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Call Transmission Efficiency in Native and Invasive Anurans: Competing Hypotheses of Divergence in Acoustic Signals

  • Diego Llusia ,

    diego_llusia@mncn.csic.es

    Affiliation Fonoteca Zoológica, Departamento de Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain

  • Miguel Gómez,

    Affiliation Fonoteca Zoológica, Departamento de Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain

  • Mario Penna,

    Affiliation Programa de Fisiología y Biofísica, Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Santiago, Chile

  • Rafael Márquez

    Affiliation Fonoteca Zoológica, Departamento de Biodiversidad y Biología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain

Call Transmission Efficiency in Native and Invasive Anurans: Competing Hypotheses of Divergence in Acoustic Signals

  • Diego Llusia, 
  • Miguel Gómez, 
  • Mario Penna, 
  • Rafael Márquez
PLOS
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Abstract

Invasive species are a leading cause of the current biodiversity decline, and hence examining the major traits favouring invasion is a key and long-standing goal of invasion biology. Despite the prominent role of the advertisement calls in sexual selection and reproduction, very little attention has been paid to the features of acoustic communication of invasive species in nonindigenous habitats and their potential impacts on native species. Here we compare for the first time the transmission efficiency of the advertisement calls of native and invasive species, searching for competitive advantages for acoustic communication and reproduction of introduced taxa, and providing insights into competing hypotheses in evolutionary divergence of acoustic signals: acoustic adaptation vs. morphological constraints. Using sound propagation experiments, we measured the attenuation rates of pure tones (0.2–5 kHz) and playback calls (Lithobates catesbeianus and Pelophylax perezi) across four distances (1, 2, 4, and 8 m) and over two substrates (water and soil) in seven Iberian localities. All factors considered (signal type, distance, substrate, and locality) affected transmission efficiency of acoustic signals, which was maximized with lower frequency sounds, shorter distances, and over water surface. Despite being broadcast in nonindigenous habitats, the advertisement calls of invasive L. catesbeianus were propagated more efficiently than those of the native species, in both aquatic and terrestrial substrates, and in most of the study sites. This implies absence of optimal relationship between native environments and propagation of acoustic signals in anurans, in contrast to what predicted by the acoustic adaptation hypothesis, and it might render these vertebrates particularly vulnerable to intrusion of invasive species producing low frequency signals, such as L. catesbeianus. Our findings suggest that mechanisms optimizing sound transmission in native habitat can play a less significant role than other selective forces or biological constraints in evolutionary design of anuran acoustic signals.

Introduction

Invasive species are drivers of ecological and evolutionary changes [16] and a leading cause of the current biodiversity decline in all biomes [711]. Examining major traits that drive a taxon to become a widespread nonindigenous species and that favour invasion has been a key and long-standing goal of invasion biology [1220]. Among other common features, organisms establishing and spreading outside their native range tend to show a high dispersal and reproductive capacity in diverse habitats and ecosystems [17,2124].

Acoustic communication performs a key function in sexual selection and reproduction in a diversity of animal taxa, including insects, anurans, birds, and mammals (e.g. [2528]). In these groups, the range of effective communication among conspecifics is constrained by the attenuation and degradation of the acoustic signals in the environments [2941]. Sound transmission in natural conditions imposes alterations in both call amplitude and fidelity that may compromise the integrity of the signal reaching the receiver. Thus, species that produce advertisement calls maximizing propagation distance to reach potential recipients would presumably broaden their communication range and increase the probability of reproduction.

In spite of the growing interest in studying the harmful effects of biological invasions [26,811], little attention has been paid to the possible consequences of acoustic intrusion of nonindigenous species in native species and communities. Recently, two studies have provided the first evidence of acoustic niche displacement generated by invasive species. Both & Grant [42] reported the effect of calls of the American Bullfrog Lithobates catesbeianus causing an increase in call frequency in Hypsiboas albomarginatus and Farina et al. [43] examined potential masking interferences of Red-billed Leiothrix Leiothrix lutea in a temperate bird community. Here we compare for the first time the relative efficiency of transmission of the advertisement calls of native and invasive species in different environments, searching for competitive advantages for acoustic communication and reproduction of introduced taxa.

Because conveying information is the basic function of sender-receiver communication systems, the assessment of signal transmission efficiency is relevant for gaining insights into the relative significance of concomitant influences driving the evolution of animal acoustic signals. Processes of divergence of species-specific advertisement calls are still poorly understood [44], and it has been proposed that the comparative approach contributes to the understanding of evolutionary patterns, shedding light on the environmental sources of divergent selection [45,46]. Accordingly, examining alterations of acoustic signals as they propagate in native and nonindigenous environments enables us to test competing hypotheses for evolutionary divergence of acoustic signals: acoustic adaptation [31,35] vs. morphological constraints [47,48]. To the best of our knowledge, no previous studies have explored this comparative approach with native and invasive species.

As predicted by the acoustic adaptation hypothesis [31,35], selective pressures lead to a signal design that favours sound transmission in the native habitats of species communicating by means of sounds. Evidence of local adaptation of acoustic signals to physical settings of the natural environments have mainly been found in birds and mammals (e.g. [30,40,4960]). According to this hypothesis, it is expected that the advertisement calls of nonindigenous species exhibit suboptimal propagation, implying an a priori reproductive disadvantage relative to native species.

Nonetheless, the design of animal acoustic signals is subjected to influences other than environmental, such as morphological constraints or phylogenetic inertia [44,47,48,6165]. Thereby, smaller-sized individuals are limited to producing comparatively higher-pitched sounds [27,48,61,66,67], although such signals generally experience more attenuation over distance in natural environments, regardless of habitat structure (e.g. [32,34]). Because of such determinants, alternatively to the acoustic adaptation hypothesis it can be hypothesized that larger-sized invasive species would probably advertise by emitting signals with lower frequency and more efficient propagation than those of native species, although the calls of invasive species have not evolved in association with the particular habitat structure of their new environment.

In this study, we test the hypothesis that the advertisement call of an invasive anuran, the American Bullfrog Lithobates catesbeianus, propagates more efficiently than that of an Iberian anuran, the Perez’s Frog Pelophylax perezi, in native habitats of the latter species, providing the invasive species an advantage for acoustic communication and mate attraction. Using sound propagation experiments, we measured the attenuation rates of pure tones (0.2–5 kHz) and playback calls across four distances (1, 2, 4, and 8 m) and over two substrates (water and soil) in seven localities within the Iberian distribution range of P. perezi, including two localities in which L. catesbeianus were introduced, but failed to establish, in the past from abandoned commercial farms. These measurements provide a framework for the study of the effective communication range of both native and invasive species, and allow us to discriminate between two competing hypotheses that may explain the optimization of sound signal transmission and evolution of acoustic signals: 1) acoustic adaptation hypothesis, i.e. signal adaptation to local habitat conferring advantages to native species, vs. 2) morphological constraints hypothesis, i.e. size constraints of sound production mechanisms, permissive for invasive species advantages. Finally, we discuss how our findings contribute to the understanding of and the prediction of the impact of invasive species with mating systems based on acoustic communication.

Materials and Methods

Study species

The American Bullfrog Lithobates catesbeianus is a voracious and aggressive ranid frog that has been introduced beyond its native range (i.e. eastern North America) to at least thirty countries around the world (seven European countries among them) in the last two centuries [6871]. Lowe et al. [72] catalogued L. catesbeianus as one of the world’s most pernicious invasive species, causing negative impacts in native amphibians in a variety ways [7381]. Associated with the relatively large body size of L. catesbeianus (adult male snout-vent length [SVL] = 90–148 mm [82]), its advertisement call contains low spectral components (dominant frequency between 0.2–0.4 kHz and secondary frequency band between 1–2 kHz [8385]), lower than those of the advertisement call of the most of Iberian native anurans (e.g. [8688]), including Perez’s Frog Pelophylax perezi (adult male SVL = 40–85 mm; dominant frequency between 2.4–2.7 kHz [89,90]). The spectral characteristics of the call of L. catesbeianus are therefore likely to be advantageous over native species in terms of propagation efficiency. Moreover, P. perezi is a widespread Iberian ranid, the closest relative of L. catesbeianus in the Iberian Peninsula [91], and is considered to be one of the most successful species among Iberian anurans, with an almost continuous distribution throughout this region [92]. In the northernmost area of its range (i.e. south west France), P. perezi has occurred in sympatry with nonindigenous populations of L. catesbeianus for decades [69,92]. Both L. catesbeianus and P. perezi are aquatic-egg layers that congregate in water bodies where they form choruses during the breeding season [89,90,9395]. Thus, L. catesbeianus and P. perezi are appropriate comparative models for assessing the competitive abilities in acoustic transmission between competing invasive and native species.

