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.

Keywords: Alligator sinensis; Archosauria; Bellow; Bioacoustics; Source-filter theory; Vocal tract resonance.

Fig. 1.

Formants of bellows produced in heliox are higher than in ambient air. (A) The spectrogram shows two calls in air (left) and two calls in heliox (right). DF, dominant frequency; F, formant (Praat settings: Method, Fourier; Window shape, Gaussian; Window length, 0.05 s; Dynamic range, 35.0 dB). (B,C) Frequencies of the first formant (B) and second formant (C) in the two conditions; boxplots represent the 25th and 75th percentiles, the centre line indicates the median, whiskers indicate the full data range (***P<0.001).

Fig. 2.

Proposed vocal tract configuration of a Chinese alligator during bellowing. Based on photographs, videos and observations, the drawing depicts the proposed arrangement of the supralaryngal vocal tract in the ‘head oblique tail arched posture’ during bellowing; terminology is derived from Britton, 2001 and Reese, 1945; further based on dissections and CT scans of American alligators by W.T.F. and S.R.

Fig. 3.

Atmosphere exchange during the experimental procedure without handling the subject. After inducing the alligator to bellow in the sealed chamber in ambient air (A), the water level was raised and the ambient air was removed (B). While lowering the water level again, the vacuum was filled with heliox (C) and the animal was subsequently stimulated to bellow in the heliox atmosphere (D).