Evolution of sleep states: Convergence or common descent?

Abstract

Sleep is an enigmatic state engaged in by all organisms studied to date. In spite of sleeps ubiquitous presence across the animal kingdom sleep manifests differently in different taxonomic groups. Interestingly, sleep in birds and mammals is composed of two distinct sub-states, slow wave sleep (SWS) and rapid eye movement (REM) sleep. Despite the presence of these similar states in birds and mammals, it has been unclear whether reptiles exhibit similar sleep states, raising the possibility that these states evolved independently in birds and mammals. In this thesis I examined various characteristics of sleep in birds and reptiles to gain a better understanding of the evolution of sleep states. Chapter 2 describes electrical signals under isoflurane anesthesia in crocodiles that are similar in some, but not all, respects to those occurring during SWS in birds. Chapter 3 demonstrates that SWS and REM sleep are present in tinamous, a member of an early evolutionary branch of birds.. Chapter 4 demonstrates that sleep responds similarly to predation risk in birds and mammals. In chapter 5, I show that, in birds, anesthetics induce brain activity in many, but not all, respects similar to SWS. These results further demonstrate that SWS and REM sleep in birds and mammals share similar regulatory mechanisms. I propose that the electrophysiological patterns observed in crocodiles reflect an ancestral form of SWS present in the common ancestor to reptiles, birds, and mammals, that was independently elaborated upon in mammals and birds.