You may have more in common with your iguana than you thought. Lizards apparently dream like we do, a team of French scientists revealed to the world Thursday in PLOS Biology.
Their study affirmed a 2016 study on bearded dragon lizards (Pogona vitticeps), which found that they have sleep states analogous to REM sleep and slow-wave sleep (or non-REM sleep). The new research by CNRS and Claude Bernard University Lyon 1, with a colleague from the French National Museum of Natural History, also expanded to study the sleep habits of the Argentine tegu (Salvator merianae) for the first time.
An important caveat is that we can't ask lizards or any other animal what it dreamed, and lizards have different physiology and neuroanatomy from humans. Parallels are an assumption.
But like us, they also need sleep. Deprived of it over time, we die. While we slumber, our bodies and brains carry out many vital activities, the scientists explain: Consolidation of knowledge acquired during the day; elimination of metabolic waste from the brain; hormone production; temperature regulation; and replenishment of energy stores.
The capacity for sleep states hasn’t been tested in every species of mammal, bird and reptile, and even some people aren’t sure they dream. In fact, they are confident they don’t. Science hasn’t decided if they’re deceitful, delusional or correct.
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But given how important all this is, it makes sense that as species evolved into the next species – in other words, throughout evolution – these patterns would be preserved and are now shared by all members of the animal kingdom.
Absent a way to talk with reptiles, we cannot be sure if our rapid-eye movements in sleep are akin to theirs, and we can't ask them what they dreamed, if they did. But the evidence is compelling.
If we accept that mammals, birds and lizards do dream, we can postulate that their common ancestor living over 350 million years ago did too. In which case, dinosaurs, the ancestors of birds, likely dreamed too.
Dream a little dream of bearded lizards
What lizards dream about, if they do, is open to interpretation. Take dogs, which are closer to humans than our iguanas are: One doggie owner will insist that its slumbering pet’s twitching paws mean he’s dreaming of the chase. Another will state with equal confidence that it’s dreaming of digging. Ten to one, both are wrong.
We are, however, now more confident that, like human beings, mammals and birds, the tested members of the squamate set has two states of sleep: rapid-eye movement (REM), which is when we dream; and the slow-wave sleep. The first thing the new study did is to recreate the result in bearded dragons, with deep-brain and electrooculogram recordings.
They then ran the whole battery of tests and more on the tegu, including a test that drugged the reptile with fluoxetine – which is known to suppress REM sleep in mammals. As with the bearded lizard, they found the tegu to have the two states of sleep.
There were differences in the pattern of the states of sleep between the tegu and bearded dragon. Clearly, different species experience sleep differently (“A diversity of sleep phenotypes,” the scientists call it).
Our REM sleep is characterized by brain and eye, much like that observed when we’re awake. Much the same applies to the bearded dragon – but not to the tegu, whose cerebral activity is “very unlike that of waking hours,” the scientists observe.
But the bottom line is that the results suggest a common origin of the two sleep states in amniotes, which means the ancestor of birds, reptiles and mammals experienced both and could dream.