Octopuses are known to sleep and change color while they do so. Now, a study published on March 25, 2021 in the journal iScience finds that these color changes are characteristic of two major alternating sleep states: an “active sleep” stage and a “quiet sleep” stage. Researchers say the findings have implications for how sleep evolves and could indicate it’s possible for octopuses to experience something that looks like dreams.
Scientists believed that only mammals and birds had two states of sleep. More recently, it has been shown that some reptiles also exhibit non-REM and REM sleep. A REM-like sleep state has also been reported in cuttlefish, a cephalopod relative of the octopus.
“This made us wonder if we could also see evidence of two sleep states in octopuses,” says lead author Sidarta Ribeiro of the Brain Institute at the Federal University of Rio Grande do Norte, in Brazil. “Octopuses have the most centralized nervous system of all invertebrates and are known to have a high learning capacity.”
To find out, the researchers captured video recordings of octopus in the lab. They found that during “quiet sleep,” the animals were still and calm, with pale skin and pupils tightened to a slit. During “active sleep” it was a different story. Animals dynamically changed the color and texture of their skin. They also moved their eyes while contracting their cupping and their body with muscle contractions.
“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,” explains Ribeiro.
The cycle repeats at approximately 30 to 40 minute intervals. To establish that these states represented sleep well, the researchers measured the octopus’ threshold of arousal using visual and tactile stimulation tests. The results of these tests showed that in both “active” and “quiet sleep” states, octopuses needed a strong stimulus to evoke a behavioral response to the alert state. In other words, they were sleeping.
The results have interesting implications for octopuses and for the evolution of sleep. They also raise intriguing new questions.
“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 around 500 million years ago”, explains the first author and graduate student Sylvia Medeiros Brain Institute, Federal University of Rio Grande do Norte, Brazil.
“If in fact two different sleep states have evolved twice independently in vertebrates and invertebrates, what are the essential evolutionary pressures that shape this physiological process?” she asks. “The independent evolution in cephalopods of an ‘active sleep’ analogous to paradoxical sleep in vertebrates may reflect an emerging property common to centralized nervous systems reaching a certain complexity.”
Medeiros also says the findings raise the possibility that octopuses are experiencing something akin to dreaming. “It is not possible to say that they are dreaming because they cannot tell us, but our results suggest that during ‘active sleep’ the octopus might experience a state similar to REM sleep, which is the state in which humans dream the most, “she says.” If octopuses do indeed dream, they are unlikely to experience complex symbolic plots like us. ‘Active sleep’ in octopus has a very long duration. short – usually a few seconds to a minute. If during this state there are any dreams going on, it should look more like little music videos, or even gifs. “
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 animal metabolism, thinking and learning.
“It’s tempting to assume that, just like in humans, dreaming in the octopus can help adapt to environmental challenges and promote learning,” says Ribeiro. “Do octopuses have nightmares? Could octopus dreams be inscribed on their vibrant skin patterns? Can we learn to read their dreams by quantifying these changes? “
Reference: “Cyclic alternation of calm and active sleep states in octopus” by Sylvia Lima de Souza Medeiros, Mizziara Marlen Matias de Paiva, Paulo Henrique Lopes, Wilfredo Blanco, Françoise Dantas de Lima, Jaime Bruno Cirne de Oliveira, Inácio Gomes Medeiros, Eduardo Bouth Sequerra, Sandro de Souza, Tatiana Silva Leite and Sidarta Ribeiro, March 25, 2021, iScience.
DOI: 10.1016 / j.isci.2021.102223
This work was supported by the State University of Rio Grande do Norte (UERN), the Coordination for the Improvement of Higher Education Personnel (CAPES), the National Council for Scientific and Technological Development (CNPq) and of the São Paulo Research Foundation (FAPESP). ) Center for neuromathematics.