Sleep patterns in a chelonian reptile (Gopherus flavomarginatus)

The ecology of sleep in non-avian reptiles

Sleep is ubiquitous in the animal kingdom and yet displays considerable variation in its extent and form in the wild. Ecological factors, such as predation, competition, and microclimate, therefore are likely to play a strong role in shaping characteristics of sleep. Despite the potential for ecological factors to influence various aspects of sleep, the ecological context of sleep in non-avian reptiles remains understudied and without systematic direction. In this review, we examine multiple aspects of reptilian sleep, including (i) habitat selection (sleep sites and their spatio-temporal distribution), (ii) individual-level traits, such as behaviour (sleep postures), morphology (limb morphometrics and body colour), and physiology (sleep architecture), as well as (iii) inter-individual interactions (intra- and inter-specific). Throughout, we discuss the evidence of predation, competition, and thermoregulation in influencing sleep traits and the possible evolutionary consequences of these sleep traits for reptile sociality, morphological specialisation, and habitat partitioning. We also review the ways in which sleep ecology interacts with urbanisation, biological invasions, and climate change. Overall, we not only provide a systematic evaluation of the conceptual and taxonomic biases in the existing literature on reptilian sleep, but also use this opportunity to organise the various ecological hypotheses for sleep characteristics. By highlighting the gaps and providing a prospectus of research directions, our review sets the stage for understanding sleep ecology in the natural world.

Bird-like propagating brain activity in anesthetized Nile crocodiles

Study Objectives

The changes in electroencephalogram (EEG) activity that characterize sleep and its sub-states-slow-wave sleep (SWS) and rapid eye movement (REM) sleep-are similar in mammals and birds. SWS is characterized by EEG slow waves resulting from the synchronous alternation of neuronal membrane potentials between hyperpolarized down-states with neuronal quiescence and depolarized up-states associated with action potentials. By contrast, studies of non-avian reptiles report the presence of high-voltage sharp waves (HShW) during sleep. How HShW relate to EEG phenomena occurring during mammalian and avian sleep is unclear. We investigated the spatiotemporal patterns of electrophysiological phenomena in Nile crocodiles (Crocodylus niloticus) anesthetized with isoflurane to determine whether they share similar spatiotemporal patterns to mammalian and avian slow waves.

Methods

Recordings of anesthetized crocodiles were made using 64-channel penetrating arrays with electrodes arranged in an 8 × 8 equally spaced grid. The arrays were placed in the dorsal ventricular ridge (DVR), a region implicated in the genesis of HShW. Various aspects of the spatiotemporal distribution of recorded signals were investigated.

Results

Recorded signals revealed the presence of HShW resembling those reported in earlier studies of naturally sleeping reptiles. HShW propagated in complex and variable patterns across the DVR.

Conclusions

We demonstrate that HShW within the DVR propagate in complex patterns similar to those observed for avian slow waves recorded from homologous brain regions. Consequently, sleep with HShW may represent an ancestral form of SWS, characterized by up-states occurring less often and for a shorter duration than in mammals and birds.

The relationship between body temperature, heart rate, breathing rate, and rate of oxygen consumption, in the tegu lizard (Tupinambis merianae) at various levels of activity

The present study determined whether EEG and/or EMG recordings could be used to reliably define activity states in the Brazilian black and white tegu lizard (Tupinambis merianae) and then examined the interactive effects of temperature and activity states on strategies for matching O2 supply and demand. In a first series of experiments, the rate of oxygen consumption (VO2), breathing frequency (fR), heart rate (fH), and EEG and EMG (neck muscle) activity were measured in different sleep/wake states (sleeping, awake but quiet, alert, or moving). In general, metabolic and cardio-respiratory changes were better indictors of the transition from sleep to wake than were changes in the EEG and EMG. In a second series of experiments, the interactive effects of temperature (17, 27 and 37 °C) and activity states on fR, tidal volume (VT), the fraction of oxygen extracted from the lung per breath (FIO2-FEO2), fH, and the cardiac O2 pulse were quantified to determine the relative roles of each of these variables in accommodating changes in VO2. The increases in oxygen supply to meet temperature- and activity-induced increases in oxygen demand were produced almost exclusively by increases in fH and fR. Regression analysis showed that the effects of temperature and activity state on the relationships between fH, fR and VO2 was to extend a common relationship along a single curve, rather than separate relationships for each metabolic state. For these lizards, the predictive powers of fR and fH were maximized when the effects of changes in temperature, digestive state and activity were pooled. However, the best r(2) values obtained were 0.63 and 0.74 using fR and fH as predictors of metabolic rate, respectively.

