Abstract
The significance of sleep and factors that affect it have been well documented, however, in light of global climate change the effect of temperature on sleep patterns has only recently gained attention. Unlike many mammals, bats (order: Chiroptera) are nocturnal and little is known about their sleep and the effects of ambient temperature (Ta) on their sleep. Consequently we investigated seasonal temperature effects on sleep behaviour and surface body temperature of free-ranging Wahlberg’s epauletted fruit bat, Epomophorus wahlbergi, at a tree roost. Sleep behaviours of E. wahlbergi were recorded, including: sleep duration and sleep incidences (i.e. one eye open and both eyes closed). Sleep differed significantly across all the individuals in terms of sleep duration and sleep incidences. Individuals generally spent more time awake than sleeping. The percentage of each day bats spent asleep was significantly higher during winter (27.6%), compared with summer (15.6%). In summer, 20.7% of the sleeping bats used one eye open sleep, and this is possibly the first evidence of one-eye-sleep in non-marine mammals. Sleep duration decreased with extreme heat as bats spent significantly more time trying to cool by licking their fur, spreading their wings and panting. Skin temperatures of E. wahlbergi were significantly higher when Ta was ≥35°C and no bats slept at these high temperatures. Consequently extremely hot days negatively impact roosting fruit bats, as they were forced to be awake to cool themselves. This has implications for these bats given predicted climate change scenarios.
Highlights
Sleep is a state that can be defined in terms of behavioural responses of sustained immobility with greatly reduced responsiveness to external stimulation, and in terms of physiological responses such as a reduction in metabolism, body temperature etc. [1,2]
We investigated the effects of seasonal variation in Ta on sleep patterns and surface body temperatures (Tskin) of free-ranging E. wahlbergi
No cooling behaviours were observed in winter, instead all bats kept their wings wrapped around their bodies. We found that both captive and free-ranging E. wahlbergi slept relatively little during their inactive phase
Summary
Sleep is a state that can be defined in terms of behavioural responses of sustained immobility with greatly reduced responsiveness to external stimulation, and in terms of physiological responses such as a reduction in metabolism, body temperature etc. [1,2]. Sleep is a state that can be defined in terms of behavioural responses of sustained immobility with greatly reduced responsiveness to external stimulation, and in terms of physiological responses such as a reduction in metabolism, body temperature etc. [1,2] It plays an important role in mammals, which can spend a quarter or more of their lives sleeping [3,4]. The amount of sleep needed and the ability to fall asleep in mammals differs between individuals and species [11,13]. Many other factors influence sleep patterns, duration and intensity e.g. light cycle, food availability, noise conditions, predation risk and temperature [2,6,14]. Sleep patterns and duration are highly susceptible to ambient heat [15] and duration generally decreases with heat stress [16,17,18]
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