Anurans are widely known for their reproductive vocalisation repertoire (Duellman & Trueb 1994). The advertisement call, emitted by males to attract females during the breeding season, is among the most extensively studied and well-known reproductive calls (Guerra et al. 2018). Additionally, there are other types of calls used in different social contexts. These calls can be categorised into distinct groups such as reproductive, aggressive, defensive, and feeding, each with its corresponding subcategories (e.g. reproductive calls are divided into advertisement, courtship, amplectant, release, post-oviposition male resealing, and rain calls) (Toledo et al. 2015, Köhler et al. 2017). Among this rich repertoire, the release call is emitted by an individual when it is improperly clasped by another, usually occurring when males mistakenly clasp other males or unreceptive females for amplexus purposes (Gerhardt 1994). The release call is considered an important source for taxonomic resolution, as it can differ among anuran species and may contain phylogenetic information to help distinguish closely related taxa (e.g. cryptic species) (Vieira et al. 2014, Mângia et al. 2019). Furthermore, release calls have the potential to facilitate interspecific communication in heterospecific amplexus (Leary 2001).

Despite the importance of the release call, few studies have focused on this aspect of neotropical hylid ecology. Moreover, for certain widespread and even common hylid genera, such as Trachycephalus (Tschudi, 1838), the existing knowledge is either insufficient or non-existent. The genus Trachycephalus comprises 18 species (Frost 2024), with only one species having its release call described: T. ‘vermiculatus’ (Barrio-Amorós & Guell 2023). The most widespread species in the genus is T. typhonius, which exhibits a wide distribution across Brazil, French Guyana, Guyana, Suriname, Argentina, Bolivia, and Venezuela (IUCN SSC 2023, Frost 2024) and shows explosive breeding behaviour (Prado et al. 2005). This species is known to have unresolved taxonomic issues and is currently recognised as a species complex. One of the few studies attempting to organise these populations was conducted by Ron et al. (2016), focusing on populations in Ecuador and Peru. The authors revalidated T. quadrangulum for populations in western Ecuador and T. macrotis in the Amazon of Ecuador and Peru. However, no comprehensive review covers the entire geographical distribution of T. typhonius. Therefore, the name T. typhonius continues to be widely used for populations in Brazil (Vaz-Silva et al. 2020, de Souza et al. 2022), underscoring the need for a more comprehensive understanding of its systematics. The significance of bioacoustics data, particularly the release call and the requirement for additional information for taxonomic resolution within T. typhonius (sensu lato), are highlighted. Herein, we provide the first description of its release call from a population in the Cerrado region of Central-Western Brazil.

Fig. 1.

Release call of Trachycephalus typhonius (ZUFMS-AMP19450, MAP-V 331, SVL 76.04 mm) from Parque Natural Municipal do Pombo, municipality of Três Lagoas, state of Mato Grosso do Sul, Brazil. Video available at  https://youtu.be/XUDJUdbOy9c.

We collected two males of T. typhonius on January 27^th and 29^th, 2024, in the Parque Natural Municipal do Pombo (20°22′25″ S; 52°36′3″ W, datum WGS84), municipality of Três Lagoas, state of Mato Grosso do Sul, Brazil. On 30^th January 2024, at approximately 10:00 a.m., while being handled for photography, the frogs began to emit release calls. At that time, the air temperature was 33.3 °C, with a relative humidity of 49% (parameters collected in the Água Clara weather station, located about 25 km from the Park, INMET 2024). Each male was gently held to simulate amplexus, with the thumb and index finger positioned in the frog's axillae, while the calls were recorded using a Tascam DR-40 digital recorder. We analysed the recordings with the software Raven Pro version 1.6.5 (K. Lisa Yang Center for Conservation Bioacoustics 2024), with the following settings: window type – Hann, window size – 216 samples, 3dB filter bandwidth – 294 Hz, brightness – 50%, contrast – 50%, overlap – 50%, DFT size – 256 samples, grid spacing – 172 Hz. We analysed the following acoustic parameters: call duration (s), pulses per call, pulse rate (pulses per second), and dominant frequency (Hz). The oscillogram and spectrogram illustrations were generated using the package seewave version 2.2.3 (Sueur et al. 2023) on the R platform version 4.3.3 (R Development Core Team 2024). Seewave settings employed were window name (Fourier transformation window) – Hanning; window length – 512 samples; overlap – 90%. Voucher specimens were measured (snout-vent length) using a digital calliper and were housed at the herpetological collection of the Federal University of Mato Grosso do Sul (ZUFMS-AMP19450 and ZUFMS-19451), Brazil. Recordings were stored as uncompressed wave files at the Mapinguari Sound Library of the Federal University of Mato Grosso do Sul (MAP-V 331 and MAP-V 332).

Table 1.

Release and advertisement call of Trachycephalus typhonius and the release call of Trachycephalus ‘vermiculatus’. Values are presented as mean, ± SD (range), SD – standard deviation; SVL – snout-vent length.

The release call of both males of T. typhonius (Fig. 1) is characterised by a multipulsed note with a duration of 0.036-0.117 s (0.073 ± 0.017), and a dominant frequency ranging from 689.06 to 947.46 Hz (812.26 ± 54.62) (Table 1). These calls consist of 5-8 pulses (6.16 ± 0.81) emitted at a rate of 0.006-0.019 pulses/s (0.012 ± 0.003). Comparatively, the advertisement call of T. typhonius exhibits a similar multipulsed note but with a longer duration (0.343-0.598 s), more pulses per call (52-87), a higher pulse rate (123-204 pulses/s), and a dominant frequency between 1,705 and 2,750 Hz (Zaracho et al. 2018) (Table 1).

In comparison to the release call of T. ‘vermiculatus’, the only known in the genus, the release call of T. typhonius exhibits a higher dominant frequency (689.06-947.46 Hz; 430.00-516.00 Hz in T. ‘vermiculatus’). Both males of T. typhonius recorded in the present study emitted the release calls inflating only one of their two vocal sacs (video available at  https://youtu.be/XUDJUdbOy9c), while the release calls registered by Barrio-Amorós & Guell (2023), both vocal sacs of T. ‘vermiculatus’ were inflated, however with about half their maximum volume.

Currently, T. ‘vermiculatus’ serves as a placeholder taxon encompassing all available names formerly attributed to Trachycephalus typhonius, ranging from Chocoan South America to southern and eastern Mexico (Frost 2024). According to Ron et al. (2016), Central American populations of this species may not be conspecific with T. typhonius, the previously assigned name, suggesting the possibility of these populations receiving a distinct designation, potentially T. vermiculatus (Barrio-Amorós & Guell 2023). Assuming the distinctiveness of these populations and their recognition as a species complex, with only two species currently reclassified for South America, there is a pressing need for a comprehensive review addressing the entire distribution range of this species. The differences in release calls observed here could serve as diagnostic synapomorphies to distinguish between the lineages. Among the 18 known species of Trachycephalus, release calls are currently described for two species, including the one presented in this study. Given the potential for taxonomic classification in the release call, it is crucial to provide additional descriptions to differentiate between species, including more T. typhonius and T. ‘vermiculatus’ populations. When combined with advertisement calls, it is evident that such calls can significantly contribute to Trachycephalus systematics.