Chorus Size Shapes Calling Effort in the Asiatic Toad (Bufo gargarizans)

Acoustic communication is a core component of reproductive behavior in anuran amphibians. Among the factors shaping male calling behavior, the social acoustic environment plays a decisive role, primarily through chorus size; yet its regulatory effect in explosive-breeding species remains poorly understood. To investigate how chorus size affects the calling behavior of the Asiatic toad (Bufo gargarizans), we conducted a controlled experiment with two chorus-size treatments, small (five individuals per box) and large (15 individuals per box), corresponding to densities of approximately 13 and 39 individuals/m², each replicated across three nights using unique individuals. We compared calling activity, call composition, and acoustic parameters between treatments. Total call number was significantly higher in large choruses, and the increase was not directly proportional to the number of males present, rising by a factor of 4.2 when group size was tripled. The proportion of two-note calls also increased, with a large effect size, although this difference was not statistically significant after multiple-comparison correction at the present sample size. None of the four acoustic parameters (call duration, dominant frequency, note duration, and note interval) showed a detectable response to chorus size, a result confirmed by mixed models fitted to all 401 recorded calls. Dominant frequency was the most conservative parameter, consistent with morphological constraint and stabilizing selection by female choice. We also show that the coefficient of variation of call duration is inflated when calls differing in note number are pooled, indicating that this metric should instead be assessed within note-number classes. Our study provides the first quantitative description of the advertisement call of B. gargarizans and indicates that males respond to a crowded acoustic environment primarily by increasing calling effort rather than by adjusting the acoustic structure of individual calls.