Mechanisms of swallowing and airway protection in infant mammals (Sus domesticus and Macaca fascicularis)

Protection of the airway, necessary for continued respiration, is a problem for mammals because of the relative positions of the oesophageal and laryngeal openings in the pharynx. In human infants, and all other mammals, infant and adult, the epiglottis contacts the posterior surface of the soft palate, providing a continuous passage from the nasopharynx to larynx. The function and movements of the epiglottis during swallows are debated as to whether the epiglottis bends to protect the airway or remains erect and leaves the airway open during the swallow. Using high-speed cineradiography, we examined swallows in detail for a precocious infant, Sus domesticus, the miniature pig, and the more altrical primate, Macaca fascicularis. Infant pigs swallowed in two different ways: down the midline of the oropharynx, over a bent epiglottis, and laterally, around an erect epiglottis, and presumably open airway. The epiglottis of infant macaques never bent, and milk always travelled laterally, through the pyriform recesses and around the larynx. The macaque airway was closed superiorily, however, when the soft palate scaled against the posterior pharyngeal wall. A hypothesis that could account for this pattern of swallowing involves an ontogenetic change from swallows travelling laterally through the pyriform recesses in young infants to swallows travelling over a bent epiglottis in more mature infants. This change would accompany maturation associated with weaning and the need to protect the airway from the larger and less fluid boluses of masticated solid food.