Vocal tract anatomy of king penguins: morphological traits of two-voiced sound production

Abstract

Background: The astonishing variety of sounds that birds can produce has been the subject of many studies aiming to identify the underlying anatomical and physical mechanisms of sound production. An interesting feature of some bird vocalisations is the simultaneous production of two different frequencies. While most work has been focusing on songbirds, much less is known about dual-sound production in non-passerines, although their sound production organ, the syrinx, would technically allow many of them to produce "two voices". Here, we focus on the king penguin, a colonial seabird whose calls consist of two fundamental frequency bands and their respective harmonics. The calls are produced during courtship and for partner and offspring reunions and encode the birds' identity. We dissected, μCT-scanned and analysed the vocal tracts of six adult king penguins from Possession Island, Crozet Archipelago. Results: King penguins possess a bronchial type syrinx that, similarly to the songbird's tracheobronchial syrinx, has two sets of vibratory tissues, and thus two separate sound sources. Left and right medial labium differ consistently in diameter between 0.5 and 3.2%, with no laterality between left and right side. The trachea has a conical shape, increasing in diameter from caudal to cranial by 16%. About 80% of the king penguins' trachea is medially divided by a septum consisting of soft elastic tissue (septum trachealis medialis). Conclusions: The king penguins' vocal tract appears to be mainly adapted to the life in a noisy colony of a species that relies on individual vocal recognition. The extent between the two voices encoding for individuality seems morphologically dictated by the length difference between left and right medial labium. The septum trachealis medialis might support this extent and could therefore be an important anatomical feature that aids in the individual recognition process.

Vocal tract shape variation contributes to individual vocal identity in African penguins

“…Similarly, we found that a lack of correlation between formant frequencies and skeletal size is observed in the African penguin, where the bird's skeleton does not anatomically constrain the trachea. Indeed, while it has been found that in the king penguin (genus Aptenodytes ) the trachea lies straight in line with the thorax [80] in the African penguin, the trachea and oesophagus form a slight loop as they exit the interclavicular air sac and are laterally displaced towards the right-hand side of the neck (electronic supplementary material, figure S2). To the best of our knowledge, the lack of correlation between formant frequencies and skeletal dimension has been reported for four other bird species, including the Humboldt ( Spheniscus humboldti ) and Magellanic ( Spheniscus magellanicus ) penguins [29], two congeneric species of the African penguin, the herring gull ( Larus argentatus [28]), and the phylogenetically distant corncrake ( Crex crex [27]).…”

Anatomical, Histological, and Electron Microscopic Structures of Syrinx in Male Budgerigars (Melopsittacus undulatus)

“…TL muscles are ill-developed and extend from the larynx to the cranial part of the tympanum under the superficial syringeal muscles. The TL muscle is strong and prominent in non-Passerine birds (Kriesell et al, 2020) but is weak and has mild role in sound control in songbirds (Warner, 1972). The ST muscles of budgerigar's syrinx in the current study are very thin and not extended from the sternum.…”

“…The vocal apparatus in birds differs considerably from that of mammals. Birds have evolved a unique phonatory organ (syrinx) that is located at the tracheal bifurcation (Getty, 1975; Nickel et al, 1977; King & Mclelland, 1984; Kriesell et al, 2020). At the developmental level, the larynx and syrinx differ considerably.…”

Morphology and Morphometry of the Magellanic Penguin (Spheniscus magellanicus) Trachea

“…The trachea was formed by the union of tracheal rings, cartilaginous and complete, with different shapes 8 and dimensions along its entire extensio [5][6][7] . In relation to the morphological aspects, mentioned that in penguins and some marine species 12,13 , a tracheal septum was present in the trachea lumen, responsible for dividing the lumen of the organ ventrodorsally along its entire length, acting against the occurrence of tracheal collapse.…”