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Research Article
A Chinese alligator in heliox: formant frequencies in a crocodilian
Stephan A. Reber, Takeshi Nishimura, Judith Janisch, Mark Robertson, W. Tecumseh Fitch
Journal of Experimental Biology 2015 218: 2442-2447; doi: 10.1242/jeb.119552
Stephan A. Reber
1Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria
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  • For correspondence: stephan.reber@univie.ac.at tecumseh.fitch@univie.ac.at
Takeshi Nishimura
2Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan
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Judith Janisch
1Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria
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Mark Robertson
3St Augustine Alligator Farm Zoological Park, St Augustine, FL 32080, USA
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W. Tecumseh Fitch
1Department of Cognitive Biology, University of Vienna, Vienna 1150, Austria
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  • For correspondence: stephan.reber@univie.ac.at tecumseh.fitch@univie.ac.at
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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.

FOOTNOTES

  • Competing interests

    The authors declare no competing or financial interests.

  • Author contributions

    S.A.R. designed the study, collected sound recordings, created the stimuli, designed the apparatus, carried out experiments, analysed the data, created the figures and wrote the manuscript; T.N. calculated parameters, participated in data analysis and helped draft the manuscript; J.J. helped carry out experiments and annotated sound files; M.R. designed and constructed the apparatus; W.T.F. helped conceive and design the study and data analysis, and co-wrote the manuscript. All authors gave final approval for publication.

  • Funding

    This research was funded by the European Research Council (ERC) Advanced Grant SOMACCA [no. 230604] and the Austrian Science Fund (FWF) [W1234-G17] to W.T.F., and a travelling grant from the Japan Society for the Promotion of Science (JSPS) [Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation] to T.N. Deposited in PMC for immediate release.

  • Supplementary material

    Supplementary material available online at http://jeb.biologists.org/lookup/suppl/doi:10.1242/jeb.119552/-/DC1

  • © 2015. Published by The Company of Biologists Ltd

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

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Keywords

  • Bioacoustics
  • Source-filter theory
  • Alligator sinensis
  • Vocal tract resonance
  • Bellow
  • Archosauria

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Research Article
A Chinese alligator in heliox: formant frequencies in a crocodilian
Stephan A. Reber, Takeshi Nishimura, Judith Janisch, Mark Robertson, W. Tecumseh Fitch
Journal of Experimental Biology 2015 218: 2442-2447; doi: 10.1242/jeb.119552
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Research Article
A Chinese alligator in heliox: formant frequencies in a crocodilian
Stephan A. Reber, Takeshi Nishimura, Judith Janisch, Mark Robertson, W. Tecumseh Fitch
Journal of Experimental Biology 2015 218: 2442-2447; doi: 10.1242/jeb.119552

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