Study sites

Sound propagation experiments were conducted in seven localities within the distribution of Pelophylax perezi in the Iberian Peninsula (Table 1, Figure 1). Study sites were selected to include different types of habitats where P. perezi breeds, such as rivers, creeks, marshes, and ponds. Measurements at three sites (El Cabaco, Las Jaras and Zarzalejo) were conducted in permanent ponds (0.1–0.9 ha) surrounded by grass pastures, bushes (Rosa spp.) and open Mediterranean forests (dominated by Quercus pyrenaica, Pinus pinea, and Quercus ilex and Fraxinus angustifolia, respectively). The substrate in El Cabaco was also partly rocky and bare. Two sites in Central Spain (Navalcarnero and Villasbuenas de Gata) included river shores having vegetation coverage of grass pastures and riparian forests (Populus nigra and Fraxinus angustifolia). The test site in Doñana was a temporary pool (1.6 ha) in the coastal salt marshes of the Doñana Biological Station. The water surface over which propagation measurements were made in Doñana had sedges (Scirpus spp.) and halophytic vegetation. Experiments in Arimbo were conducted in a creek with rocky shores and grass pastures. In two localities (Navalcarnero and Villasbuenas de Gata) Lithobates catesbeianus was introduced in the past from abandoned commercial farms, but established populations have not been confirmed [9698].

LocalityLat (N), Long (W)Test date  Air temp. (°C)Humidity (%)Habitat
Arimbo37°10', 7°51'30/03/096–1190–96Creek, rocky shore, grass pastures
Doñana36°59', 6°26'26/03/0917–2351–78Coastal salt marshes, sedges
El Cabaco40°32', 6°09'03/06/0916–1952–87Pond, bare soil, grass pastures
Las Jaras37°58', 4°50'01/04/0914–2047–70Pond, bushes, open forests
Navalcarnero40°10', 3°57'11/06/0915–2365–94River, grass pastures
Villasbuenas de Gata40°09', 6°39'01/06/0926–3035–77River, riparian forests
Zarzalejo40°32', 4°08'04/06/0912–2344–78Pond, bushes, grass pastures

Table 1. Environmental conditions in the study sites of propagation of advertisement calls of Pelophylax perezi and Lithobates catesbeianus in the Iberian Peninsula.

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Figure 1. Distribution range of Pelophylax perezi and location of the study sites in the Iberian Peninsula.

Grey circles correspond to the 10 x 10 km UTM squares (Universal Transverse Mercator coordinate system) with occurrence of P. perezi [136,137]. White circles correspond to the study sites: 1) Arimbo, 2) Doñana, 3) El Cabaco, 4) Las Jaras, 5) Navalcarnero, 6) Villasbuenas de Gata, and 7) Zarzalejo.

https://doi.org/10.1371/journal.pone.0077312.g001

Measurements were recorded between March and June of 2009, corresponding to the breeding season of the study species in this region, so that the study sites showed the same environmental conditions in which calling activity takes place. To avoid interference from calling individuals, the propagation experiments were conducted a few hours before sunset, prior to the peak of calling activity of the resident population. As air temperature and humidity have negligible effects on sound propagation at the frequencies and distances from the source that we tested [41,99,100], these measurements are assumed to apply also to night hours. During the experiments, the atmospheric conditions were stable and only slight gusts of wind occurred occasionally, during which measurements were temporally suspended. Air and surface water temperature and relative humidity were measured every 5 min with data loggers (HOBO Pendant 64K and HOBO Pro V-2, Onset Computer Corporation, Cape Cod, MA, USA) during the entire testing period. Background noise was also recorded for 45 s before and after the measurements with the microphone of a sound level meter (B&K 2238 Mediator, Brüel & Kjær, Nærum, Denmark) positioned on the propagation transects.

Ethical statements

This study was carried out in public and protected areas. Permits to work and use of facilities in the protected area Doñana Biological Station were granted by ICTS Doñana (CSIC). Experiments did not include animal collection, capture, manipulation, or disturbance, endangered or protected species, and only involved the emission of recorded sounds. Therefore, this study was not submitted to approval for Institute of Animal Care and Use Committee (IACUC), and no specific permits were required according to national and local regulations.

Broadcast signals

Propagation efficiency of the study species were measured by pre-recorded signals played back through loudspeakers installed in natural environments, as used in previous studies (e.g. [32,33,40,41,53,101]). This experimental procedure was selected because it has shown lack of significant differences in attenuation average rates by comparing with those obtained from natural calls [102].

A 3 min audio file (44.1 KHz and 16 bit) containing advertisement calls of 7 Pelophylax perezi and 6 Lithobates catesbeianus, 29 pure tones, and white noise was used for propagation experiments. The calls of P. perezi (Figure 2a) were recorded with a digital audio tape recorder (DA-P1 TASCAM, Montebello, CA, USA) and directional microphones (Sennheiser M66 and M80, Wedemark, Germany) between May and June of 2004 in El Casar (Guadalajara, Spain; N 40° 42’, W 3° 25’) at air temperatures of 16–20 °C. The calls of L. catesbeianus (Figure 2b) were recorded with a digital audio field recorder (Marantz PMD 660, Kanagawa, Japan) and directional microphones (Sennheiser MKH 70, Wedemark, Germany; and Telinga Pro 6, Uppsala, Sweden) in May of 2008 in the Mammoth Cave National Park (Kentucky, USA; N 37° 09’, W 86° 06’) at air temperatures of 24 °C. The vocalizations were recorded 2–10 m from the calling individuals. From each individual, 6 calls were included in the playback recording to account for intra-individual variation. All signals were previously filtered between 0 and 300 Hz and 100% peak normalized to standardize their relative amplitude with Raven 1.2 (Cornell University, Ithaca, NY, USA). For P. perezi, mean ± SD (minimum-maximum) of dominant frequency (Hz) and call duration (ms) were 2678.3 ± 279.5 (2153.3–3186.9) and 430 ± 183 (181–862), respectively. For L. catesbeianus, mean ± SD (minimum-maximum) of dominant frequency (Hz) and call duration (ms) were 1405.2 ± 552.6 (430.7–1981.1) and 641 ± 101 (453–862), respectively. These recordings are deposited in the collection of the Fonoteca Zoológica (Zoological Record Library of the National Museum of Natural Sciences, MNCN-CSIC, Madrid, Spain), collection codes 9086–9092 (P. perezi) and 8188–8189 (L. catesbeianus). In addition to the advertisement calls, 29 distinct pure tones of 0.5 s each and white noise of 3 s were also included in the playback recording in order to examine the transmission properties of the study sites for specific sound frequencies. The series of pure tones was composed of 23 tones between 200 and 2500 Hz, in steps of 100 Hz, and 6 tones between 2500 and 5000 Hz, in 500 Hz intervals. Signal generation and call editing was performed with Audacity 1.3.6 (SourceForge) and Peak Pro 5.2 (BIAS, Petaluma, CA, USA).

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Figure 2. Oscillograms and power spectra of representative advertisement calls for sound propagation experiments: (A) Pelophylax perezi and (B) Lithobates catesbeianus.

Air temperatures during recordings of these calls were 20 °C and 24 °C, respectively. Figures generated with Seewave software (3170 FFT size, 90% overlap, A-weighting; [105]).

https://doi.org/10.1371/journal.pone.0077312.g002

Experimental procedures

The playback calls and pure tones were broadcast with a self-powered loudspeaker (Explorer Pro 7500, Anchor Audio, Carlsbad, CA, USA) connected to a laptop computer (Mac iBook G4, Apple Inc., Cupertino, CA, USA) and placed at positions typically occupied by calling males of Pelophylax perezi on the shores of water bodies at the study sites. The broadcast level was adjusted by setting a 1 kHz pure tone at 75 dB Peak SPL (dB re 20 μPa) at 0.5 m, measured with a sound level meter B&K 2238 Mediator (Brüel & Kjær, Nærum, Denmark). Distortion products measured for pure tones at 0.5 m from the loudspeaker were at least 30 dB below the amplitude of the tones generated. The frequency response of the loudspeaker was within ± 8 dB between 0.2 and 5 kHz (within ± 3 dB between 0.2 and 1.1 kHz and within ± 5 dB between 1.2 and 5 kHz). The unequal frequency response of the loudspeaker did not affect measurements, since attenuation was obtained by subtracting amplitudes of signals at a given distance from amplitudes of the same signal at 0.5 m from the loudspeaker (see below).

