Lizards may sleep in stages just like humans, according to new research. Scientists have found that bearded dragons may also dream, similar to mammals.
Amniotes, which include birds, reptiles and mammals, are a group of tetrapod vertebrates whose eggs could survive outside water. This allows them to colonize the land and diversify into several different animal groups.
In this latest study, the researchers wanted to see just how similar the reptilian brain was to the mammalian one. Because the reptilian brain has a simpler, ancestral design, it makes it far easier to study.
The researchers wanted to understand the cortical function, dynamics and computation of the reptilian brain. During experiments, the scientists noticed that the brain activity recorded from resting lizards during the night oscillated between two states. They then wondered if they were seeing REM and then slow-wave sleep.
The research team found that the Australian dragon had many common features with mammalian sleep. More specifically, it had a phase characterized by low frequency/high amplitude average brain activity and rare and bursty neuronal firing. There was also a phase that had awake-like brain activity with rapid eye movements.
With that said, there were interesting differences. Lizard sleep rhythm was extremely regular and fast in comparison to humans. Also, the lizards' slow-wave and REM-sleep had roughly the equal durations during each cycle. In humans, there are differences in the amount of slow-wave and REM-sleep.
These latest findings, in particular, show that a common ancestor is likely.
"Positing convergent evolution (two or three times in amniote evolution) of a complex phenomenon such as sleep brain dynamics is a lot less plausible than imagining a common origin," said Gilles Laurent of the Max Planck Institute for Brain Research, one of the researchers involved in the study. "Given the early branching out of the reptiles, additional evidence from several of reptilian branches such as turtles, lizards, or crocodiles will only increase the probability that we are looking at a common origin."
The results reveal a bit more about evolution and the fact that a common origin may be the most likely.
The findings were published in the April 29 issue of the journal Science.
