Scientists have long known that octopuses sleep and that they also change color while they do so. Maybe they even dream. Now, Brazilian researchers have discovered that the color changes are not random, but correspond to two alternating sleep states: a stage of "quiet sleep" and another of "active sleep" very similar to our REM phase. This makes it possible for these cephalopods to experience something similar to dreams.
Until recently, the two sleep states were only attributed to mammals, birds and, more recently known, also to some reptiles. A REM-like state has even been reported in a cuttlefish, a cephalopod relative of the octopus.
"That led us to wonder if we could also see evidence of two sleep states in octopuses," says Sidarta Ribeiro of the Brain Institute at the Federal University of Rio Grande do Norte. "Octopuses have the most centralized nervous system of all invertebrates and are known to have a high learning capacity," he points out.
To find out, the researchers videotaped octopuses in the lab. They discovered that during "quiet sleep" the animals were still and calm, with pale skin and the pupils of their eyes contracted into a slit. However, "active sleep" was very different. The animals dynamically changed the color and texture of their skin. They also moved their eyes while contracting their suction cups and their body with muscle spasms.
"What makes it more interesting is that this 'active sleep' occurs mainly after a long 'quiet sleep', usually more than 6 minutes, and that it has a characteristic periodicity," says Ribeiro.
The cycle would repeat at intervals of approximately 30 to 40 minutes. To establish that these states actually represented sleep, the researchers measured the octopuses' arousal threshold using visual and tactile stimulation tests. The results of those tests showed that in both the "active" and "quiet sleep" states, the octopuses needed a strong stimulus to evoke a behavioral response compared to the alert state. In other words, they were sleeping.
"The alternation of sleep states observed in Octopus insularis seems quite similar to ours, despite the enormous evolutionary distance between cephalopods and vertebrates, with an early divergence of lineages about 500 million years ago," says Sylvia Medeiros, first author. of the study, published in iScience. In his opinion, "the independent evolution in cephalopods of 'active sleep' analogous to REM sleep in vertebrates may reflect an emergent property common to centralized nervous systems that reach a certain complexity."
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Medeiros also says the findings raise the possibility that octopuses experience something similar to dreaming. "It is not possible to say that they are dreaming because they cannot tell us that, but our results suggest that during 'active sleep' the octopus could experience a state analogous to REM sleep, which is the state during which humans dream the most," he explains. the researcher. "If octopuses really dream, it is unlikely that they experience complex symbolic plots like we do. 'Active sleep' in the octopus has a very short duration, typically a few seconds to a minute. If during this state there is any continuous dreaming, it should be more like small video clips, or even gifs,” he explains.
In future studies, researchers would like to record neural data from cephalopods to better understand what happens when they sleep. They are also curious about the role of sleep in animals' metabolism, thinking and learning.
"It is tempting to speculate that, as in humans, dreaming in the octopus may help adapt to environmental challenges and promote learning," says Ribeiro. «Do octopuses have nightmares? Could octopuses' dreams be inscribed in their dynamic skin patterns? "Could we learn to read your dreams by quantifying these changes?" he asks.