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
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