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In diurnal species with short breeding seasons, an extension of diel activity to the night hours could be favoured to maximize mating opportunities, but individuals must deal with physiological and behavioural constraints. We tested this hypothesis in the Yungas Red-belly Toad ( Melanophryniscus rubriventris (Vellard, 1947)). We registered the diel pattern of male calling activity in two localities using automated recording systems, and related it to abiotic factors such as temperature, relative air humidity, and precipitation. The diel pattern of vocalization was mainly diurnal. Interestingly though, nocturnal calling activity was a common feature, representing between 40% and 43% of call records in both localities. Vocal activity was significantly influenced by time of the day and presence of rainfall. Calling males showed high plasticity, with activity in the entire environmental range of relative air humidity and temperature. Nocturnal calling seems to play an important role in the mating strategy of males, and it is probably more frequent in the genus Melanophryniscus than currently assumed. We discuss the implications of our findings in relation to different aspects of ecology of the species and suggest that bright colouration in M. rubriventris might result from a compromise between several nonconflicting functions (e.g., aposematism and thermoregulation). [ABSTRACT FROM AUTHOR]