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Dominy, N. J., & Melin, A. D. (2020). Liminal Light and Primate Evolution. Annu. Rev. Anthropol., 49(1).
Abstract: The adaptive origins of primates and anthropoid primates are topics of enduring interest to biological anthropologists. A convention in these discussions is to treat the light environment as binary—night is dark, day is light—and to impute corresponding selective pressure on the visual systems and behaviors of primates. In consequence, debate has tended to focus on whether a given trait can be interpreted as evidence of nocturnal or diurnal behavior in the primate fossil record. Such classification elides the variability in light, or the ways that primates internalize light in their environments. Here, we explore the liminality of light by focusing on what it is, its many sources, and its flux under natural conditions. We conclude by focusing on the intensity and spectral properties of twilight, and we review the mounting evidence of its importance as a cue that determines the onset or offset of primate activities as well as the entrainment of circadian rhythms.
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Murugavel, B., Kelber, A., & Somanathan, H. (2021). Light, flight and the night: effect of ambient light and moon phase on flight activity of pteropodid bats. J Comp Physiol A Neuroethol Sens Neural Behav Physiol, in press.
Abstract: Fruit-feeding pteropodid bats roost under varying light conditions. Some roost in trees with high exposure to daylight (> 1000 lx), while others roost in dark caves (< 0.1 lx). To understand the effect of ambient light intensity and moon phase on flight activity, we examined flight times across five lunar cycles in three pteropodid species whose roosts differ in daylight exposure. We found significant interspecific differences in flight emergence and termination times. All species initiated flights after sunset but Rousettus leschenaultii, which typically roosts in caves, delayed emergence (40 +/- 11 min) more than the two tree-roosting species Pteropus giganteus (16 +/- 6 min) and Cynopterus sphinx (19 +/- 7 min). R. leschenaultii terminated flights earlier (30 +/- 7 min before sunrise) than P. giganteus (11 +/- 11 min) and C. sphinx (16 +/- 10 min). All individuals from P. giganteus and C. sphinx roosts emerged within less than an hour, while emergence times were more spread out in the R. leschenaultii colony. Peak emergence times differed across moon phases in the cave-roosting R. leschenaultii but not in the other species. Flight activity in R. leschenaultii is restricted to comparatively lower light levels than the tree-roosting species. The observed interspecific differences suggest that bat species, sharing same landscapes may respond differently to light pollution.
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Spitschan, M., Aguirre, G. K., Brainard, D. H., & Sweeney, A. M. (2016). Variation of outdoor illumination as a function of solar elevation and light pollution. Sci Rep, 6, 26756.
Abstract: The illumination of the environment undergoes both intensity and spectral changes during the 24 h cycle of a day. Daylight spectral power distributions are well described by low-dimensional models such as the CIE (Commission Internationale de l'Eclairage) daylight model, but the performance of this model in non-daylight regimes is not characterised. We measured downwelling spectral irradiance across multiple days in two locations in North America: One rural location (Cherry Springs State Park, PA) with minimal anthropogenic light sources, and one city location (Philadelphia, PA). We characterise the spectral, intensity and colour changes and extend the existing CIE model for daylight to capture twilight components and the spectrum of the night sky.
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Walmsley, L., Hanna, L., Mouland, J., Martial, F., West, A., Smedley, A. R., et al. (2015). Colour As a Signal for Entraining the Mammalian Circadian Clock. PLoS Biol, 13(4), e1002127.
Abstract: Twilight is characterised by changes in both quantity (“irradiance”) and quality (“colour”) of light. Animals use the variation in irradiance to adjust their internal circadian clocks, aligning their behaviour and physiology with the solar cycle. However, it is currently unknown whether changes in colour also contribute to this entrainment process. Using environmental measurements, we show here that mammalian blue-yellow colour discrimination provides a more reliable method of tracking twilight progression than simply measuring irradiance. We next use electrophysiological recordings to demonstrate that neurons in the mouse suprachiasmatic circadian clock display the cone-dependent spectral opponency required to make use of this information. Thus, our data show that some clock neurons are highly sensitive to changes in spectral composition occurring over twilight and that this input dictates their response to changes in irradiance. Finally, using mice housed under photoperiods with simulated dawn/dusk transitions, we confirm that spectral changes occurring during twilight are required for appropriate circadian alignment under natural conditions. Together, these data reveal a new sensory mechanism for telling time of day that would be available to any mammalian species capable of chromatic vision.
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