Anuran accents: Continental-scale citizen science data reveal spatial and temporal patterns of call variability

Abstract

Many animals rely on vocal communication for mating advertisement, territorial displays, and warning calls. Advertisement calls are species-specific, serve as a premating isolation mechanism, and reinforce species boundaries. Nevertheless, there is a great deal of interspecific variability of advertisement calls. Quantifying the variability of calls among individuals within a species and across species is critical to understand call evolution and species boundaries, and may build a foundation for further research in animal communication. However, collecting a large volume of advertisement call recordings across a large geographic area has traditionally posed a logistical barrier. We used data from the continental-scale citizen science project FrogID to investigate the spatial and temporal patterns of call characteristics in six Australian frog species. We found intraspecific call variability in both call duration and peak frequency across species. Using resampling methods, we show that variability in call duration and peak frequency was related to the number of individuals recorded, the geographic area encompassed by those individuals, and the intra-annual time difference between those recordings. We conclude that in order to accurately understand frog advertisement call variation, or "anuran accents," the number of individuals in a sample must be numerous (N ≥ 20), encompass a large geographic area relative to a species' range, and be collected throughout a species' calling season.

Keywords: advertisement call; bioacoustics; frog; geographic variation; isolation by distance; temporal variation.

FIGURE 1

Study species, geographic range, and spectrograms of representative advertisement calls. Relative amplitudes over time are as follows: Crinia insignifera, 3 calls: ±500, 7 s; Crinia parinsignifera, 2 calls: ±500, 6 s; Limnodynastes dorsalis, 1 call: ±20, 2 s; Limnodynastes peronii, 2 calls: ±6, 8 s; Litoria chloris, 1 call, 11 notes: ±1, 8 s; Litoria xanthomera, 1 call, 14 notes: ±25, 15 s. Photographs by Jodi Rowley (C. parinsignifera, Lim. peronii, Lit. chloris, Lit. xanthomera) and Stephen Mahoney (C. insignifera, Lim. dorsalis)

FIGURE 2

Call duration and peak frequency, measured as the mean value for each individual analyzed, clustered by genus and species. Shapes denote genera: Crinia as squares, Limnodynastes as circles, and Litoria as triangles. Colors distinguish species within each genus: green versus blue

FIGURE 3

As the number of individuals measured increases, deviation of measured variance from actual variance decreases. Shapes and line types denote genera: Crinia as squares with solid lines, Limnodynastes as circles with dashed lines, and Litoria as triangles with dotted lines. Colors distinguish species within each genus: green versus blue.

FIGURE 4

Vocal variability correlates with (a) intra‐annual time difference and (b) geographic area. Shaded area around each linear model represents standard error, which varies based on number of individuals within a given section of the model. Colors distinguish models for call duration (green) versus peak frequency (blue)

FIGURE 5

Correlation coefficients for call duration and peak frequency as a function of intra‐annual time difference and geographic area. Error bars represent 95% confidence intervals. Colors distinguish model estimates for call duration (green) versus peak frequency (blue)