Octopuses, or octopi, are fascinating because of their tentacular intelligence, which brings them close to human beings. But that’s not all they have in common: according to a new Japanese-American study, these sea creatures are also capable of dreaming.
Researchers at the Okinawa Institute of Science and Technology (OIST) and the University of Washington examined the brain activity and skin-colour changes of 29 octopuses belonging to the nocturnal “Octopus laqueus” species. They found that, during the day, the octopuses closed their eyes and adopted a resting posture akin to sleep.
Every hour or so, these animals also underwent rapid changes in skin colour lasting about a minute, as well as changes in respiratory rate, and body and eye movements.
These observations enabled the scientists to deduce that octopuses go through two different sleep stages: quiet sleep and active sleep. It is in the latter phase that contractions of different parts of the octopus’s body and rapid changes in the texture and pattern of its skin occur, as the researchers describe in their paper recently published in the journal “Nature”.
To test this hypothesis, the biologists verified whether the octopuses were really asleep in the active sleep stage by subjecting them to various external stimuli. In particular, they tapped on the tanks in which the animals were resting to observe their reactions.
The sea creatures reacted differently when they were awake or in a quiet or active sleep phase. The research team also found that, if the octopuses were deprived of sleep for two days, they subsequently entered the active sleep stage earlier and more frequently.
This finding suggests the existence of a homeostatic process, which is one of the key criteria for sleep.
Reliving past experiences
In addition, probes recording the neuronal activity of octopuses revealed the existence of brain waves resembling sleep spindles. Sleep spindles are a set of waves thought to protect sleep by inhibiting sensory input.
Although their exact function is still a mystery, even in humans, scientists believe these spindles contribute to the consolidation of memories. Biologists have noted that, in octopuses, these waves occur in brain regions associated with learning and memory, suggesting that they potentially have a function similar to that in humans.
These sleep spindles could explain the changes in octopus skin patterns observed by scientists. Indeed, the animals can imitate their environment by modifying the texture and appearance of their epidermis instantly and in a variety of ways, thanks to thousands of pigmented cells called chromatophores. This defence mechanism enables them to hide from potential predators.
During the active sleep stage, the scientists observed that octopuses alternate the same skin patterns. This phenomenon could be explained by the fact that the animals use their sleep to train their camouflage skills, or simply to ensure their chromatophores are working properly.
But another hypothesis is that octopuses could be reliving and learning from past experiences, such as hunting or hiding from a predator, and reactivating the skin pattern associated with each memory. In other words, they could be dreaming.
“In this sense, while humans can verbally report what kind of dreams they had only once they wake, the octopuses’ skin pattern acts as a visual readout of their brain activity during sleep,” said Dr Sam Reiter, associate professor at OIST.
“We currently don’t know which of these explanations, if any, could be correct. We are very interested in investigating further,” he added.