The broadcast signals were recorded with the microphone of a sound level meter (B&K 2238 Mediator, Brüel & Kjær, Nærum, Denmark) fitted with a 10-meter extension cable, a foam wind-shield and a digital audio field recorder (Marantz PMD-660, Kanagawa, Japan), and placed successively at distances of 0.5, 1, 2, 4, and 8 m from the loudspeaker in two type of substrates: water and soil. Measurements on the water substrate were conducted with the microphone supported on a tripod and positioned 5–10 cm above the water surface of the breeding habitats. Measurements on the soil substrate were conducted with the microphone placed 5–10 cm above the land of the surrounding area of the water bodies. The distances between the microphone and the loudspeaker were measured using a laser distance meter (Leica DISTO Classic5, Leica Geosystems AG, Heerbrugg, Switzerland). These distances were chosen to facilitate calculations of excess attenuation at distances doubling the preceding one and to allow comparisons with previous studies [38,41,102]. The recording level of the audio recorder was kept constant for all measurements, and at distant locations from the loudspeaker the sensitivity of the sound level meter was increased in the discrete steps provided by the instrument, in order to input detectable signals into the audio recorder. To calibrate the recordings, a 1-kHz pure tone at 94 dB SPL from a portable calibrator (B&K 4231, Brüel & Kjær, Nærum, Denmark) was recorded at the beginning and end of each transect. At the end of the playback, 3 s of white noise was emitted to check congruence with attenuation of specific frequency tones, and 1 min of background noise was recorded to estimate the signal-to-noise ratio.

Signal analysis

Signal attenuation is a major effect of sound transmission that may interrupt effective communication if signals are attenuated below auditory thresholds of recipients. Thus, excess attenuation (i.e. attenuation in excess of that expected due to spherical spreading) of pure tones and playback calls was used as measure of sound propagation [3133,38,40,41,102] and was calculated for distances of 1, 2, 4, and 8 m, relative to measurements at 0.5 m. First, sound amplitudes of the playback calls and pure tones recorded in propagation experiments were measured using Sound Ruler software [103]. Sound pressure levels (SPLs; dB re 20 μPa) of the recorded signals were determined relative to the value of the recorded calibration tone. Second, values predicted by spherical spreading were calculated with the equation: spherical transmission loss (dB) = 20 log [dB RMS SPL at far distance (m) / dB RMS SPL at 0.5 (m)]. Finally, these values were subtracted from the actual transmission loss (i.e. the differences between SPLs at 0.5 m from the loudspeaker and those measured at the corresponding farther distances). Positive and negative values of excess attenuation indicated that the sound attenuated at higher and lower rates, respectively, relative to the SPLs predicted by spherical transmission loss for each distance. In addition, maximum dB RMS and peak SPL of the background noise were calculated for 15 s periods of the noise recording.

Statistical analysis

To examine the physical properties of the habitats in which sound transmission tests were carried out, differences in excess attenuation of pure tones among localities, substrates (water and soil), distances (1, 2, 4, and 8m), and frequencies of the pure tones were statistically compared using four-way repeated measures ANOVA (P < 0.05). The frequency factor was pooled in five categories to reduce the number of groups, and was added as a between-subjects factor. The categories of frequency band were selected based on the spectral structure of the advertisement calls of both study species: F1) pure tones between 0.2 and 0.5 kHz that corresponds to the fundamental frequency of the calls of Lithobates catesbeianus; F2) pure tones between 0.6 and 1 kHz that encompass the frequency band between the fundamental and dominant frequencies of L. catesbeianus; F3) pure tones between 1.1 and 2 kHz that corresponds to the dominant frequency of L. catesbeianus; F4) pure tones between 2.1 and 3 kHz that include the frequencies of the advertisement calls of Pelophylax perezi; and F5) pure tones between 3.5 and 5 kHz, with frequencies above the previous ranges. Excess attenuation values of pure tones that were entered into the ANOVA test corresponded to single measurements at each distance, substrate and study site.

To analyse the differences in excess attenuation of the playback calls, a similar four-way ANOVA was computed with repeated measures on three factors (distance, substrate and locality) and one between-subjects factor (species). The species factor allows assessing the differences in sound propagation between the advertisement calls from the native and the invasive species. For the statistical analysis, the excess attenuation values obtained for the six calls of each species at a given distance, substrate and study site were converted to a linear scale (N/m2) to calculate averages and then reconverted to decibels.

All interactions between factors were also considered in the analyses of both pure tones and playback calls. Normality and sphericity assumptions were assessed with Mauchly’s test and Shapiro-Wilk test. The excess attenuation values were log transformed to attain normality. When the sphericity assumption failed, Huynh-Feldt corrected P-values were reported. The differences among levels of factors were evaluated using multiple comparisons with Bonferroni correction. Average linear scale values were used in statistical analyses and average decibel values in figure design. Statistical analyses were conducted using SPSS 19.0 software (IBM Corporation, Armonk, NY, USA). Figures were composed with R [104,105].

Results

Attenuation of pure tones

Measurements of propagation of pure tones showed that frequencies in the low range generally experienced lower attenuation rates, although the patterns of attenuation were markedly different across substrates and localities (Figures 3 and 4). Transmission of pure tones over water surface at frequencies below 2 kHz (F1–F3, within the spectral range of the call of Lithobates catesbeianus) resulted often in less attenuation that expected due to spherical spreading, with excess attenuation sometimes reaching values below -10 dB. However, excess attenuation showed a sharp increase at about 2–3 kHz (F4), corresponding to the dominant frequency of the advertisement call of Pelophylax perezi (Figure 3). In contrast with the water substrate, measurements of propagation of pure tones on the soil substrate were highly variable, with larger differences in attenuation among distances and study sites, and smaller differences among frequency categories, as shown in Figure 4. Mean, SD and range of excess attenuation of pure tones for each substrate and frequency category are summarized in Table 2.

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Figure 3. Excess attenuation (dB) of pure tones over water substrate in the study sites.

Measurements recorded at 1 m (circles), 2 m (triangles), 4 m (crosses), and 8 m (diamonds) relative to SPLs (dB re 20 μPa) at 0.5 m from the loudspeaker.

https://doi.org/10.1371/journal.pone.0077312.g003

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Figure 4. Excess attenuation (dB) of pure tones over soil substrate in the study sites.

Measurements recorded at 1 m (circles), 2 m (triangles), 4 m (crosses), and 8 m (diamonds) relative to SPLs (dB re 20 μPa) at 0.5 m from the loudspeaker.

https://doi.org/10.1371/journal.pone.0077312.g004

Frequency (kHz)Excess attenuation (dB)
SubstrateCategoriesRangeMeanSDMinMax
WaterF10.2–0.51.765,61-5.2810.18
F20.6–1.00.575,20-5.979.99
F31.1–2.0-2.743,53-16.1210.76
F42.1–3.01.491,68-19.5219.55
F53.5–5.06.710,05-12.1622.33
SoilF10.2–0.50.220,12-18.7515.00
F20.6–1.02.701,03-18.6519.60
F31.1–2.06.572,80-18.0926.22
F42.1–3.09.392,77-11.4126.96
F53.5–5.06.261,13-13.9520.97
TotalF10.2–0.51.022,43-18.7515.00
F20.6–1.01.700,33-18.6519.60
F31.1–2.03.113,82-18.0926.22
F42.1–3.06.313,69-19.5226.96
F53.5–5.06.490,58-13.9522.33

Table 2. Excess attenuation (dB) of pure tones measured at four distances (1, 2, 4, and 8 m) on water and soil substrates of seven study sites in the Iberian Peninsula.

