Laryngeal and soft palate valving in the harbour seal (Phoca vitulina)

Abstract

Effective ‘valving’ in the upper aerodigestive tract (UAT) is essential to temporarily separate the digestive and respiratory pathways. Marine mammals are largely dedicated to feeding underwater, and in many cases swallowing prey whole. In seals, little work has been done to explore the anatomy and function of the upper aerodigestive tract in the context of valving mechanisms that function to separate food and air pathways. Here we use videofluoroscopy, gross dissection, histology and CT renderings to explore the anatomy of the larynx and soft palate in the harbour seal (Phoca vitulina), and generate models for how valving mechanisms in the head and neck may function during breathing, phonating, diving and swallowing. Harbour seals have an elevated larynx and the epiglottis may rise above the level of the soft palate, particularly in pups when sucking. In addition, the corniculate and arytenoid cartilages with associated muscles form most of the lateral margins of the laryngeal inlet and vestibule, and move independently to facilitate airway closure. The corniculate cartilages flex over the laryngeal inlet beneath the epiglottis to completely close the laryngeal vestibule and inlet. The vocal folds are thick and muscular and the medial margin of the folds contains a small vocal ligament. The soft palate has well-defined levator veli palatini muscles that likely function to elevate the palate and close the pharyngeal isthmus during feeding. Our results support the conclusion that harbour seals have evolved UAT valving mechanisms as adaptations to a marine environment that are not seen in terrestrial carnivores.

Vocal plasticity in harbour seal pups

“…In addition, future replications of the current experiment could test the effect of louder playbacks on the amplitude shift [ 15 ] and apply various masking frequency bands to test whether different frequencies could induce an upward versus downward shift in f 0 . As a complement to behavioural experiments, anatomical work could investigate the elastic properties of seal larynges to establish upper and lower anatomical boundaries for f 0 production [ 50 , 88 ]. Finally, neurobiological work should track purported direct cortico-laryngeal connections in seal pups, and compare them against closely related Caniformia not capable of f 0 plasticity [ 78 , 87 ].…”

Vocal plasticity in harbour seal pups

“…Therefore, other experimental designs could spur the seals to perform amplitude shifts or temporal modifications, or instead even stronger frequency adaptations. As a complement to behavioural experiments, anatomical work could investigate the elastic properties of seal larynges to establish upper and lower anatomical boundaries for F0 production [100, 101]. Finally, neurobiological work should track purported direct cortico-laryngeal connections in seal pups, and compare them against closely related Caniformia not capable of F0 plasticity [90, 99].…”

Can harbor seals (Phoca vitulina) discriminate familiar conspecific calls after long periods of separation?

“…Alternative explanations for the absence of effect could be that our experiment was underpowered or not well-enough targeted to this species’ ecology. Harbor seals are subject of increasing scientific work; empirical evidence on vocal learning, sound production, time perception, hearing, auditory memory, and social dynamics, to name a few, is growing ( Heinrich, Dehnhardt & Hanke, 2016 ; Ravignani et al, 2016 ; Kastelein, Helder-Hoek & Terhune, 2018 ; Wilson et al, 2018 ; Adams et al, 2020 ; Galatius et al, 2020 ; Heinrich, Ravignani & Hanke, 2020 ; Borda et al, 2021 ). Once a clearer socioecological picture is available, we hope that our experiment will be modified to properly target the threshold of long-term memory capacities in harbor seals.…”