Individual variation in two types of advertisement calls of Pacific tree frogs, Hyliola (=Pseudacris) regilla, and the implications for sexual selection and species recognition

Abstract

Anuran advertisement calls play a fundamental role in social interactions related to reproduction, like territory defense and mate attraction. As such, advertisement calls are often shaped by natural and sexual selection. Therefore, to understand the evolution, function, and mechanisms of signal production and perception, it is important to characterize sources and patterns of variation in call properties. We recorded and analyzed the two types of advertisement calls produced by Pacific tree frogs, Hyliola regilla: diphasic and monophasic calls. Specifically, we examined (i) the relationship between call properties and both body size and physical condition, (ii) patterns of within- and among-individual variation in call properties, and (iii) acoustic differences between diphasic and monophasic calls. While only one property of diphasic calls correlates with physical condition, spectral properties of both types of calls correlate with body size. Patterns of within- and among-individual variation were similar between call types and, overall, consistent with patterns reported for closely related species. Monophasic calls have lower frequencies, are delivered faster, and have higher call effort and duty cycle than diphasic calls. We discuss our results in relation to sexual selection and formulate hypotheses about the evolution, function, and mechanisms of acoustic communication in Pacific tree frogs.

Keywords: Hyliola regilla; Hyliola sierra; Pacific chorus frog; Pacific tree frog; advertisement call; animal communication.

Figure 1.

Acoustical analysis of diphasic and monophasic calls. Spectrogram (a) and waveform (b) of two consecutive diphasic (left) and monophasic (right) calls recorded from the same individual. Close-ups of one call (c) and two pulses (d) of each call type. (e) Average power spectra of the calls in (c). Spectrograms in (a) and power spectra in (e) were generated using 75%-overlap Hann windows with an FFT size of 512 and 1024 points, respectively. Call period (i) was used to calculate call rate and call duty cycle. Measurements associated with the overall shape of the call include the call duration(ii), time to P1 (iii), time to P2 (iv) and P1/P2 relative amplitude (v). Measurements obtained from individual pulses of both types of calls include the pulse period (vi), which was used to calculate the instantaneous pulse rate, pulse duration (vii), pulse rise time (viii), pulse fall time (ix), 50% rise time (x), and 50% fall time (xi). From the average power spectrum over the entire call (e), and using the slice tool for individual pulses (not shown), we measured the fundamental frequency (xii), the dominant frequency (xiii), and their relative amplitude (xiv). Additional measurements for diphasic calls shown here include the duration of phase 1 (xv), the inter-phase interval (xvi), and the duration of phase 2 (xvii). See main text for detailed descriptions of these and additional measurements not shown here.

Figure 2.

Patterns of within- and among-individual variation in acoustic properties. (a) Call properties with higher values of within-individual variation also have higher values of among-individual variation in diphasic (left; r=0.79, p<0.0001) and monophasic calls (right; r=0.88, p<0.0001). (b) However, the extent to which among-individual variation exceeds within-individual variation (i.e., CV_A/CV_w) is higher for properties with low within-individual variation in diphasic (r=−0.64; p<0.0001) and monophasic (r=−0.53; p=0.001) calls. Dashed lines in (b) represent same amount of within- and among-individual variation (i.e., CV_A/CV_w=1). Symbol colors represent call properties classified as static in white (CV_w<5%), intermediate (5%≤CV_w≤12%) in gray, and dynamic (CV_w>12%) in black in both (a) and (b).

Figure 3.

Compared to diphasic calls, monophasic calls have lower frequencies, higher call efforts, and are delivered at faster rates. Raw (dots), median (horizontal line), inter-quartile range (box), and non-outlier range (whiskers) values of principal components (PC) one through three (a) and one acoustic property that loads heavily on the corresponding PC and is significantly different between call types (b). Monophasic calls have significantly higher values of PC1, which loads inversely on spectral properties of the calls, higher values of PC2, which loads heavily on properties associated to the amount of sound produced per call (i.e., number of pulses, all effort, call duty cycle), and lower values of PC3, which loads inversely on call rate and measurements of instantaneous pulse rate. See Table 2, Table SM2, and main text for details.