Do android reptiles dream of electric flies? If Philip K. Dick were alive today, would he write a post-apocalyptic novel based on the latest news that some reptiles dream like mammals and birds do? Would the movie be called Everglade Runner?
One of the defining signs of dreaming is REM – rapid eye movement – during sleep. It’s been seen in humans, other mammals and birds, but not in reptiles. Yet birds and reptiles evolved from common ancestors. If birds do it, shouldn’t reptiles do it too?
Gilles Laurent and a team of researchers at the Max Planck Institute for Brain Research in Frankfurt, Germany, decided to find out. They chose to study the bearded dragon lizard of Australia (Pogona vitticeps) because it started its own branch on the evolutionary tree prior to the split that led to dinosaurs and birds.
In their previous studies of reptilian brains, Laurent and his group team recorded brain activity and found that sleeping or resting reptiles show changes in brain activity at regular intervals. The next step was to prove it’s REM, slow-wave sleep (deep non-dreaming sleep) and a dream state.
The team found that the bearded dragons had the low frequency/high amplitude average brain activity that indicates REM and the bursts of neural firing that indicates slow-wave sleep and that they were controlled by the dorsal ventricular ridge (the reptilian version of the hippocampus), the same brain area that controls dreaming in humans.
While these findings indicated the dragons were dreaming, their dream states were noticeably different than those in mammals and birds. Reptilian sleep cycles lasted only a minute while they can last over an hour in humans. Also, the time periods of dreaming and non-dreaming deep sleep were equal in the dragons while they are varied and irregular in birds and mammals.
All of this strongly suggests that reptiles do dream (possibly of flies, although their tongues don’t flick during sleep likes dogs’ legs run) but their sleep state is more primitive than that of mammals and birds.
Would the fact that mammals, birds and reptiles dream prove they have a common ancestor, Dr. Laurent?
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 evidence, thus far, points to an origin of REM and slow-wave sleep at least as far back as the common ancestor of reptiles, birds and mammals, which lived about 320 million years ago.
Waiter, leave the fly in my soup.