The color changes exhibited by octopuses while they sleep are characteristic of two main alternate sleep states: a stage of “active sleep“and a stage of” squiet sleep“.
The researchers say this finding, published in iScience, has implications for the evolution of sleep and could indicate that octopuses may experience something akin to dreams.
Scientists used to think that only mammals and birds had two sleep states. More recently, it was shown that some reptiles also show non-REM and REM sleep. A REM-like sleep state was also reported in the sepia, a cephalopod relative of the octopus.
“That led us to wonder if we might also see evidence of two sleep states in octopuses,” says lead author. Sidarta Ribeiro from the Brain Institute of the Federal University of Rio Grande do Norte, Brazil. “Octopuses have the most centralized nervous system of all invertebrates and are known to have a high capacity for learning.”
To find out, researchers captured video recordings of octopuses in the laboratory. They found that during “quiet sleep” the animals were still and silent, with pale skin and the pupils of the eyes constricted into a slit. During “active sleep”, it was a different story. The animals dynamically changed the color and texture of their fur. They also moved their eyes as they contracted their suction cups and their body in muscle spasms.
“What makes it more interesting is that this ‘active sleep’ occurs mainly after a long ‘quiet sleep’, generally of more than 6 minutes, and that it has a characteristic periodicity,” says Ribeiro.
The cycle would repeat at approximately 30-40 minute intervals. To establish that these states did indeed represent sleep, the researchers measured the octopuses’ arousal threshold using visual and tactile stimulation tests. The results of these tests showed that both in the “active” state and in the “quiet sleep” state, octopuses needed a strong stimulus to evoke a behavioral response compared to alertness. In other words, they were sleeping.
The findings have interesting implications for octopuses and for the evolution of sleep. They also raise intriguing new questions.
“The alternation of sleep states observed in the insularis octopus looks pretty similar to ours, despite the enormous evolutionary distance between cephalopods and vertebrates, with an early divergence of lineages around 500 million years ago “, says the first author and graduate student Sylvia Medeiros Instituto del Cerebro at the Federal University of Rio Grande do Norte, Brazil.
“If in fact two different sleep states evolved twice independently in vertebrates and invertebrates, what are the essential evolutionary pressures that shape this physiological process?” he wonders. “The independent evolution in cephalopods of an ‘active sleep’ analogous to REM sleep in vertebrates may reflect an emergent property common to centralized nervous systems that reach a certain complexity “.
Medeiros also says the findings raise the possibility that octopuses experience something akin to dreaming. “It is not possible to affirm that they are dreaming because they cannot tell us that, but our results suggest that during ‘active sleep’ the octopus may experience a state analogous to REM sleep, which is the state during which humans dream the most“he says.” If octopuses really dream, they are unlikely to experience complex symbolic plots like we do. The ‘active sleep’ in the octopus has a very short duration, typically a few seconds to a minute. If there are any dreams in progress during this state, it should be more like little video clips, or even gifs. ”
In future studies, the 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.
“ANDIt’s tempting to speculate that, just like in humans, dreaming of the octopus can help adapt to environmental challenges and promote learning“says Ribeiro.” Do octopuses have nightmares? Could the dreams of octopuses be inscribed in their dynamic skin patterns? Could we learn to read your dreams by quantifying these changes? ”
And next to these unknowns, perhaps others wonder if octopuses dream of “electric sheep”, as did the writer Philip K. Dick of the androids in the title of the sci-fi novel that inspired Ridley Scott’s movie ‘Blade runner‘.