The frequency categories F1 and F3 correspond to the spectral range of the advertisement call of Lithobates catesbeianus, while F4 corresponds to that of the advertisement calls of Pelophylax perezi. The average excess attenuation data were not calculated directly from the averages of SPLs (dB re 20 μPa), but were obtained after transforming dB values to a linear scale and then converting back to dB.
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The repeated measures ANOVA revealed that three of the factors considered (frequency, distance, and locality) showed significant effects on the attenuation rates of pure tones (Table 3). The frequency of the pure tones had a considerable influence on excess attenuation (F4, 24 = 17.70, P < 0.001). The lower frequency categories exhibited the lowest attenuation rates (mean = 1.02 dB for F1; mean = 1.70 dB for F2; mean = 3.11 dB for F3), and differed significantly in attenuation from the higher frequency categories (P < 0.05, in all cases; mean = 6.31 dB for F4; mean = 6.49 dB for F5). Transmission of pure tones was also affected by the distance (F3, 72 = 23.06, P < 0.001). Measurements of propagation at the largest distance (8 m) showed a significant increase in the rates of excess attenuation relative to shorter distances (P < 0.001, in all cases). Among the study sites, significant differences were found between El Cabaco, Villasbuenas de Gata, and Zarzalejo and the rest of localities (P < 0.01) and between El Cabaco and Zarzalejo (P = 0.014), the attenuation of pure tones being similar in the remaining of pairwise comparisons. Overall, the excess attenuation was similar between water and soil substrates (F1, 24 = 3.15, P = 0.089), but they largely differed in interaction with the rest of the variables (P < 0.001). Moreover, all interactions among factors in the model were also highly significant (Table 3).

FactorFdfP
Frequency17.704, 24< 0.001
Distance23.063, 72< 0.001
Substrate3.151, 240.089
Locality37.136, 144< 0.001
Locality * Frequency6.7624, 144< 0.001
Locality * Substrate36.026, 144< 0.001
Locality * Distance32.1418, 432< 0.001
Substrate * Frequency6.564, 240.001
Substrate * Distance28.493, 72< 0.001
Distance * Frequency13.7612, 72< 0.001

Table 3. Four-way repeated measures ANOVA of the excess attenuation (dB) of pure tones in the study site of propagation of advertisement calls of Pelophylax perezi and Lithobates catesbeianus in the Iberian Peninsula.

Huynh-Feldt corrected P-values were reported when the sphericity assumption failed, as shown by Mauchly’s test.
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Attenuation of advertisement calls

Attenuation of playback calls followed patterns concordant with measurements of pure tones (Figures 5 and 6). Signals with lower spectral components, namely the advertisements calls of Lithobates catesbeianus, present lower overall attenuation rates, although patterns of propagation were largely variable depending on localities, substrates, and distances. Mean, SD and range of excess attenuation of advertisement calls of both species for each substrate and distance are summarized in Table 4.

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Figure 5. Excess attenuation (dB) of playback calls over water substrate in the study sites.

Measurements recorded at 1 m (circles), 2 m (triangles), 4 m (crosses), and 8 m (diamonds) relative to SPLs (dB re 20 μPa) at 0.5 m from the loudspeaker. Each symbol represents the average for six calls of a species at a given distance. Abbreviations: Pp: Pelophylax perezi, Lc: Lithobates catesbeianus.

https://doi.org/10.1371/journal.pone.0077312.g005

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Figure 6. Excess attenuation (dB) of playback calls over soil substrate in the study sites.

Measurements recorded at 1 m (circles), 2 m (triangles), 4 m (crosses), and 8 m (diamonds) relative to SPLs (dB re 20 μPa) at 0.5 m from the loudspeaker. Each symbol represents the average for six calls of a species at a given distance. Abbreviations: Pp: Pelophylax perezi, Lc: Lithobates catesbeianus.

https://doi.org/10.1371/journal.pone.0077312.g006

Excess attenuation (dB)
SubstrateDistance (m)MeanSDMinMax
P. pereziWater12.174,74-5.8210.60
21.203,66-5.9510.72
4-0.801,89-9.529.30
83.421,89-8.8218.13
mean1.636,83
Soil11.378,71-4.907.39
24.752,08-9.8714.78
44.256,34-4.4511.25
88.962,43-1.6617.91
mean5.277,91
L. catesbeianusWater1-0.6210,79-4.113.15
2-4.829,43-11.39-1.27
4-1.643,10-9.788.02
80.280,72-7.1511.03
mean-1.507,76
Soil10.8210,10-6.034.11
23.968,15-1.7310.46
41.534,24-5.289.76
85.024,97-4.1212.41
mean3.0014,23

Table 4. Excess attenuation (dB) of advertisement calls of Pelophylax perezi and Lithobates catesbeianus measured at four distances (1, 2, 4, and 8 m) on water and soil substrates of seven study sites in the Iberian Peninsula.

The average excess attenuation data were not calculated from the averages of SPLs (dB re 20 μPa), but were obtained after transforming dB values to a linear scale and then converting back to dB.
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Excess attenuation of advertisement calls differed significantly across species, substrate, distance, and locality, as indicated by four-way repeated measures ANOVA (Table 5). The calls of L. catesbeianus experienced significantly lower attenuation rates than those of Pelophylax perezi (F1, 11 = 22.87, P = 0.001). Mean ± SD (minimum-maximum) of excess attenuation was 1.04 ± 2.6 dB (-11.39–12.41 dB) for L. catesbeianus, while 3.64 ± 2.5 dB (-9.87–18.13 dB) for P. perezi. As shown in Figure 7, these differences were found both for water and soil substrates (i.e. interaction between species and substrate: F1, 11 = 1.36, P = 0.267). Nevertheless, the interaction between species and site was found to be significant (F6, 66 = 34.17, P < 0.001), and the calls of P. perezi showed on average better propagation in riparian habitats, such as Villasbuenas de Gata and Navalcarnero, while the calls of L. catesbeianus suffered less attenuation in ponds or marshes, such as El Cabaco, Las Jaras, and Doñana (Figures 5 and 6).

FactorFdfP
Species22.871, 110.001
Substrate227.471, 11< 0.001
Distance202.213, 33< 0.001
Locality92.986, 66< 0.001
Locality * Species34.176, 66< 0.001
Locality * Substrate10.826, 660.001
Locality * Distance142.4918, 198< 0.001
Substrate * Species1.361, 110.267
Substrate * Distance373.433, 33< 0.001
Distance * Species7.973, 33< 0.001

Table 5. Four-way repeated measures ANOVA of the excess attenuation (dB) of playback calls in the study site of propagation of advertisement calls of Pelophylax perezi and Lithobates catesbeianus in the Iberian Peninsula.

Huynh-Feldt corrected P-values were reported when the sphericity assumption failed, as shown by Mauchly’s test.
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Figure 7. Averages of excess attenuation (dB) of playback calls for the seven study sites.

Measurements for Pelophylax perezi (circles) and Lithobates catesbeianus (squares) emitted over water (open symbols) and over soil (filled symbols) substrates.

https://doi.org/10.1371/journal.pone.0077312.g007

The ANOVA revealed that type of substrate was the factor with the greatest effect (F1, 11 = 227.47, P < 0.001), showing water surface as a substrate with remarkably better proprieties for sound transmission than soil substrate in both species (Figure 7). Excess attenuation of playback calls was also highly affected by distance (F3, 33 = 202.21, P < 0.001). Differences in attenuation were found among all positions within the propagation transect (P < 0.01, in all cases), with larger excess attenuation at the farthest distance between sound source and recipient (8 m; Figure 7). Significant interactions were found among all the factors considered, except the latter mentioned interaction between species and substrate. The relationship between excess attenuation and playback calls of both species varied among distances and localities (Table 5).

Discussion

Despite being broadcast in nonindigenous habitats, the advertisement calls of the invasive species were, overall, transmitted more efficiently than those of the native species, as shown by sound propagation experiments in different environments. Lower attenuation rates were found for acoustic signals of Lithobates catesbeianus in both aquatic and terrestrial substrates, and in most of the study sites. The less efficient propagation of the signals of the native species suggests an absence of optimal relationships between native environments and propagation of acoustic signals, in contrast to the acoustic adaptation hypothesis [31,35], proposed from evidence in other vertebrates. Our results are in general agreement with an increasing number of studies examining this prediction in other anurans from temperate and tropical habitats that have not found relationships between habitat structures and call characteristics [3739,41,61,106108]. In anurans, this apparent lack of optimization is likely related to the restricted distances over which anurans communicate as compared to birds or mammals [27]. Thus, our findings imply that adaptive mechanisms optimizing sound transmission in native habitats may play a less significant role than other selective forces or biological constraints in evolutionary design of acoustic signal in anurans. This suggestion has previously been proposed for tropical anuran species [48,61], and is also concordant with findings in some bird species [47,67,109].

The different call transmission efficiency between the invasive and native species may be due to the lower frequency contents of the spectra of the advertisement calls of the L. catesbeianus (with spectral peaks at about 0.5 and 1.8 kHz) relative to those of the Pelophylax perezi (with peaks at about 1 and 3 kHz). As revealed by the analysis of propagation of pure tones, the observed patterns of call transmission were consistent with properties of the different study sites for sound propagation over different frequency ranges. It has been well established that particular spectral and temporal features, may increase attenuation and degradation process of acoustic signals through environment, regardless of habitat structure [30,32,33,3537,39,41,110]. In general, higher frequency sounds experience more attenuation over distance than lower frequency sounds, and amplitude-modulated signals restrict sound transmission relative to tonal signals (e.g. [32,34]).

