A Chinese alligator in heliox: formant frequencies in a crocodilian

Abstract

Crocodilians are among the most vocal non-avian reptiles. Adults of both sexes produce loud vocalizations known as ‘bellows’ year round, with the highest rate during the mating season. Although the specific function of these vocalizations remains unclear, they may advertise the caller's body size, because relative size differences strongly affect courtship and territorial behaviour in crocodilians. In mammals and birds, a common mechanism for producing honest acoustic signals of body size is via formant frequencies (vocal tract resonances). To our knowledge, formants have to date never been documented in any non-avian reptile, and formants do not seem to play a role in the vocalizations of anurans. We tested for formants in crocodilian vocalizations by using playbacks to induce a female Chinese alligator (Alligator sinensis) to bellow in an airtight chamber. During vocalizations, the animal inhaled either normal air or a helium/oxygen mixture (heliox) in which the velocity of sound is increased. Although heliox allows normal respiration, it alters the formant distribution of the sound spectrum. An acoustic analysis of the calls showed that the source signal components remained constant under both conditions, but an upward shift of high-energy frequency bands was observed in heliox. We conclude that these frequency bands represent formants. We suggest that crocodilian vocalizations could thus provide an acoustic indication of body size via formants. Because birds and crocodilians share a common ancestor with all dinosaurs, a better understanding of their vocal production systems may also provide insight into the communication of extinct Archosaurians.

Ontogeny of the Middle-Ear Air-Sinus System in Alligator mississippiensis (Archosauria: Crocodylia)

“…Likewise, the subtympanic foramen is a feature common to the middle ear of all extant crocodylians (and also many extinct taxa within the clade Mesoeucrocodylia). That the resonant properties of the middle ear remain remarkably constant during ontogeny is in contrast to the recent discovery that vocal tract resonance produces formant frequencies as a function of size [ 52 ]. Thus far we have only documented correlations of auditory resonance with cochlear sensitivity and juvenile vocalization frequencies in Alligator mississippiensis , and further analyses are needed to see if the correlation holds for other extant crocodylian taxa.…”

Vocal plasticity in a reptile

“…Vocal signalling is a less common feature in reptiles than in anurans, with many taxa producing only non-vocal sounds, such as percussive sounds or hissing, generated by forceful expiration of air without involving the vocal cords. Among the vocal reptiles (some chelonians, some lizardsmost notably geckos, and crocodilians (reviewed in [19,40])), the functional morphology of their vocal systems are quite diverse, as are the complexity of signals produced, which range from noisy grunts that require little control of glottal tension to frequency-modulated tonal sounds requiring active control of the vocal cords [19]. Basal sound production mechanisms, such as hissing or grunting are more inflexible in their production than tonal, harmonic and frequency modulated sounds, which are modified by changes in tension of the vibratory tissues, and thus the evolution of more sophisticated vocal anatomy may be a first step towards the evolution of flexible acoustic communication systems, such as those of birds, mammals, and now geckos.…”

Humans recognize emotional arousal in vocalizations across all classes of terrestrial vertebrates: evidence for acoustic universals

“…The signal produced by the source is subsequently filtered by the resonances of the supralaryngeal vocal tract (the 'filter') with certain frequencies being enhanced or attenuated. Source vibration determines the fundamental frequency of the vocalization (F 0 ), and filter resonances shape its spectral content, producing concentrations of acoustic energy in particular frequency bands (called 'formants') [6][7][8][9][10][11]. For instance, when humans vocalize, air passes from the lungs through an opening between the vocal folds, causing them to vibrate.…”