This record contributes to the discussion about calling behaviors of snakes, since it was previously stated that their sounds were composed only of vocal emissions without temporal structure, amplitude, or spectral modulation. As it is a widely distributed Amazonian species, the record of this behavioral act highlights the importance of careful observations on the behavior of snakes, given that vocalization appears to be highly costly to these animals that are devoid of developed vocal cords, which are rare to be observed due to other passive defensive behaviors with lower energetic cost (e.g., hiding the head and curling up)1.
The hissing, common to many families, is characterized by not having a temporal or spectral structure, which can be considered broadband white noise2. The vocalization in our records was made during the exhalation of air through the larynx, similar to the vocalization of other snakes, such as Pituophis melanleucurus (Daudin, 1803) (Colubridae); P. melaolecurus presents a modulated temporal series corresponding to the time and number of exhalations, issuing a defensive call with a frequency of 2000-9500 Hz3. Young et al. (1995) described the structures of the larynx of P. melaolecurus corresponding to vocal cords, allowing controlled and modulated sound emission in this species. This observance raises the hypothesis that the same could be observed in D. catesbyi, in which the specialization of body structures allows for a complex call.
However, the modulation observed in D. catesbyi differs from all patterns observed for snakes resembling the agonistic call of anuran amphibians, which could characterize an evolutionary mimicry of this behavior. Evolutionary convergence and acoustic mimicry are observed in different groups, mainly in the context of defensive behaviors, which can be described between snakes4. Some species of birds and snakes5 in this study demonstrate a possible mimetic character with the agonistic vocalization of anurans by D. catesbyi, which must be confirmed with more studies.
The audible frequency perceptible by snakes is low, in which sound perception occurs mainly through seismic vibrations or, according to some authors, through the detection of sound waves in the air by mechanoreceptors in the skin scales6. However, the evolution of a trait related to sound production may have arisen through the selection of this characteristic. Despite the deafness of snakes, there is possibly a way in which the vocalization frequency is selected by predator pressure. In addition, intraspecifically, vocal emission via the larynx can generate internal vibrations perceptible to the snakes' auditory system, which, when vocalizing, vibrate the laryngeal cartilage and vocal cord. Our hypothesis is that structured vocal emission through laryngeal air exhalation may be a characteristic shared by other species of the Colubridae family and that sound emission through different pathways may be correlated with different families of snakes, requiring more knowledge on the subject.