Sound propagation experiments of the present study rather than being consistent with the acoustic adaptation hypothesis support the morphological constraints hypothesis [47,48]. This hypothesis proposes that morphological features, such as body size, mass of vocal cords or size of other sound production structures, which evolve from multiple concomitant selective pressures, exert a more substantial influence on call spectral parameters, and hence on call transmission efficiency, than mechanisms of signal adaptation optimizing transmission efficiency in native habitats. Body size is a major factor influencing spectral properties of the advertisement calls, as it affects the size of sound production mechanisms, so that spectral properties of signals are usually a function of body size both within and among species using similar sound production mechanisms [27,48,61,66,67]. As L. catesbeianus is around twice the averaged size of P. perezi [82,89,90], the spectral differences between acoustic signals of the two species are presumably due to differences in body size. As such, the smaller body size of the native species relative to invasive species imposes constraints to the propagation of the advertisement calls even within its indigenous range.

The lack of a strict correspondence between signal quality and environment is likely to render native sound-communicating vertebrates particularly vulnerable to intrusion generated by invasive species producing low frequency signals of high amplitude, such as L. catesbeianus. The efficient signal propagation of its advertisement calls is likely to confer a competitive advantage to this invasive species in acoustic communication and reproduction, favouring processes of establishing and spreading outside their native range. Acoustic communication plays a crucial role in species recognition and mate attraction and selection in anurans and other acoustic communicating animals [2528], and hence it is expected that a better call transmission efficiency results in an increase of the range of effective communication between conspecifics and of the probability of reproductive success. However, to confirm this prediction it would be necessary to assess the remaining features of the communication system of the study species, such as hearing thresholds, sound pressure levels of the advertisement calls, or sender-receiver distances within breeding choruses, given that they might counteract or increase the specific constraints in signal propagation. This information is available for L. catesbeianus, and indicates that this species is able to communicate over long distances of about 60 m (e.g. [111115]). However, this information is not available for most of temperate anurans, including P. perezi, which prevents comparative analyses. Thus, future studies are needed to confirm that the properties of the advertisement call of L. catesbeianus may contribute to the invasion process.

Nonindigenous populations of L. catesbeianus cause negative impacts in native amphibians due to direct predation [7477], resource competition [73,76,80], and pathogen carriage, such as Batrachochytrium dendrobatidis [78,79,81]. Although there is growing evidence that the noise interference generated by abiotic sources [116120], biotic sources [121125], and anthropogenic sources [126129] have diverse consequences for vocal activity of vertebrates, few studies have examined the possible consequences of acoustic competition of nonindigenous species. Such competition could be significant, especially if these species exhibit calls with (1) high sound amplitude, (2) frequency spectra overlapping native calls, and (3) low attenuation in their transmission through the environment. The advertisement calls of L. catesbeianus seem to combine these three characteristics. The first evidence suggesting an effect of the vocalizations of introduced L. catesbeianus on acoustic communication of native populations has recently been reported for the neotropical tree frog Hypsiboas albomarginatus, which increases call frequency in response to invasion of its acoustic niche by calls of L. catesbeianus [42]. Because of the female preference for lower frequency signals recorded in several anurans (e.g. [27,28,130134]), a shift to higher frequencies could have a negative effect on the reproduction of native anurans. Moreover, interference also affects parameters of vocal activity other than frequency, such as amplitude, call duration, and rate of emission of vocalizations in different vertebrates (e.g. [135]), effects that may be experienced by native sound–communicating communities confronting foreign acoustic intrusions. Future studies should provide more extensive assessments of effects of such potential exposures.

Acknowledgments

We want to thank ICTS Doñana (CSIC) and Consejería de Medio Ambiente (Junta de Andalucía) for grant us permits to work and use of facilities in Doñana Biological Station. We are particularly grateful to N. Mendizábal, A. Arias, G. Tena, G. Palomar, and L. Arregui for their help in the field. Valuable comments from Peter M. Narins and an anonymous reviewer on this manuscript were really appreciated.

Author Contributions

Conceived and designed the experiments: DL MP RM. Performed the experiments: DL. Analyzed the data: MG DL. Contributed reagents/materials/analysis tools: RM. Wrote the manuscript: DL MG MP RM.

