title: 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.
authors.affiliation: Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria stephan.reber@univie.ac.at tecumseh.fitch@univie.ac.at.
authors.forename: Stephan A
authors.initials: SA
authors.lastname: Reber
authors.affiliation: Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Nishimura
authors.affiliation: Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria.
authors.forename: Judith
authors.initials: J
authors.lastname: Janisch
authors.affiliation: St Augustine Alligator Farm Zoological Park, St Augustine, FL 32080, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Robertson
authors.affiliation: Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria stephan.reber@univie.ac.at tecumseh.fitch@univie.ac.at.
authors.forename: W Tecumseh
authors.initials: WT
authors.lastname: Fitch
mesh_terms: D000489:Alligators and Crocodiles / Q000502:physiology*; D000818:Animals; D049628:Body Size; D003463:Cues; D005260:Female; D006371:Helium; D010100:Oxygen; D013018:Sound Spectrography; D014828:Vocalization, Animal / Q000502:physiology*
chemical_list: D006371:Helium; C038949:heliox; D010100:Oxygen
keywords: Alligator sinensis; Archosauria; Bellow; Bioacoustics; Source-filter theory; Vocal tract resonance