True, you may never see an octopus slip into some pyjamas or snuggle under a duvet, but new research has revealed the cephalopods closely mirror humans while sleeping.
A new study from Brazil’s University of Rio Grande do Norte has revealed, similar to people, the eight-limbed creatures have two major alternating sleep states: an ‘active sleep’ stage and a ‘quiet sleep’ stage.
When observing octopuses sleeping in a lab setting, researchers found that during ‘quiet sleep’, the animals were motionless, their pupils contracted.
However, during ‘active sleep’, they were seen to change their skin colour and texture, and – akin to humans during rapid eye movement (REM) sleep – to move their eyes and experience muscle twitches.
Although it could not be confirmed, these findings indicate that octopuses may be able to dream in their sleep.
“Our results suggest that during 'active sleep' the octopus might experience a state analogous to REM sleep, which is the state during which humans dream the most," said lead researcher and graduate student Sylvia Medeiros.
“If octopuses indeed dream, it is unlikely that they experience complex symbolic plots like we do. 'Active sleep' in the octopus has a very short duration – typically from a few seconds to one minute.
"If during this state there is any dreaming going on, it should be more like small videoclips, or even gifs."
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Whether octopuses dream or not, the study raises major questions about the very nature of sleep. As humans and octopuses evolved almost independently (their lineages diverged around 500 million years ago), the similarity between their sleep stages asks why both animals first adapted this behaviour.
As Medeiros said: "If in fact two different sleep states evolved twice independently in vertebrates and invertebrates, what are the essential evolutionary pressures shaping this physiological process?"
"The independent evolution in cephalopods of an 'active sleep' analogous to vertebrate REM sleep may reflect an emerging property common to centralised nervous systems that reach a certain complexity."
Reader Q&A: How many hearts does an octopus have?
Asked by: Louise Denver, Southampton
Octopuses have three hearts: one pumps blood around the body; the other two pump blood to the gills. The reason for this impressive cardiac hardware probably comes down to the unusual composition of their blood.
Unlike vertebrates that have iron-rich haemoglobin packed into red blood cells, octopuses – along with some tarantulas, scorpions and horseshoe crabs – have copper-rich haemocyanin dissolved directly in their blood (this means their blood is blue!).
Haemocyanin is less efficient than haemoglobin as an oxygen transporter. The three hearts help to compensate for this by pumping blood at higher pressure around the body to supply the octopuses’ active lifestyle.
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