References

  1. 1. Vermeij GJ (1996) An agenda for invasion biology. Biol Conserv 78: 3–9. doi:10.1016/0006-3207(96)00013-4.
  2. 2. Fritts TH, Rodda GH (1998) The role of introduced species in the degradation of island ecosystems: a case history of Guam. Annu Rev Ecol Syst 29: 113–140. doi:10.1146/annurev.ecolsys.29.1.113.
  3. 3. Mack RN, Simberloff D, Lonsdale WM, Evans H, Clout M et al. (2000) Biotic invasions: causes, epidemiology, global consequences, and control. Ecol Appl 10: 689–710. doi:10.1890/1051-0761(2000)010[0689:BICEGC]2.0.CO;2.
  4. 4. Mooney HA, Cleland EE (2001) The evolutionary impact of invasive species. Proc Natl Acad Sci U S A 98: 5446–5451. doi:10.1073/pnas.091093398. PubMed: 11344292.
  5. 5. Crooks JA (2002) Characterizing ecosystem-level consequences of biological invasions: the role of ecosystem engineers. Oikos 97: 153–166. doi:10.1034/j.1600-0706.2002.970201.x.
  6. 6. MacDougall AS, Turkington R (2005) Are invasive species the drivers or passengers of change in degraded ecosystems? Ecology 86: 42–55. doi:10.1890/04-0669.
  7. 7. Wilson EO (1992) The diversity of life. Cambridge: Harvard University Press. 427 p.
  8. 8. Vitousek PM, D’Antonio CM, Loope LL, Westbrooks R (1996) Biological invasions as global environmental change. Am Sci 84: 468–478.
  9. 9. Pimentel D, Lach L, Zuniga R, Morrison D (2000) Environmental and economic costs of nonindigenous species in the United States. BioScience 50: 53–65.
  10. 10. Sax DF, Gaines SD, Brown JH (2002) Species invasions exceed extinctions on islands worldwide: a comparative study of plants and birds. Am Nat 160: 766–783. doi:10.1086/343877. PubMed: 18707464.
  11. 11. Clavero M, García-Berthou E (2005) Invasive species are a leading cause of animal extinctions. Trends Ecol Evol 20: 110. doi:10.1016/j.tree.2005.01.003. PubMed: 16701353.
  12. 12. Elton CS (1958) The ecology of invasions by animals and plants. London: Methuen. 196 p.
  13. 13. Baker HG (1965) Characteristics and modes of origin of weeds. In: HG BakerGL Stebbins. The genetics of colonizing species. New York: Academic Press. pp. 147–168.
  14. 14. Drake JA, Mooney HA, di Castri F, Groves RH, Kruger FJ et al. (1989) Biological invasions. A global perspective. Chichester: Wiley. 525 p.
  15. 15. Goodwin BJ, McAllister AJ, Fahrig L (1999) Predicting invasiveness of plant species based on biological information. Conserv Biol 13: 422–426. doi:10.1046/j.1523-1739.1999.013002422.x.
  16. 16. Blackburn TM, Duncan RP (2001) Determinants of establishment success in introduced birds. Nature 414: 195–197. doi:10.1038/35102557. PubMed: 11700555.
  17. 17. Kolar CS, Lodge DM (2001) Progress in invasion biology: predicting invaders. Trends Ecol Evol 16: 199–204. doi:10.1016/S0169-5347(01)02101-2. PubMed: 11245943.
  18. 18. Sol D, Timmermans S, Lefebvre L (2002) Behavioural flexibility and invasion success in birds. Anim Behav 63: 495–502. doi:10.1006/anbe.2001.1953.
  19. 19. Heger T, Trepl L (2003) Predicting biological invasions. Biol Invasions 5: 313–321.
  20. 20. Buddenhagen CE, Chimera C, Clifford P (2009) Assessing biofuel crop invasiveness: a case study. PLOS ONE 4: e5261. doi:10.1371/journal.pone.0005261. PubMed: 19384412.
  21. 21. O'Connor RJ (1986) Biological characteristics of invaders among bird species in Britain. Philos Trans R Soc Lond B Biol Sci 314: 583–598. doi:10.1098/rstb.1986.0074.
  22. 22. Veltman CJ, Nee S, Crawley MJ (1996) Correlates of introduction success in exotic New Zealand birds. Am Nat 147: 542–557. doi:10.1086/285865.
  23. 23. Reichard SH, Hamilton CW (1997) Predicting invasions of woody plants introduced into North America. Conserv Biol 11: 193–203. doi:10.1046/j.1523-1739.1997.95473.x.
  24. 24. Shirley SM, Kark S (2009) The role of species traits and taxonomic patterns in alien bird impacts. Glob Ecol Biogeogr 18: 450–459. doi:10.1111/j.1466-8238.2009.00452.x.
  25. 25. Bradbury JW, Vehrencamp SL (1998) Principles of animal communication. Sunderland: Sinauer Associates. 917 p.
  26. 26. Fay RR, Popper AN (1999) Comparative hearing: fish and amphibians. New York: Springer-Verlag. 441 p.
  27. 27. Gerhardt HC, Huber F (2002) Acoustic communication in insects and anurans: common problems and diverse solutions. Chicago: The University of Chicago Press. 531 p.
  28. 28. Narins PM, Feng AS, Fay RR, Popper AN (2007) Hearing and sound communication in amphibians. New York: Springer Verlag. 362 p.
  29. 29. Chapuis C (1971) Un exemple de l’influence du milieu sur les émissions vocales des oiseaux: l’évolution des chants en forêt équatoriale. Terre Vie 118: 183–202.
  30. 30. Jilka A, Leisler B (1974) Die Einpassung dreier Rohrsängerarten (Acrocephalus schoenobaenus, A. scirpaceus, A. arundinaceus) in ihre Lebensräume in bezug auf das Frequenzspektrum ihrer Reviergesänge. J Ornithol 115: 192–212. doi:10.1007/BF01643290.
  31. 31. Morton ES (1975) Ecological sources of selection on avian sounds. Am Nat 109: 17–34. doi:10.1086/282971.
  32. 32. Marten K, Marler P (1977) Sound transmission and its significance for animal vocalization. I. Temperate habitats. Behav Ecol Sociobiol 2: 271–290. doi:10.1007/BF00299740.
  33. 33. Marten K, Quine D, Marler P (1977) Sound transmission and its significance for animal vocalization. II. Tropical forest habitats. Behav Ecol Sociobiol 2: 291–302. doi:10.1007/BF00299741.
  34. 34. Waser PM, Waser MS (1977) Experimental studies of primate vocalization: Specializations for long-distance propagation. Z Tierpsychol 43: 239–263.
  35. 35. Wiley RH, Richards DG (1978) Physical constraints on acoustic communication in the atmosphere: implications for the evolution of animal vocalizations. Behav Ecol Sociobiol 3: 69–94. doi:10.1007/BF00300047.
  36. 36. Richards DG, Wiley RH (1980) Reverberations and amplitude fluctuations in the propagation of sound in a forest: implications for animal communication. Am Nat 115: 381–399. doi:10.1086/283568.
  37. 37. Ryan MJ, Cocroft RB, Wilczynski W (1990) The role of environmental selection in intraspecific divergence of mate recognition signals in the cricket frog, Acris crepitans. Evolution 44: 1869–1872. doi:10.2307/2409514.
  38. 38. Penna M, Solís R (1998) Frog call intensities and sound propagation in the South American temperate forest region. Behav Ecol Sociobiol 42: 371–381. doi:10.1007/s002650050452.
  39. 39. Kime NM, Turner WR, Ryan MJ (2000) The transmission of advertisement calls in Central American frogs. Behav Ecol 11: 71–83. doi:10.1093/beheco/11.1.71.
  40. 40. Perla BS, Slobodchikoff CN (2002) Habitat structure and alarm call dialects in Gunnison's prairie dog (Cynomys gunnisoni). Behav Ecol 13: 844–850. doi:10.1093/beheco/13.6.844.
  41. 41. Penna M, Márquez R, Bosch J, Crespo EG (2006) Nonoptimal propagation of advertisement calls of midwife toads in Iberian habitats. J Acoust Soc Am 119: 1227–1237. doi:10.1121/1.2149769. PubMed: 16521783.
  42. 42. Both C, Grant T (2012) Biological invasions and the acoustic niche: the effect of bullfrog calls on the acoustic signals of white-banded tree frogs. Biol Lett 8: 714–716. doi:10.1098/rsbl.2012.0412. PubMed: 22675139.
  43. 43. Farina A, Pieretti N, Morganti N (2013) Acoustic patterns of an invasive species: the Red-billed Leiothrix (Leiothrix lutea Scopoli 1786) in a Mediterranean shrubland. Bioacoustics. In press doi:10.1080/09524622.2012.761571.
  44. 44. Wilkins MR, Seddon N, Safran RJ (2013) Evolutionary divergence in acoustic signals: causes and consequences. Trends Ecol Evol 28: 156–166. PubMed: 23141110.
  45. 45. Brooks DR, McLennan DA (1991) Phylogeny, ecology, and behavior: a research program in comparative biology. Chicago: University of Chicago Press. 434 p.
  46. 46. Harvey PH, Pagel MD (1991) The comparative method in evolutionary biology. Oxford: Oxford University Press. 239 p.
  47. 47. Ryan MJ, Brenowitz EA (1985) The role of body size, phylogeny, and ambient noise in the evolution of bird song. Am Nat 126: 87–100. doi:10.1086/284398.
  48. 48. Ryan MJ (1986) Factors influencing the evolution of acoustic communication: biological constraints. Brain Behav Evol 28: 70–82. doi:10.1159/000118693. PubMed: 3567542.
  49. 49. Bowman RI (1979) Adaptive morphology of song dialects in Darwin's finches. J Ornithol 120: 353–389. doi:10.1007/BF01642911.
