The darker the night, the brighter the stars: consequences of nocturnal brightness on amphibian reproduction

The effects of the moon on organisms have been documented in several animal groups. However, few field studies have considered the real amount of light intensity produced by the interaction of the moon and cloudiness as determinant of nocturnal activity patterns. Many nocturnal animals may present an endogenous cycle with lunar periodicity but others are believed to respond directly to changing moonlight by synchronizing foraging and reproductive behaviours as well as predatory and anti-predatory strategies. In this study, we investigated whether and how natural environmental brightness affects the oviposition and nocturnal activity of Hyla intermedia. This is one of the few field studies, and the first attempt on amphibians, that tests the effect of measured environmental brightness on the reproductive behaviour of animals. Overall, the nocturnal activity and oviposition strategy of tree frogs appears to be not intrinsically related to the synodic lunar cycle; rather, the moon acts indirectly as the primary source of environmental light by determining an abrupt drop in tree frog activity when light increases beyond a threshold. Tree frogs showed a shift in microhabitat use by occupying concealed habitats in very low light condition, and open habitats when moonlight was enough to allow interindividual visual recognition. In this way, males likely counterbalance the risk of being conspicuous to predators by the increased chances to be detected and localized by females through both acoustic and visual stimuli. As for oviposition, tree frogs showed a laying-egg activity conditioned by the environmental brightness, with a clear preference towards dark nights as resulting by the interactive effect of weather condition and moon phases. We speculate that the evolution of behaviours involving a trade-off between activity and safety could be behind the “moonlight phobia” showed by the studied tree frog