How variable and stress-sensitive is sleep expression in lizards?

Abstract

Sleep expression in wild animals is regulated by ecological context. Yet, how sleep differs between populations of a species that experience different environmental challenges such as urbanization, remains to be fully understood. Robust quantification of sleep in small animals, such as lizards, has so far been methodologically difficult, limiting population-level comparisons. Using recently developed miniature loggers, we recorded sleep in 19 wild-caught individuals of the South Indian rock agama lizard (Psammophilus dorsalis). We compared individuals captured from urban and rural environments and measured electro-oculogram (EOG)-derived sleep parameters: sleep duration, bout frequency, bout duration, and interval between bouts. We then subjected all individuals to an acute stressor (handling-restraint) and examined changes in sleep expression. We found that this species slept mostly at night, with an average total sleep time of 11h 10min. EOG-derived sleep characteristics did not differ substantially between populations, although daytime sleep bouts were more consolidated (shorter interval between bouts) in urban-origin lizards. Contrary to evidence from mammals, acute restraint stress did not impact sleep, overall or for either population. To complement these measures, we also quantified the latency to arousal after exposure to vibration stimuli in a separate set of urban and rural lizards (n = 12 each) and found that urban individuals responded marginally quicker at night compared to rural individuals, demonstrating greater vigilance and/or more fragmented sleep. Overall, we show that sleep characteristics of wild animals in a common garden can be highly conserved within a species, with limited context-dependent variation between populations.