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Phenotype constrains the vocal tract in the most dimorphic mammal, the southern elephant seal (Mirounga leonina)

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Canadian Journal of Zoology
Authors:
Laura Redaelli at Marine and Environmental Science Centre [MARE-Madeira]
Laura Redaelli
  • Marine and Environmental Science Centre [MARE-Madeira]
Filippo Galimberti
Filippo Galimberti
Simona Sanvito
Simona Sanvito
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Abstract and Figures

The study of mammal acoustic communication was revolutionized by the application of the source–filter theory, originally developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates a structural link between phenotype and acoustic formants, providing a basis for honest signaling. The phenotype–formant link was validated in many species, but the phenotype–vocal tract link was rarely assessed. We used two dimensional (2D) videogrammetry to estimate the vocal tract length of wild southern elephant seal (Mirounga leonina (Linnaeus, 1758)) males during their normal vocalization behavior. We showed that (i) the vocal tract can be measured noninvasively in a large wild mammal; (ii) the vocal tract depends on the structural phenotype (age, body length, and skull size); (iii) the nasal tract is more related to the structural phenotype than the buccal tract; and (iv) the dependence on size, and body length in particular, is stronger than the dependence on age. Altogether, the phenotypic constraint on the vocal tract provides the anatomical basis for honest signaling in elephant seals.
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Article
Phenotype constrains the vocal tract in the most
dimorphic mammal, the southern elephant seal
(Mirounga leonina)
Laura Redaelli a, Filippo Galimberti b, and Simona Sanvito b
aDipartimento di Scienze Naturali, Università degli Studi di Milano, Via Festa del Perdono 7, 20122, Milano, Italy; bElephant Seal
Research Group, Sea Lion Island, FIQQ 1ZZ, Falkland Islands
Corresponding author: Laura Redaelli (email: laura.redaelli.95@gmail.com)
Abstract
The study of mammal acoustic communication was revolutionized by the application of the source–filter theory, originally
developed for human speech. The theory states that the vocal tract is constrained by body anatomy and, therefore, creates
a structural link between phenotype and acoustic formants, providing a basis for honest signaling. The phenotype–formant
link was validated in many species, but the phenotype–vocal tract link was rarely assessed. We used two dimensional (2D)
videogrammetry to estimate the vocal tract length of wild southern elephant seal (Mirounga leonina (Linnaeus, 1758)) males
during their normal vocalization behavior. We showed that (i) the vocal tract can be measured noninvasively in a large wild
mammal; (ii) the vocal tract depends on the structural phenotype (age, body length, and skull size); (iii) the nasal tract is more
related to the structural phenotype than the buccal tract; and (iv) the dependence on size, and body length in particular, is
stronger than the dependence on age. Altogether, the phenotypic constraint on the vocal tract provides the anatomical basis
for honest signaling in elephant seals.
Key words: vocal tract, age, body length, skull size, vocalizations, source–filter theory, honest signaling, marine mammals,
southern elephant seal, Mirounga leonina
Résumé
L’application de la théorie source–filtre, proposée à l’origine pour la parole humaine, a révolutionné l’étude de la commu-
nication acoustique chez les mammifères. Cette théorie postule que le tractus vocal est contraint par l’anatomie du corps et
établit donc un lien structural entre le phénotype et les formants acoustiques, fournissant ainsi une base pour la signalisation
honnête. Si le lien phénotype–formants a été validé chez de nombreuses espèces, le lien phénotype–tractus vocal a rarement
été évalué. Nous utilisons la vidéogrammétrie bidimensionnelle (2D) pour estimer la longueur du tractus vocal d’éléphants
de mer austraux (Mirounga leonina (Linnaeus, 1758)) mâles à l’état sauvage durant leur comportement de vocalisation normal.
Nous démontrons que (i) le tractus vocal peut être mesuré de manière non invasive chez un grand mammifère à l’état sauvage,
(ii) le tractus vocal dépend du phénotype structural (âge, longueur du corps et taille du crâne), (iii) le tractus nasal est plus
étroitement relié au phénotype structural que le tractus buccal, (iv) la relation à la taille et la longueur du corps en particulier
est plus forte que la relation à l’âge. Dans l’ensemble, la contrainte imposée par le phénotype sur le tractus vocal fournit le
fondement anatomique de la signalisation honnête chez les éléphants de mer. [Traduit par la Rédaction]
Mots-clés : tractus vocal, âge, longueur du corps, taille du crâne, vocalisations, théorie source–filtre, signalisation honnête,
mammifères marins, éléphant de mer austral, Mirounga leonina
Introduction
Acoustic signals may convey honest information about the
emitter’s phenotypic traits, such as body size or age, and
can be used for assessment by the receiver (Taylor and Reby
2010). Signals are defined as “honest” if they provide accu-
rate information to the receivers, either about the signaler it-
self (i.e., advertisement calls) or about the environment (i.e.,
alarm calls; Fitch and Hauser 2003). The structural basis of
acoustic honest signaling had been uncertain for a long while
(Maynard Smith and Harper 2003;Seyfarth and Cheney 2017),
until the source–filter theory, originally developed for hu-
man speech (Fant 1960), was applied to animal communica-
tion. The source–filter theory provides a robust framework to
study the anatomical constraints that may generate honesty
in acoustic communication (Taylor and Reby 2010;Taylor et
al. 2016), and suggests which traits may be more promising to
376 Can. J. Zool. 100: 376–388 (2022) | dx.doi.org/10.1139/cjz-2021-0188
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