With its eight legs wrapped around itself as if in a hug and its eye pupils narrowed to a slit, the octopus breathes evenly, its body a uniform whitish gray.
Moments later it begins to change color – a mesmerizing shift between burnt orange and rust red. Its eyes, muscles and sucker pads twitching as if it may be experiencing a particularly vivid dream.
Brazilian scientists say the shifts in color, behavior and movement are evidence of a sleep cycle – with the octopus switching between active and quiet sleep just as humans switch between deep sleep and REM sleep – named for the rapid eye movements we experience in this state.
The findings, published Thursday in the journal iScience, show how sleep may have evolved in a similar way in very different creatures and suggests that octopuses may experience something akin to a dream.
“It is not possible to affirm that octopuses dream because they cannot tell us that, but our results suggest that during ‘Active sleep’ the octopus experiences a state analogous to REM sleep, which is the state during which humans dream the most,” said the study authors Sidarta Ribeiro and Sylvia Medeiros in an email.
Ribeiro is a professor of neuroscience at the Brain Institute of the Federal University of Rio Grande do Norte, and Medeiros is a doctoral student at the same university.
Scientists used to think that only mammals and birds experienced different sleep states – think of a sleeping cat twitching as though it were chasing a bird in the backyard. More recent research, however, has revealed some reptiles and cuttlefish – another cephalopod and relative of the octopus – show non-REM and REM-like sleep.
Octopuses have a very different brain structure to humans, but they share some of the same functions as mammal brains. The creatures have special learning abilities – including being able to solve problems and other sophisticated cognitive abilities, the authors said.
They said investigating octopus sleep was a “vantage point” for comparing them neurobiologically and psychologically with mammals – with the sleep similarities likely a consequence of “the very taxing mental loads experienced by these separate groups of animals.”
The octopus has long been a source of human fascination. Video footage from 2019 of an octopus called Heidi changing color as she slept in a tank had scientists wondering if the creatures could really dream. The Netflix documentary “My Octopus Teacher” has also showcased the creatures’ unique abilities.
Dreaming in GIFs not movies
How were the researchers sure the octopuses they studied were asleep and not just resting? The researchers videoed four members of the Octopus insularis species in their lab and studied the animals’ behavior over a period of more than 50 days. The octopuses were very sensitive to very weak stimuli when they were alert, but in both sleep states they needed a strong visual or tactile stimulus to evoke a behavioral response, the scientists said.
Octopuses usually change their skin color for camouflage or for communication but during sleep, environmental factors no longer trigger these patterns. The researchers inferred that the color changes during sleep results from independent brain activity.
The study found that the octopus experiences active sleep after a long episode of quiet sleep. In the case of an octopus, the long period is usually more than six minutes.
“If octopuses indeed dream, it is unlikely that they experience complex symbolic plots like we do. ‘Active sleep’ in the octopus has a very short duration (typically from a few seconds to one minute),” the authors said via email. “If during this state there is any dreaming going on, it should be more like small videoclips, or even gifs.”