Acoustic allometry, background stream noise and its relationship with large-bodied and voiceless rheophilic frogs

Anurans that breed in rivers and cascade streams need to deal with the influence of the low-frequency water flow noise that could interfere with their acoustic communication. To overcome that, some species display visual signals or emit high-pitched calls. However, there is a constraint related to body size [snout-vent length (SVL)] that limits the anurans' calling frequency range. Thus, to evaluate if water noise influences this allometric relationship, we analysed hylodid frogs’ SVL, dominant frequencies of advertisement call, and background noise to which individuals are submitted. Although Hylodes and Crossodactylus have differences in morphology and natural history, we found negative acoustic allometry in both genera. This allometry may be part of the explanation of the absence of advertisement calls in Megaelosia and Phantasmarana genera. Based on the large body lengths of Megaelosia and Phantasmarana species, if they could call, it would be low-pitched and acoustically masked by the stream noise. Besides, we found a positive relationship between Hylodes phyllodes advertisement call and background sound pressure level (SPL), thus, the analysed individuals, emitted louder calls when submitted to louder background SPLs. This study exemplifies the complexity of acoustic allometry in rheophilic species: we evidenced a strategy for intraspecific communication effectiveness in rivers and streams with broad water flow background noise; and how acoustic allometry coped with background noise could be implying in unexpected trait evolution, as lack of vocalization in anurans.