  50. 50. Hunter ML, Krebs JR (1979) Geographical variation in the song of the great tit (Parus major) in relation to ecological factors. J Anim Ecol 48: 759–785. doi:10.2307/4194.
  51. 51. Gish SL, Morton ES (1981) Structural adaptations to local habitat acoustics in Carolina wren songs. Z Tierpsychol 56: 74–84.
  52. 52. Shy E (1983) The relation of geographical variation in song to habitat characteristics and body size in North American tanagers (Thraupinae: Piranga). Behav Ecol Sociobiol 12: 71–76. doi:10.1007/BF00296935.
  53. 53. Brown CH, Schwagmeyer PL (1984) The vocal range of alarm calls in Thirteen-lined Ground Squirrels. Z Tierpsychol 65: 273–288.
  54. 54. Anderson ME, Conner RN (1985) Northern Cardinal song in three forest habitats in eastern Texas. Wilson Bull 97: 436–449.
  55. 55. Sorjonen J (1986) Factors affecting the structure of song and the singing behaviour of some northern European passerine birds. Behaviour 98: 286–304. doi:10.1163/156853986X01017.
  56. 56. Waas JR (1988) Song pitch-habitat relationships in white-throated sparrows: cracks in acoustic windows? Can J Zool 66: 2578–2581. doi:10.1139/z88-379.
  57. 57. Handford P (1988) Trill rate dialects in the rufous-collared sparrow, Zonotrichia capensis, in northwestern Argentina. Can J Zool 66: 2658–2670. doi:10.1139/z88-391.
  58. 58. Handford P, Lougheed SC (1991) Variation in duration and frequency characters in the song of the rufous-collared sparrow, Zonotrichia capensis, with respect to habitat, trill dialects and body size. Condor 93: 644–658. doi:10.2307/1368196.
  59. 59. Tubaro PL, Segura ET (1995) Geographic, ecological and subspecific variation in the song of the Rufous-browed Peppershrike (Cyclarhis gujanensis). Condor 97: 792–803. doi:10.2307/1369187.
  60. 60. Kirschel ANG, Blumstein DT, Cohen RE, Buermann W, Smith TB et al. (2009) Birdsong tuned to the environment: green hylia song varies with elevation, tree cover, and noise. Behav Ecol 20: 1089–1095. doi:10.1093/beheco/arp101.
  61. 61. Zimmerman BL (1983) A comparison of structural features of calls of open and forest habitat frog species in the Central Amazon. Herpetologica 39: 235–246.
  62. 62. Ryan MJ (1988) Constraints and patterns in the evolution of anuran acoustic communication. In: B. FritzschMJ RyanW. WilczynskiT. HetheringtonW. Walkowiak. The evolution of the amphibian auditory system. New York: Wiley. pp. 637–677.
  63. 63. Cocroft RB, Ryan MJ (1995) Patterns of advertisement call evolution in toads and chorus frogs. Anim Behav 49: 283–303. doi:10.1006/anbe.1995.0043.
  64. 64. Fitch WT, Hauser MD (2002) Unpacking "honesty": vertebrate vocal production and the evolution of acoustic signals. In: A. SimmonsRR FayAN Popper. Acoustic Communication. New York: Springer Verlag. pp. 65–137.
  65. 65. Goicoechea N, de la Riva I, Padial JM (2010) Recovering phylogenetic signal from frog mating calls. Zool Scripta 39: 141–154. doi:10.1111/j.1463-6409.2009.00413.x.
  66. 66. Wallschläger D (1980) Correlation of song frequency and body weight in passerine birds. Cell Mol Life Sci 36: 412. doi:10.1007/BF01975119.
  67. 67. Badyaev AV, Leaf ES (1997) Habitat associations of song characteristics in Phylloscopus and Hippolais warblers. Auk 114: 40–46. doi:10.2307/4089063.
  68. 68. Lever C (2003) Naturalized amphibians and reptiles of the world. New York: Oxford University Press. 344 p.
  69. 69. Ficetola GF, Thuiller W, Miaud C (2007) Prediction and validation of the potential global distribution of a problematic alien invasive species — the American bullfrog. Divers Distrib 13: 476–485. doi:10.1111/j.1472-4642.2007.00377.x.
  70. 70. Santos-Barrera G, Hammerson G, Hedges B, Joglar R, Inchaustegui S et al. (2009) Lithobates catesbeianus. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.1. Available: http://www.iucnredlist.org. Accessed 10 May. p. 2013.
  71. 71. Kraus F (2009) Alien reptiles and amphibians: a scientific compendium and analysis. New York: Springer Verlag. 564 p.
  72. 72. Lowe S, Browne M, Boujdelas S, De Poorter M (2000). 100 of the world's worst invasive alien species. A selection from the Global Invasive Species Database. Auckland: International Union for Conservation of Nature. 12 p.
  73. 73. Werner EE, Wellborn GA, McPeek MA (1995) Diet composition in postmetamorphic bullfrogs and green frogs: implications for interspecific predation and competition. J Herpetol 29: 600–607. doi:10.2307/1564744.
  74. 74. Kiesecker JM, Blaustein AR (1998) Effects of introduced bullfrogs and smallmouth bass on microhabitat use, growth, and survival of native red-legged frogs (Rana aurora). Conserv Biol 12: 776–787. doi:10.1046/j.1523-1739.1998.97125.x.
  75. 75. Kiesecker JM, Blaustein AR, Miller CL (2001) Potential mechanisms underlying the displacement of native red-legged frogs by introduced bullfrogs. Ecology 82: 1964–1970. doi:10.1890/0012-9658(2001)082[1964:PMUTDO]2.0.CO;2.
  76. 76. Kats LB, Ferrer RP (2003) Alien predators and amphibian declines: review of two decades of science and the transition to conservation. Divers Distrib 9: 99–110. doi:10.1046/j.1472-4642.2003.00013.x.
  77. 77. Pearl CA, Adams MJ, Bury RB, McCreary B (2004) Asymmetrical effects of introduced bullfrogs (Rana catesbeiana) on native ranid frogs in Oregon. Copeia. 11–20.
  78. 78. Hanselmann R, Rodríguez M, Fajardo-Ramos L, Aguirre A, Kilpatrick A et al. (2004) Presence of an emerging pathogen of amphibians in introduced bullfrogs Rana catesbeiana in Venezuela. Biol Conserv 120: 115–119. doi:10.1016/j.biocon.2004.02.013.
  79. 79. Garner TWJ, Perkins MW, Govindarajulu P, Seglie D, Walker S et al. (2006) The emerging amphibian pathogen Batrachochytrium dendrobatidis globally infects introduced populations of the North American bullfrog, Rana catesbeiana. Biol Lett 2: 455–459. doi:10.1098/rsbl.2006.0494. PubMed: 17148429.
  80. 80. Cook DG, Jennings MR (2007) Microhabitat use of the California red-legged frog and introduced bullfrog in a seasonal marsh. Herpetologica 63: 430–440. doi:10.1655/0018-0831(2007)63[430:MUOTCR]2.0.CO;2.
  81. 81. Greenspan SE, Calhoun AJK, Longcore JE, Levy MG (2012) Transmission of Batrachochytrium dendrobatidis to wood frogs (Lithobates sylvaticus) via a bullfrog (L. catesbeianus) vector. J Wildl Dis 48: 575–582. doi:10.7589/0090-3558-48.3.575. PubMed: 22740523.
  82. 82. Shirose LJ, Brooks RJ, Barta JR, Desser SS (1993) Intersexual differences in growth, mortality, and size at maturity in bullfrogs in central Ontario. Can J Zool 71: 2363–2369. doi:10.1139/z93-332.
  83. 83. Capranica RA (1965) The evoked vocal response of the bullfrog: a study of communication by sound. Cambridge: MIT Press. 110 p.
  84. 84. Bee MH, Gerhardt HC (2001) Neighbour-stranger discrimination by territorial male bullfrogs (Rana catesbeiana): I. Acoustic basis. Anim Behav 62: 1129–1140. doi:10.1006/anbe.2001.1851.
  85. 85. Bee MH (2004) Within-individual variation in bullfrog vocalizations: Implications for a vocally mediated social recognition system. J Acoust Soc Am 116: 3770–3781. doi:10.1121/1.1784445. PubMed: 15658727.
  86. 86. Paillette M (1969) Les signaux acoustiques de Hyla meridionalis Boettger (Amphibiens, Anoures). C R Soc Biol 163: 74–80.
  87. 87. Márquez R, Bosch J (1995) Advertisement calls of the midwife toads Alytes (Amphibia, Anura, Discoglossidae) in Spain. J Zool Syst Evol Res 33: 185–192.
  88. 88. Castellano S, Cuatto B, Rinella R, Rosso A, Giacoma C (2002) The advertisement call of the European treefrogs (Hyla arborea): a multilevel study of variation. Ethology 108: 75–89. doi:10.1046/j.1439-0310.2002.00761.x.
  89. 89. García-París M, Montori A, Herrero P (2004) Amphibia, Lissamphibia. Fauna Ibérica, vol. 24. Madrid: Museo Nacional de Ciencias Naturales (CSIC). 640 p.
  90. 90. Schneider H, Steinwarz D (1990) Mating call and territorial calls of the Spanish lake frog, Rana perezi (Ranidae, Amphibia). Zool Anz 225: 265–277.
  91. 91. Frost DR, Grant T, Faivovich J, Bain RH, Haas A et al. (2006) The amphibian tree of life. Bull Am Museum Nat Hist 297: 1–291. doi:10.1206/0003-0090(2006)297[0001:TATOL]2.0.CO;2.
  92. 92. Llorente GA, Montori A, Carretero MA, Santos X (2002) Rana perezi. In: JM PleguezuelosR. MárquezM. Lizana. Atlas y Libro Rojo de los anfibios y reptiles de España. Madrid: Dirección General de la Conservación de la Naturaleza-Asociación Herpetológica Española. pp. 126–128.
