Do lizards dream? They very well might – rapid eye movement (REM) and slow-wave sleep, once thought only to be found in birds and mammals, have now been seen in lizard brains.
A study from the Max Planck Institute for Brain Research in Frankfurt, Germany recorded the brain activity of five lizards. From the data emerged a sleep cycle pattern similar to humans' but never before seen in reptiles.
The work, published in Science, places the evolution of REM and slow-wave sleep to a common ancestor of mammals, birds and reptiles hundreds of millions of years ago.
Some 300 million years ago, small, lizard-like creatures evolved into groups of animals called synapsids – which would give rise to mammals – and sauropsids. Sauropsids produced reptiles as well as birds, which diverged around 200 million years ago.
Some evolutionary biologists suspected REM and slow-wave sleep evolved independently in birds and mammals – convergent evolution driven by having warm blood.
But what if such sleep cycles existed in a common ancestor? To test this, a team of researchers led by Mark Shein-Idelson looked at lizards.
Lepidosaurs – lizards' ancestral line – emerged some 40 million years before birds. Thus, if slow-wave and REM sleep first cropped up in a common ancestor of birds and mammals, and was conserved to today, they should also see them in lizards.
So Shein-Idelson and colleagues implanted probes in the brains of Australian bearded dragons (Pogona vitticeps) to measure their neural activity during sleep.
Over a number of weeks, they found oscillating patterns in the sleeping lizards' brain activity, but of much shorter time periods than seen in humans. Where a person may experience four or five sleep cycles a night, each around 90 minutes in duration, the lizards' cycles were much shorter – around 80 seconds – and they went through around 350 cycles per sleep.
"Although similar to mammalian sleep, lizard [slow-wave sleep] and REM [sleep] resemble a stripped-down version of the richer mammalian repertoire," they write.
Where we might spend a quarter of our total sleep time in REM sleep, the lizards spent more than half their time in that state. And incredibly, along with brain activity, the researchers also saw the lizards fluttering their eyelids – an action associated with dreaming in humans. So – do lizards dream too?
Well, it's a bit early to say for sure, but the study's senior author Gilles Laurent thinks it's a possibility.
"If you forced me to speculate and to use a loose definition of dreaming, I'd speculate that those dreams are about recent notable events: insects, maybe a place where there are good insects, an aggressive male in the next terrarium, et cetera," Laurent told Reuters.
"If I were an Australian dragon living in Frankfurt, I'd be dreaming of a warm day in the sun."
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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