Most of the time, when an octopus sleeps, its skin is pale and the muscles relaxed. From time to time, however, the animal will begin to twitch, tighten the suction cups, move its eyes and change color. The explanation for this is now provided by a new study, according to which the eight-armed cephalopods have active and passive sleep phases that alternate periodically – similar to humans. From the point of view of the researchers, it is even plausible that octopuses dream during active sleep phases. The result is also interesting with regard to the evolution of sleep.
Pet owners can occasionally see their four-legged friend move their paws or make noises while they sleep. It has also been shown that birds and reptiles have what is known as REM sleep – the phases in which people dream most. This phase is named after the rapid eye movements that are typical. Studies in animals and humans show that REM sleep is related to learning processes and could play a role in coping with stress.
Less excitable in sleep
Researchers led by Sylvia Medeiros from the Federal University of Rio Grande do Norte in Brazil have now used video observations on four octopuses to show that they too go through alternating sleep phases: for most of their sleeping time, the octopuses are pale in color and largely motionless. In between, however, they change their color and their muscles on the body and suction cups contract. The researchers report that increased eye movements can also be observed during this phase.
In order to verify that these active phases are actually sleep and that the octopuses don’t just wake up every now and then, the researchers presented them with visual stimuli: On a computer screen next to the aquarium, they played videos of crabs – one of their favorite foods – for them. If the octopuses were awake, they were interested in the video, if they slept on the other hand, they showed no or a significantly delayed reaction. In the case of an octopus, which was not enthusiastic about the video even when awake, the researchers instead knocked on the pool with increasing intensity. Here, too, it was shown that when the octopus slept, it needed significantly stronger stimuli before it reacted. This was even more true for active sleep phases than for passive ones. Apparently it was actually sleep.
What do octopuses dream?
“What makes it even more interesting is that this ‘active sleep’ usually occurs after a long ‘quiet sleep’ – generally longer than six minutes – and that it has a characteristic periodicity,” says Medeiro’s colleague Sidarta Ribeiro. According to the observations made by the researchers, the cycle repeats itself at approximately 30 to 40-minute intervals. The active sleep phases are usually very short – typically a few seconds to a minute. Based on the findings on REM sleep in humans and various vertebrates, the researchers speculate that the octopuses experience something similar to dreams during active sleep.
“It is not possible to confirm that they are dreaming because they cannot tell us, but our results suggest that during ‘active sleep’ the octopus might experience a state analogous to REM sleep, which is the state , in which people dream most, ”says Medeiros. As for the content of the octopus dreams, she says: “If octopuses actually dream, it is unlikely that they will experience complex symbolic actions as we do.” The active sleep phases are too short for that. “If people dream at all in this state, they tend to be in the form of small video clips or even gifs,” says Medeiros.
New look at the evolution of sleep
The discovery that octopuses have different phases of sleep and may even dream sheds new light on the evolution of sleep. “The change in sleep states that we have observed in the octopus seems to be very similar to ours, despite the enormous evolutionary distance between cephalopods and vertebrates,” says Medeiros. Given that the evolutionary lines of octopus and humans separated about 500 million years ago, it seems unlikely that the various phases of sleep go back to their last common ancestors.
“If the different sleep states actually developed twice independently of one another in vertebrates and invertebrates, the question arises: What are the essential evolutionary driving forces that shape this physiological process?” Says Medeiros. “The independent evolution of active sleep in cephalopods analogous to REM sleep in vertebrates could reflect a property that is common to all central nervous systems that reach a certain level of complexity.”
In future studies, the researchers want to record neural data from cephalopods to better understand what is going on during their sleep. They also want to clarify the role that sleep plays in animals’ metabolism, thinking and learning. “It is tempting to speculate that dreaming in the octopus, just as it does in humans, could help adapt to environmental challenges and encourage learning,” says Ribeiro.
Source: Sylvia Medeiros (Federal University of Rio Grande do Norte) et al., IScience, doi: 10.1016 / j.isci.2021.102223