase, as these creatures, about the size of a fingernail, have movable retinal tubes to redirect their gaze.
In newly emerged spiders of the species, researchers said, these movements can be observed more directly through their temporarily translucent exoskeleton.
In the new study, using cameras trained to observe jumping spiders at night, researchers observed periodic bouts of retinal movements along with limb twitches.
Researchers also report “stereotyped curling behavior” in a jumping spider that included wide contractions of limbs toward the sternum.
They said the observations provide parallels to REM sleep in terrestrial vertebrates such as humans and other mammals.
“Observed retinal movements were consistent, including regular durations and intervals, with both increasing during the night,” researchers wrote in the study.
“Given the regularity of twitches and leg curls and their co-occurrence with retinal movements, both movement types appeared to be different expressions of the same active sleep-like phase,” researchers explained.
They said the findings challenge conventional wisdom on REM sleep.
“Observed retinal movements were consistent, including regular durations and intervals, with both increasing during the night,” researchers noted.
“Eye movement patterns during REM sleep have been hypothesized to be directly linked to the visual scene you experience while dreaming—begging the deeper question of whether jumping spiders can experience visual dreams,” researchers concluded.
While sleep is a common trait seen across creatures in the animal kingdom, it remains to be determined whether REM-like sleep is equally universal.
It is also unclear how these sleep phases are expressed in less visual species, researchers pointed out.