Bioacoustics of Hydrolaetare schmidti

whereas distress calls produced by females were previously described by Hödl & Gollmann (1986) from Central Brazilian Amazonia. When we compare the female and male distress calls of the species, there is variation related to note length (male: 100–1366 ms; female: 202–480 ms) and dominant frequency (male: 376–2121 Hz; female: 900–4500 Hz). The much wider value range of note length in our sample might simply reflect a larger sample size of analyzed notes ( Hödl & Gollmann 1986 did not report the number of analyzed notes) or varying distress levels of different individuals while manipulated ( Martins & Haddad 1988 ). Lower frequencies in anuran calls could be associated with larger body sizes, which cannot explain lower call frequencies of male distress calls of H. schmidti , given that females are reported to be larger than males ( Souza & Haddad 2003 ). Despite this, male frogs can have disproportionally larger larynges than females ( Guerra et al. 2014 ), which might reflect in an increased area density of vocal cords, thus a lower call fundamental frequency ( Martin 1971 ). The first quantitative description of the advertisement call from H. schmidti was based on one male from French Guiana ( Lescure & Marty 2000 ). The advertisement call described in our study based on seven males from the Amazonian savanna of Amapá is consistent with the first call description of the species, with total overlap in all temporal and spectral traits analyzed, which can be explained by the relatively short geographic distance, even though these two populations of H. schmidti inhabit different ecosystems of Amazonian lowland forests and savannas.

In total, we quantified 24 distress calls and 84 advertisement calls from eight males (data are presented as mean ± SD), of which six produced advertisement calls, one produced distress calls, and one produced both call types . The distress call ( Fig. 1A–B ) of H. schmidti is composed of one type of note when manipulated in the field. The amplitude peak varies from 21 to 88 (58.4 ± 3.0) % of the note, whose length varies from 100 to 1366 (548.1 ± 50.0) ms. The note has a variable envelope and generally exhibits a harmonic structure with sound energy distributed over a wide range of frequencies ( Fig. 1A ), even though some notes can have inharmonic portions at start and/or end lengths. On one occasion, the note displays a pulsatile envelope with individual pulse units recognized at the end of the note and without clear harmonic intervals ( Fig. 1B ). The dominant frequency ranges from 376 to 2121 (874.8 ± 285.5) Hz, generally coinciding with the third and fourth harmonics (together, 57% of the analyzed notes). Of 43% of analyzed notes, 17% coincide with the fundamental harmonic, 13% with the fifth harmonic, 9% with the sixth harmonic, and 4% with the eight harmonic. The advertisement call ( Fig. 1C ) was heard and recorded in the first hours of the night (18:00–20:00 h). The call consists of a single note emitted 4 to 10 (7.6 ± 2.4) per minute. It has an elliptical-shaped envelope reflecting the gradual increase in sound amplitude until its peak around the mid-length of the note, followed by a gradual decrease in amplitude throughout the second half of the note. The amplitude peak varies from 30 to 76 (60.5 ± 11.7) % of the note, whose length varies from 866 to 1893 (1364.3 ± 251.7) ms. The dominant frequency ranges from 258 to 850 (484.7 ± 147.2) Hz, with sound energy emphasized in the first three harmonics. Among the analyzed notes, the dominant frequency coincides with the fundamental (46%), second (43%) or third (11%) harmonic.