Environmental and social factors influence chorusing behaviour in a tropical frog: examining various temporal and spatial scales

Many animals use conspicuous display to attract mates, and there should be selection for displays to occur at times and places that maximise the probability of mating, while minimising energetic costs and predator attraction. To select the best times for display, individuals may use environmental cues, the presence of other individuals, or both, but few studies have examined these sources of variation in display activity. In this study, we examined physical environmental and social factors triggering displays in a tropical, terrestrially breeding frog, Cophixalus ornatus. To measure the influence of physical environmental conditions on calling activity, we recorded temperature, rainfall, moon illumination/visibility, humidity, barometric pressure and intensity of calling activity throughout a breeding season at six locations along a 560-m transect. The intensity of calling varied daily, seasonally, and at a small spatial scale. Variation in calling activity from day to day was large. There was also a strong seasonal trend in calling activity: few males called at the start of the season, activity peaked shortly after the beginning of the season, and then declined linearly from the peak to the end of the season. There was also consistent variation among sites along the transect, which may have been due to variations in frog density at each site, or to consistent microscale variations in physical conditions, or both. After statistically removing consistent local variation among sites, a principal components analysis suggested that a maximum of 35.8% of the variation in calling activity among days was due to factors common to all sites, such as weather, moon illumination, or large-scale social facilitation (e.g. of choruses by other choruses). The remainder of the variation among sites (64.2%) was due to site-specific factors, such as small-scale social facilitation or unmeasured, apparently stochastic effects, such as microenvironmental physical factors that do not vary consistently over sites. Regressions of environmental variables on residual calling activity (after removing consistent effects of site and season), alone or in combination, accounted for very little of the variation in the number of calling males (maximum 10%). Thus, our data, showing strong seasonal effects and consistent variation among sites combined with large amounts of variation in the number of calling males at small spatial scales, suggest that environmental conditions, such as temperature, rainfall, moon illumination and barometric pressure, which act over large spatial scales, may determine the overall environmental envelope within which calling can occur but do not account for most of the variation in the number of calling males on a day-to-day or site-to-site basis. Similarly, variations in the number of calling males at small spatial scales suggest that social facilitation is a relatively unimportant trigger for displays on a large scale in these frogs. On the other hand, our data suggest that social facilitation may have important effects on variation in the number of calling males on a day-to-day and site-to-site basis. We used playback experiments to assess whether the sound of calling could initiate displays. We played either a taped chorus or white noise in areas where few (zero to two) males were calling. The number of calling males increased both during and after the chorus stimulus, whereas there was no increase in calling in response to white noise. These data suggest that examining variation in calling activity at small spatial scales can reveal the sources of variation for the number of calling males, and indicate that, in these frogs, males tend to use the calling of other individuals as a cue to determine when to display. Received: 19 October 1999 / Revised: 30 June 2000 / Accepted: 26 August 2000