A dog wails softly in its sleep and jolts awake. An octopus shifts color in the dark as it does when camouflaging while awake. A spider, drifting toward sleep, folds its legs and its eyes flicker. The lion rumbles in a dream, even as it rests. For centuries, dreaming was thought to be uniquely human. Today, scientists increasingly accept that other species dream as well.
Sleep and sleep-like states occur across the animal kingdom, and recent studies convincingly show sleep-like states in arthropods, nematodes, and even cnidarians. This observation comes from a study led by researchers from Harvard University, the University of Florence, and the Max Planck Institute for Animal Behavior, published in the Proceedings of the National Academy of Sciences.
Nevertheless, the presence of distinct sleep stages across different groups remains uncertain, especially regarding rapid eye movement REM sleep. The researchers acknowledge that most work to date has focused on terrestrial vertebrates, particularly mammals and birds.
“The clearest indicator of REM sleep is eye movement,” the study notes, but eye movements in many species are not as easily observed or interpreted. REM-like processes appear to have evolved in a limited set of lineages, a factor that complicates cross-species comparisons.
A notable finding is that spiders in the Salticidae family possess mobile retina tubes, allowing them to direct their gaze. In newborns, retinal movements can even be seen through their translucent exoskeletons.
animals with imagination
Researchers describe a condition in spiders that closely resembles REM sleep in humans, characterized by periodic retinal movements during night rest, limb spasms, and habitual leg curling.
Retinal movement episodes were consistent, with regular durations and intervals that tended to increase as the night progressed.
The presence of such REM-like behaviors in a visually driven and highly divergent lineage like spiders challenges current ideas about this sleep state, according to the researchers.
Dreaming may signal imaginative capacity in animals
Comparisons across these diverse origins raise questions about the brain’s visual processing, the origins of REM sleep, and its function. One of the most intriguing questions is whether dreaming reflects an imaginative ability in animals and the capacity to experience the world from their own perspective.
Most dreams in humans occur during REM. The question remains whether spiders, given their REM-like sleep, also dream. Other teams are studying species such as lizards, cuttlefish, or zebrafish, and in all of them a phase similar to human REM has been observed.
REM sleep also features rapid eye movements, temporary muscle paralysis, periodic body contractions, and rises in brain activity, breathing, and heart rate.
The exact purpose of REM remains uncertain. Scientists think it plays a role in memory and learning, helping the brain create and reorganize memories and even regulate its temperature.
Zebrafish and bearded dragons
In healthy young adults, REM sleep accounts for roughly a quarter of rest time. REM sleep was first observed in human infants in 1953 and was soon identified in other mammals, including primates, cats, mice, horses, sheep, pigs, and several other species.
Scientists suspect that octopuses and squids dream
Evidence suggests that octopuses and squid experience REM-like states during sleep, with arms and eyes moving, breathing rate rising, and color changes that imply imagined imagery. REM-like phases have also been observed in zebrafish and bearded dragons, indicating that dream-like activity may extend across distant evolutionary branches.
Additionally, some pigeon studies show REM-like contractions during rest that remind researchers of re-experiencing waking events, prompting discussions about dreaming in birds.
Given the presence of sleep-stage markers in evolutionarily distant species, scientists propose these stages arose hundreds of millions of years ago.
All these signals lead researchers to consider that animals may be dreaming; however, they emphasize the need for continued, rigorous studies to establish scientific proof.
Some scientists caution that what is observed might not be REM sleep in nonhuman animals, even if it resembles it closely. This remains a topic with ongoing debate and investigation in the field.
Reference work: PNAS study on REM-like sleep and animal dreaming.
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