It is sometimes perceived that the cues affecting chorusing behavior in frogs are simple combinations of weather variables, and that closely related or sympatric species should respond to these cues in similar ways. We investigated these ideas by examining chorusing behavior in two species of tropical Australian microhylid frogs, Austrochaperina robusta and Cophixalus ornatus. Both species have small adult body sizes, are terrestrial breeding, direct-developers, and extensively co-occur in the Australian Wet Tropics. We used timed tape recorders to monitor calling activity over two breeding seasons (October to March) during 1995–96 and 2001–02. Calling activity was recorded at three times each night at seven locations along a 550 m transect. In the 2001–2002 seasons, dataloggers recorded temperature, humidity, and rainfall, and we calculated moon phase each night. Individuals of C. ornatus called more times per minute than individuals of A. robusta, and called at much more consistent levels across nights, in a seasonal pattern that closely followed a quadratic curve in both seasons. Calling activity by A. robusta males could occur at any level on any night of the breeding season, and varied greatly among nights. Principal components analyses for the 2001–02 data showed that 78.1% of variation in A. robusta calling activity could be due to factors common to the entire transect, while such factors could account for 62.8% of the variation in calling by C. ornatus. This result suggests that weather influences calling activity of A. robusta males more than of C. ornatus males. Canonical discriminant function analyses and partial rank correlations of the relationships between environmental variables and calling activity over the season indicated that initiation of calling in A. robusta males was strongly affected by rainfall, and continuation of calling by rainfall and humidity. Calling initiation in C. ornatus males was less correlated with weather variables, and appeared most strongly affected by humidity. Calling levels within species were correlated across the three recording times each night, but much less strongly than would be expected if caused by responses to weather. Calling activity also was correlated between species at each time of night, and a substantial proportion of this correlation did not appear to be caused by common responses to the weather variables we measured. We suggest that in both species weather conditions affect the probability of initiation of calling activity, and that initiation and continuation of calling are probably also affected by intraspecific and perhaps interspecific social facilitation.

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