  93. 93. Emlen ST (1976) Lek organization and mating strategies in the bullfrog. Behav Ecol Sociobiol 1: 283–313. doi:10.1007/BF00300069.
  94. 94. Howard RD (1978) The evolution of mating strategies in bullfrogs, Rana catesbeiana. Evolution 32: 850–871. doi:10.2307/2407499.
  95. 95. Ryan MJ (1980) The reproductive behavior of the bullfrog (Rana catesbeiana). Copeia. 108–114.
  96. 96. García-París M (1991) Primeros datos sobre Rana catesbeiana en España. Revista Española de Herpetología 5: 89–92.
  97. 97. Ayllón E, Barbera JC (2001) Seguimiento de la evolución de la granja abandonada de rana toro Rana catebeiana Shaw, 1802 en el municipio de Navalcarnero. Madrid: Sociedad para la Conservacion de los Vertebrados. 10 p.
  98. 98. Cabana M, Fernández D (2010) Nueva vía de entrada de rana toro (Lithobates catesbeianus) en la Península Ibérica. Boletín de la Asociación Herpetológica Española 21: 101–104.
  99. 99. Harris CM (1966) Absorption of sound in air versus humidity and temperature. J Acoust Soc Am 40: 148–159. doi:10.1121/1.1910031.
  100. 100. Piercy JE, Daigle GA (1991) Sound propagation in the open air. In: CM Harris. Handbook of Acoustical Measurements and Noise Control. New York: McGraw-Hill. pp. 3.1–3.26.
  101. 101. De la Torre S, Snowdon CT (2002) Environmental correlates of vocal communication of wild pygmy marmosets, Cebuella pygmaea. Anim Behav 63: 847–856. doi:10.1006/anbe.2001.1978.
  102. 102. Penna M, Llusia D, Márquez R (2012) Propagation of natural toad calls in a Mediterranean terrestrial environment. J Acoust Soc Am 132: 4025–4031. doi:10.1121/1.4763982. PubMed: 23231131.
  103. 103. Gridi-Papp M (2007) SoundRuler: Acoustic Analysis for Research and Teaching. Available: http://soundruler.sourceforge.net. Accessed 25 May 2011.
  104. 104. R Development Core Team (2012) R: a language and environment for statistical computing. R Foundation for Statistical Computing. Available: . http://www.R-project.org. Accessed 15 November 2012.
  105. 105. Sueur J, Aubin T, Simonis C (2008) Seewave: a free modular tool for sound analysis and synthesis. Bioacoustics 18: 213–226. doi:10.1080/09524622.2008.9753600.
  106. 106. Bosch J, De la Riva I (2004) Are frog calls modulated by the environment? An analysis with anuran species from Bolivia. Can J Zool 82: 880–888. doi:10.1139/z04-060.
  107. 107. Ey E, Fischer J (2009) The “acoustic adaptation hypothesis”— a review of the evidence from birds, anurans and mammals. Bioacoustics 19: 21–48. doi:10.1080/09524622.2009.9753613.
  108. 108. Gómez M (2012) Efecto del hábitat en la propagación de las llamadas reproductivas. Estudio comparativo en Hyla arborea (Linnaeus, 1758), e H. meridionalis (Boettger, 1874). Master Thesis Universidad Autónoma de Madrid.
  109. 109. Wiley RH (1991) Associations of song properties with habitats for territorial oscine birds of eastern North America. Am Nat 138: 973–993. doi:10.1086/285263.
  110. 110. Mockford EJ, Marshall RC, Dabelsteen T (2011) Degradation of rural and urban great tit song: testing transmission efficiency. PLOS ONE 6: e28242. doi:10.1371/journal.pone.0028242. PubMed: 22174781.
  111. 111. Feng AS, Narins PM, Capranica RR (1975) Three populations of primary auditory fibers in the bullfrog (Rana catesbeiana): their peripheral origins and frequency sensitivities. J Comp Physiol A 100: 221–229. doi:10.1007/BF00614532.
  112. 112. Lewis ER, Leverenz EL, Koyama H (1982) The tonotopic organization of the bullfrog amphibian papilla, an auditory organ lacking a basilar membrane. J Comp Physiol A 145: 437–445. doi:10.1007/BF00612809.
  113. 113. Megela-Simmons A (1984) Behavioral vocal response thresholds to mating calls in the bullfrog, Rana catesbeiana. J Acoust Soc Am 76: 676–681. doi:10.1121/1.391254. PubMed: 6333442.
  114. 114. Megela-Simmons A, Moss CF, Daniel KM (1985) Behavioral audiograms of the bullfrog (Rana catesbeiana) and the green tree frog (Hyla cinerea). J Acoust Soc Am 78: 1236–1244. doi:10.1121/1.392892. PubMed: 3877086.
  115. 115. Boatright-Horowitz SL, Horowitz SS, Simmons AM (2000) Patterns of vocal interactions in a bullfrog (Rana catesbeiana) chorus: preferential responding to far neighbors. Ethology 106: 701–712. doi:10.1046/j.1439-0310.2000.00580.x.
  116. 116. Penna M, Pottstock H, Velasquez N (2005) Effect of natural and synthetic noise on evoked vocal responses in a frog of the temperate austral forest. Anim Behav 70: 639–651. doi:10.1016/j.anbehav.2004.11.022.
  117. 117. Penna M, Hamilton-West C (2007) Susceptibility of evoked vocal responses to noise exposure in a frog of the temperate austral forest. Anim Behav 74: 45–56. doi:10.1016/j.anbehav.2006.11.010.
  118. 118. Grafe TU, Preininger D, Sztatecsny M, Kasah R, Dehling JM et al. (2012) Multimodal communication in a noisy environment: a case study of the Bornean rock frog Staurois parvus. PLOS ONE 7: e37965. doi:10.1371/journal.pone.0037965. PubMed: 22655089.
  119. 119. Narins PM, Feng AS, Lin W-Y, Schnitzler H-U, Denzinger A et al. (2004) Old World frog and bird vocalizations contain prominent ultrasonic harmonics. J Acoust Soc Am 115: 910–913. doi:10.1121/1.1636851. PubMed: 15000202.
  120. 120. Feng AS, Narins PM, Xu C-H, Lin W-Y, Yu Z-L et al. (2006) Ultrasonic communication in frogs. Nature 440: 333–336. doi:10.1038/nature04416. PubMed: 16541072.
  121. 121. Alexander RD (1961) Aggressiveness, territoriality, and sexual behavior in field crickets (Orthoptera: Gryllidae). Behaviour 17: 130–223. doi:10.1163/156853961X00042.
  122. 122. Lopez PT, Narins PM, Lewis ER, Moore SW (1988) Acoustically induced call modification in the white-lipped frog, Leptodactylus albilabris. Anim Behav 36: 1295–1308. doi:10.1016/S0003-3472(88)80198-2.
  123. 123. Weary DM, Lambrechts MM, Krebs JR (1991) Does singing exhaust male great tits? Anim Behav 41: 540–542. doi:10.1016/S0003-3472(05)80860-7.
  124. 124. Jehle R, Arak A (1998) Graded call variation in male Asian cricket frogs (Rana nicobariensis). Bioacoustics 9: 35–48. doi:10.1080/09524622.1998.9753378.
  125. 125. Bosch J, Márquez R (2001) Call timing in male-male acoustical interactions and female choice in the midwife toad Alytes obstetricans. Copeia. 169–177.
  126. 126. Sun JW, Narins PM (2005) Anthropogenic sounds differentially affect amphibian call rate. Biol Conserv 121: 419–427. doi:10.1016/j.biocon.2004.05.017.
  127. 127. Parris KM, Schneider A (2009) Impacts of traffic noise and traffic volume on birds of roadside habitats. Ecol Soc 14: 29.
  128. 128. Parris KM, Velik-Lord M, North JM (2009) Frogs call at a higher pitch in traffic noise. Ecol Soc 14: 25.
  129. 129. Kaiser K, Hammers JL (2009) The effect of anthropogenic noise on male advertisement call rate in the neotropical treefrog, Dendropsophus triangulum. Behaviour 146: 1053–1069. doi:10.1163/156853909X404457.
  130. 130. Ryan MJ (1980) Female mate choice in a neotropical frog. Science 209: 523–525. doi:10.1126/science.209.4455.523. PubMed: 17831371.
  131. 131. Gerhardt HC, Schneider H (1980) Mating call discrimination by females of the treefrog Hyla meridionalis on Tenerife. Behav Processes 5: 143–149. doi:10.1016/0376-6357(80)90061-3.
  132. 132. Schneider H (1982) Phonotaxis bei weibchen des kanarischen Laubfrosches, Hyla meridionalis. Zool Anz 208: 161–174.
  133. 133. Gerhardt HC (1994) The evolution of vocalization in frogs and toads. Annu Rev Ecol Sys 25: 293–324. doi:10.1146/annurev.es.25.110194.001453.
  134. 134. Márquez R (1995) Female choice in the midwife toads (Alytes obstetricans and A. cisternasii). Behaviour 132: 151–161.
  135. 135. Nemeth E, Brumm H (2009) Blackbirds sing higher pitched songs in cities: adaptation to habitat acoustics or side effect of urbanization? Anim Behav 78: 637–641. doi:10.1016/j.anbehav.2009.06.016.
  136. 136. Pleguezuelos JM, Márquez R, Lizana M (2002) Atlas y Libro Rojo de los anfibios y reptiles de España. Madrid: Dirección General de Conservación de la Naturalezaa y Asociación Herpetológica Española. 587 p.
  137. 137. Loureiro A, Ferrand de Almeida N, Carretero MA, Paulo OS (2008) Atlas dos anfíbios e répteis de Portugal. Lisboa: Instituto da Conservação da Natureza e da Biodiversidade. 257 p.