Call acoustics reflect body size across four clades of anurans

Abstract

An inverse relationship between body size and advertisement call frequency has been found in several frog species. However, the generalizability of this relationship across different clades and across a large distribution of species remains underexplored. We investigated this relationship in a large sample of 136 species belonging to four clades of anurans (Bufo, Hylinae, Leptodactylus and Rana) using semi‐automatic, high‐throughput analysis software. We employed two measures of call frequency: fundamental frequency (F0) and dominant frequency (DF). The slope of the relationship between male snout‐vent length (SVL) and frequency did not differ significantly among the four clades. However, Rana call at a significantly lower frequency relative to size than the other clades, and Bufo call at a significantly higher frequency relative to size than Leptodactylus. Because the relationship between F0 and body size may be more straightforwardly explained by biomechanical constraints, we confirmed that a similar inverse relationship was observed between F0 and SVL. Finally, spectral flatness, an indicator of the tonality of the vocalizations, was found to be inversely correlated with SVL, contradicting an oft‐cited prediction that larger animals should have rougher voices. Our results confirm a tight and widespread link between body size and call frequency in anurans, and suggest that laryngeal allometry and vocal fold dimensions in particular are responsible.

The use of bioacoustics in anuran taxonomy: theory, terminology, methods and recommendations for best practice

“…Body size is usually strongly correlated with spectral traits, and this correlation also holds between species, suggesting that fundamental and dominant frequencies are under morphological constraints, with smaller frogs (with shorter vocal cords) producing calls at higher frequencies (Gerhardt & Huber 2002;Gingras et al 2013). This correlation is almost universal in anurans, with few exceptions (Sullivan 1984;Sullivan & Malmos 1994;Lingnau & Bastos 2007), including frogs that can actively adjust frequency depending on the context (see above).…”

“…The spectral flatness, also known as tonality coefficient or Wiener entropy, is calculated as the geometric mean of the power spectrum divided by its arithmetric mean (Dubnov 2004). This measurement has not been tested so far as a tool for species delimitation in taxonomic approaches, but similar to dominant frequency, spectral flatness has been found to be inversely related to SVL in three of four frog clades studied (Gingras et al 2013).…”

“…One of these has been pointed out by Gingras et al (2013): the spectral flatness, a quantitative measure of tonality, has turned out to be valuable in the identification of different clades of frogs. The spectral flatness, also known as tonality coefficient or Wiener entropy, is calculated as the geometric mean of the power spectrum divided by its arithmetric mean (Dubnov 2004).…”

Five mechanisms of sound symbolic association

“…It is important to note that while many studies have found a relationship between fundamental frequency and body size in several species (e.g., Bowling et al, 2017;Charlton & Reby, 2016;Gingras, Boeckle, Herbst, & Fitch, 2013;Hauser, 1993;Wallschläger, 1980), others have not (e.g., Patel, Mulder, & Cardoso, 2010;Rendall, Kollias, Ney, & Lloyd, 2005;Sullivan, 1984). As noted by Bowling et al (2017), a relevant factor seems to be the range in body sizes 16 The authors theorize that two separate, but potentially related, processes may be at work.…”

Female gray treefrogs, Hyla versicolor, are responsive to visual stimuli but unselective of stimulus characteristics

“…Because the vocal sac is inflated for the duration of the call, the duration of visible inflation of the vocal sac could provide redundant information to females on call duration, a frequent target of female choice in the acoustic modality (Gerhardt et al, 2000). In addition, the size of the vocal sac is likely to be correlated to overall male body size, a characteristic that is often correlated with the dominant frequency of acoustic signals (Gingras et al, 2013), but also potentially assessed visually by females. Finally, certain characteristics of vocal sacs may be signals that are entirely independent of the qualities assessed in the acoustic modality.…”