Unveiling the acoustic repertoire of true boas: hisses resemble white noise and indicate individual identity

Abstract

Abstract Although hissing is a common strategy of defence used by snakes, especially of the Phytonidae and Boidae families, few studies have described in detail the acoustics components of this behaviour. Here we gathered eight recordings from six different boas from Brazil and extracted acoustic measurements from their hissing sounds. We also examined the potential of our measurements to encode individual identity using discriminant function analyses (DFAs) and to correlate with body length using linear mixed models (LMMs). Boas produce from four to 12 hisses per minute (7.13 ± 3.04) evoked by a negative stimulus, a visual looming stimulus generated by researcher’s approach. Each hiss usually has two parts (the first louder part and the second softer part) that are basically broadband white noises split by a short silent interval. The first hiss part also has more frequency modulations and narrower bandwidth than the second hiss part. The DFAs correctly assigned all hisses to their correspondent individuals in the testing data. The first discriminant function explained most of the variance (⩾84%) in the discrimination between groups for the entire hiss and for both hiss parts in the training data subset. Frequency parameters and spectral entropy (for entire hiss and hiss part 1) and the mean frequency and duration (for hiss part 2) were the strongest absolute loadings in the DFAs. Intraspecific morphological traits (e.g. sexual dimorphism and ontogenetic variations) may influence the sound parameters, as detected by individual variations and the tendency for larger individuals to produce lower frequency hisses. Further studies may investigate these aspects in detail, including anatomy of the snake’s larynx and experiments with different types of predators.