Malondialdehyde suppresses cerebral function by breaking homeostasis between excitation and inhibition in turtle Trachemys scripta

The levels of malondialdehyde (MDA) are high in the brain during carbonyl stress, such as following daily activities and sleep deprivation. To examine our hypothesis that MDA is one of the major substances in the brain leading to fatigue, the influences of MDA on brain functions and neuronal encodings in red-eared turtle (Trachemys scripta) were studied. The intrathecal injections of MDA brought about sleep-like EEG and fatigue-like behaviors in a dose-dependent manner. These changes were found associated with the deterioration of encoding action potentials in cortical neurons. In addition, MDA increased the ratio of γ-aminobutyric acid to glutamate in turtle's brain, as well as the sensitivity of GABAergic neurons to inputs compared to excitatory neurons. Therefore, MDA, as a metabolic product in the brain, may weaken cerebral function during carbonyl stress through breaking the homeostasis between excitatory and inhibitory neurons.

Topographical distribution of the locus coeruleus and raphe nuclei in the lizard Ctenosaura pectinata: Functional implications on sleep

The nature of sleep in reptiles has traditionally created intense discussion and has originated some controversy. Nevertheless, some authors have described a sleep phase analogous to sleep in endotherm vertebrates. It is known that in mammals, the locus coeruleus and raphe nuclei, located in the brain stem, are functionally related to the sates vigilance regulation. In contrast the presence of two sleep phases in the lizard Ctenosaura pectinata similar to slow wave sleep and rapid eye movement sleep have been described. Therefore we carried out studies of the brain stem of C. pectinata to search for cellular groupings related to the regulation of these sleep phases. We identified and described the topographical distribution of the locus coeruleus and raphe nuclei in the lizard C. pectinata. Results show that these nuclei that have been functionally related to vigilance states in mammals, are also present in C. pectinata. These nuclei are formed by fairly well defined cellular groupings placed in the brain stem.

Time-related interdependence between low-frequency cortical electrical activity and respiratory activity in lizard,Gallotia galloti

Electroencephalograms of medial cortex and electromyograms of intercostal muscles (EMG-icm) were simultaneously recorded in the lizard, Gallotia galloti, during two daily time periods (at daytime, DTP: 1200-1600 h; by night, NTP: 0000-0400 h), to investigate whether a relationship exists between the respiratory and cortical electrical activity of reptiles, and, if so, how this relationship changes during the night rest period. Testing was carried out by studying interdependence between cortical electrical and respiratory activities, by means of linear and nonlinear signal analysis techniques. Both physiological activities were evaluated through simultaneous power signals, derived from the power of the low-frequency band of the electroencephalogram (pEEG-LF), and from the power of the EMG-icm (pEMG-icm), respectively. During both DTP and NTP, there was a significant coherence between both signals in the main frequency band of pEMG-icm. During both DTP and NTP, the nonlinear index N measured significant linear asymmetric interdependence between pEEG-LF and pEMG-icm. The N value obtained between pEEG-LF vs. pEMG-icm was greater than the one between pEMG-icm vs. pEEG-LF. This means that the system that generates the pEEG-LF is more complex than the one that generates the pEMG-icm, and suggests that the temporal variability of power in the low-frequency cortical electrical activity is driven by the power of the respiratory activity.

On the evolution of waking and sleeping

1. The aim of this paper is to present a new hypothesis to explain the evolution of the sleeping and waking states. 2. We propose that the reptilian waking state and the mammalian slow wave sleep are homologous states. 3. We also propose that instead of looking at the polygraphic sleep as a new evolutive acquisition of mammals and birds, it seems more convenient to look at the full waking state; the "advanced wakefulness" as the true new evolutionary acquisition of these animals. 4. These conclusions are reached after examining some available reports of slow wave electroencephalogram in waking reptiles, some other reports showing signs of rapid eye movement sleep in this same group and the coevolution between sleep states and thermoregulation. Finally, a clear parallelism between sleep ontogeny and phylogeny is shown under the light of the proposed hypothesis.

The functions of sleep: further analysis

Most theories addressed to the functions of sleep are proposed primarily according to the results in one area of sleep research and may not be compatible with the results in other areas of sleep research. This paper provides a new theory regarding the functions of sleep by integratively analyzing different areas of sleep research. First, it concludes from the phylogenetic studies and other related sleep research that sleep in mammals has at least one obligatory function which cannot be accomplished during waking. It also shows that the synchronized sleep (SS) period plays a critical role in accomplishing the obligatory functions of sleep and that the obligatory functions of sleep are related to the brain. Then it points out that adjusting and reorganizing emotional behaviors is a very important function of SS. Finally, this theory suggests that the gradual accumulation of various randomly learned memories in the limbic structures would inevitably imbalance and disorganize emotional behaviors so that sleep should be developed in evolution to adjust and reorganize emotions and so that the functions of SS for memory and emotional regulation are the obligatory functions of sleep. Although phylogenetic studies suggest that (PS) may not play obligatory functions across all mammals, there is no doubt that, in tight correlation with SS, PS also plays very important roles in memory and emotion which, however, are different from the corresponding SS roles in those mammals possessing PS.