Rorqual whale nasal plugs: protecting the respiratory tract against water entry and barotrauma

Evolutionary novelties underlie sound production in baleen whales

Baleen whales (mysticetes) use vocalizations to mediate their complex social and reproductive behaviours in vast, opaque marine environments1. Adapting to an obligate aquatic lifestyle demanded fundamental physiological changes to efficiently produce sound, including laryngeal specializations2-4. Whereas toothed whales (odontocetes) evolved a nasal vocal organ5, mysticetes have been thought to use the larynx for sound production1,6-8. However, there has been no direct demonstration that the mysticete larynx can phonate, or if it does, how it produces the great diversity of mysticete sounds9. Here we combine experiments on the excised larynx of three mysticete species with detailed anatomy and computational models to show that mysticetes evolved unique laryngeal structures for sound production. These structures allow some of the largest animals that ever lived to efficiently produce frequency-modulated, low-frequency calls. Furthermore, we show that this phonation mechanism is likely to be ancestral to all mysticetes and shares its fundamental physical basis with most terrestrial mammals, including humans10, birds11, and their closest relatives, odontocetes5. However, these laryngeal structures set insurmountable physiological limits to the frequency range and depth of their vocalizations, preventing them from escaping anthropogenic vessel noise12,13 and communicating at great depths14, thereby greatly reducing their active communication range.

Heterochronic maturation of anatomical plugs for protecting the airway in rorqual whales (Balaenopteridae)

Recently, a unique mechanism for protecting the airway during lunge feeding was discovered in rorqual whales (Balaenopteridae). This mechanism is based on an oral plug structure in the soft palate with similarities in musculo-fatty composition to the nasal plugs protecting the respiratory tract of rorquals from water entry and barotrauma during diving. As a follow-up, we present here a developmental series on fetal, prenatal, juvenile and adult specimens across five species of rorquals, showing differential maturation of the nasal and oral respiratory protection plugs. Nasal plugs are fully formed to serve an immediate crucial function at birth. By contrast, the soft palate remains muscular until the onset of solid food intake, where a musculo-fatty oral plug is developed.

Anatomical mechanism for protecting the airway in the largest animals on earth

Separation of respiratory and digestive tracts in the mammalian pharynx is critical for survival. Food must be kept out of the respiratory tract, and air must be directed into the respiratory tract when breathing.¹ Cetaceans have the additional problem of feeding while underwater. Lunge-feeding baleen whales (rorquals) open the mouth while swimming at high speeds to engulf a volume of prey-laden water as large as their own body² and experience tremendous forces as water floods the mouth. How the respiratory tract is protected in the pharynx during engulfment and while swallowing a massive slurry of tiny living prey remains unknown, despite its importance to survival. By dissecting adult and fetal fin whales, we determined that a large musculo-fatty structure passively seals the oropharyngeal channel. This "oral plug" is not observed in other animals, and its position indicates it must be shifted to allow swallowing; it is a part of the soft palate and can only shift posteriorly and dorsally. Elevation of the oral plug allows food transfer to the pharynx and protects the upper airways from food entry. The laryngeal inlet in the floor of the pharynx is sealed by laryngeal cartilages, and the muscular laryngeal sac moves upward into the laryngeal cavity, completely occluding the airway. The pharynx is dedicated to the digestive tract during swallowing, with no connection between upper and lower airways. These adaptations to facilitate swallowing were a critical development in the evolution of large body size in these, the largest animals on earth.

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

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 UAT in the context of valving mechanisms that function to separate food and air pathways. Here we use videofluoroscopy, gross dissection, histology and computed tomography (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 probably 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.