Octopuses have two stages of sleep, including an active one in which they appear to dream, according to one of the most conclusive studies of their brainwaves and skin patterns yet.
When people sleep, we cycle through four stages. One of these is the rapid eye movement (REM) stage where brain activity increases and we dream, and the others where our brains are more quiet are called non-rapid eye movement (NREM). Birds and mammals like mice also cycle between a REM and a NREM stage, and video recordings of octopuses have suggested they might do the same.
To confirm this is the case, at the University of Washington in Seattle and her colleagues used implanted electrodes to record brain signals of nine octopuses (Octopus laqueus) while awake and asleep.
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During sleep, octopuses go through short periods where they twitch and聽thrash around and their skin rapidly switches between different patterns. Researchers have suggested this is the octopuses dreaming or having nightmares.
Meshulam and the team found that during these periods octopus brain activity closely resembles that while awake 鈥� a resemblance that is a hallmark of REM sleep in mammals. In the other, more quiet stage the octopuses鈥� brains were a lot less active. The brainwaves the researchers did detect in this stage were in areas of the brain thought to be associated with learning and memory.
The researchers then used an algorithm to compare the skin patterns that three octopuses displayed during 98 bouts of active sleep compared with when they were awake. They found that, for a given octopus, patterns it displayed during active sleep precisely aligned with those that it took on while it was awake and intentionally camouflaging to avoid danger.
Octopuses started off pale and plain and then seemed to cycle through all the patterns they use to keep themselves camouflaged while awake, says Meshulam. She says that this may be a sign that they are re-learning or practising these patterns.
Octopuses evolved intelligence completely separately from mammals, so evidence that they also sleep in stages may shed light on how sleep evolved in general, says at Washington State University.
鈥淭he implications are so huge, if there鈥檚 REM sleep for cephalopods, then there must be some basic common needs for this sleep state for all animals,鈥� says Frank.
The brainwave recording is an experimental breakthrough, as putting probes into an animal that is soft, shapeless and must be in salty water is notoriously difficult, says at the Rockefeller University in New York.
However, to insert the monitoring probes into the octopuses brains and keep the animals from interfering with them, the researchers had to use invasive techniques, says Frank. That could have disturbed the octopuses sleep and affected the results, he says.
at the Okinawa Institute of Science and Technology in Japan and part of the team says this concern is valid, but he and his colleagues compared observations from octopuses that had the procedure and those that didn鈥檛, such as how often they have an active sleep period and how their skin changes, and found largely similar behaviour.
Nature
