Most animals sleep, although we still don’t quite know why. Scientists know that some animals, such as mammals, birds, and some species of reptiles, have a stage called “active” sleep, or what is known as REM (rapid eye movement) sleep. A new study, published March 25 in the open access journal iScience, sought to find out whether this “active” sleep stage also occurs in sleeping octopuses, which are as different from humans as possible.
“The motivation for studying the octopus is to see if an animal that is so far away from us, but has a very complex nervous system and very, very sophisticated behaviors, is whether this animal could also have active sleep,” says the lead author. Sidarta. Ribeiro, director of the Brain Institute of the Federal University of Rio Grande do Norte, Brazil.
The study findings suggest that it is. “We discovered that the octopus has two states of sleep, one calm and one active, just like humans,” says Ribeiro.
“We are in the middle of a revolution in understanding what the dream does for us and for organisms in general. And that’s why this article is so great, ”says Marcos Frank, a neuroscientist at Washington State University who knows Ribeiro but was not involved in the study. Frank has worked on similar issues in sepia. “Clues have been accumulating over the last 20 years or so that sleep states that are similar to human sleep can occur in other organisms that evolved independently of us.”
Led by Sylvia Lima de Souza Medeiros, a graduate student at the Brain Institute, the team of researchers studied four adult insularis octopuses, a species that lives on the north coast of Brazil. After capturing the animals and acclimating them to a laboratory environment, the team recorded them using computational methods to determine what was happening in the different states of sleep.
They found a “calm” state, which was already known, in which the octopus is “completely pale, almost completely immobile, with very smooth, sweet, gentle movements; the eyes are closed, the pupils are closed, ”says Ribeiro.
Interestingly, the team noted that about every half hour, the sea creatures’ state seemed to change: their eyes, arms, and suckers began to move freely. And, along with this movement, there were changes in the color and texture of the skin. “The colors of the skin will change drastically.”
In 2019, a viral clip from a PBS documentary showed an octopus, Heidi, shaking, changing color, and seemingly dreaming. Scientists at the time were cautious, given the absence of data showing that octopuses have REM-like stages of sleep. In the iScience study, to show whether these octopuses actually slept, the researchers stimulated them with images and crab vibrations and measured their response. The animals’ late response or total lack of feedback suggested that they were, in fact, asleep during the “quiet” and “active” stages.
The authors don’t know if these octopuses from the iScience study were dreaming; even in human beings, dreams are difficult to study. But because the “active” sleep state in octopuses generally only lasted up to a minute, “if there are any dreams during this state, it should be more like little video clips or even gifs,” lead author Medeiros explained in a sentence.
One limitation of the study, says Ribeiro, is that they only studied four animals (which “was already a lot of work”). Also, the best way to measure brain activity is using an electroencephalogram or an EEG, which requires temporarily placing or implanting electrodes on the head. But that’s difficult to do with an octopus, whose head is slippery and boned, and which tends to tear off unwanted accessories. Therefore, the team was unable to measure brain activity that could provide further confirmation of their findings. “It’s also a limitation in our studies,” says Frank.
Every time a trait evolves independently, like the eye or the wing, “it’s really exciting, because it really tells you something about why that adaptation happened in the first place,” he says. “In the case of sleep, it is extremely exciting, because we don’t really understand the function of sleep.” The fact that this REM-like experience also exists in octopuses, says Frank, “indicates that this state must be doing something important,” and understanding it could help unravel the mystery of why we sleep.