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The remarkable vocal anatomy of the koala (Phascolarctos cinereus): insights into low-frequency sound production in a marsupial species

journal contribution
posted on 2023-06-09, 11:28 authored by Roland Frey, David Reby, Guido Fritsch, Benjamin D Charlton
Koalas are characterised by a highly unusual vocal anatomy, with a descended larynx and velar vocal folds, allowing them to produce calls with disproportionately low frequencies. Here we use advanced imaging techniques, histological data, classical macroscopic dissection and behavioural observations to provide the first detailed description and interpretation of male and female koala vocal anatomy. We show that both males and females have an elongated pharynx and soft palate, resulting in a permanently descended larynx. In addition, the hyoid apparatus has a human-like configuration in which paired dorsal, resilient ligaments suspend the hyoid apparatus from the skull, while the ventral parts tightly connect to the descended larynx. We also show that koalas can retract the larynx down into the thoracic inlet, facilitated by a dramatic evolutionary transformation of the ventral neck muscles. First, the usual retractors of the larynx and the hyoid have their origins deep in the thorax. Second, three hyoid muscles have lost their connection to the hyoid skeleton. Third, the genioglossus and geniohyoid muscles have greatly increased in length. Finally, the digastric, omohyoid and sternohyoid muscles, connected by a common tendinous intersection, form a guiding channel for the dynamic down-and-up movements of the ventral hyoid parts and the larynx. We suggest that these features evolved to accommodate the low resting position of the larynx and assist in its retraction during call production. We also confirm that the edges of the intra-pharyngeal ostium have specialised to form the novel, extra-laryngeal velar vocal folds, which are much larger than the true, intra-laryngeal vocal folds in both sexes, but more developed and specialised for low frequency sound production in males than in females. Our findings illustrate that strong selection pressures on acoustic signalling not only lead to the specialisation of existing vocal organs, but can also result in the evolution of novel vocal structures in both sexes.

History

Publication status

  • Published

File Version

  • Accepted version

Journal

Journal of Anatomy

ISSN

0021-8782

Publisher

Wiley

Issue

4

Volume

232

Page range

575-595

Department affiliated with

  • Psychology Publications

Full text available

  • Yes

Peer reviewed?

  • Yes

Legacy Posted Date

2018-01-09

First Open Access (FOA) Date

2019-02-19

First Compliant Deposit (FCD) Date

2018-01-09

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