The Morphology and Function of the Hyoid Region in the Tree Sloths, Bradypus and Choloepus

Abstract

The sloth hyoid region has unique structural features, associated with inverted feeding and dependence upon the tongue and lips for food manipulation and ingestion. The hyoid apparatus functions as a unit and is large and robust. The basihyal is strongly bound to the skull posterior in comparison to other mammals. The anterior digastric is enlarged and fused medially, covering the mylohyoid in Choloepus but exposing its anterior aspect in Bradypus. Posteriorly, the muscle fuses with the sternohyoid to form a wide sternomandibularis with a loose tendinous connection to the basihyal. The proposed model of hyoid function predicts that this arrangement aids an inverted sloth in opening the mouth fully against gravity while permitting the hyoid to move anteriorly during tongue extension independent of the sternomandibularis. In upright feeding, contraction of the posterior digastric-sternomandibularis complex allows a similar function. In all postures, increasing gape improves the ability of the hyoglossus and styloglossus muscles to act jointly in moving the tongue laterally. Hyoid retraction, in all postures, occurs with contraction of the modified sternothyroid, which arises from the mid-dorsum of the manubrium sternum and inserts on the ossified thyroid cartilage. The sloth palate also facilitates inverted feeding. The mucosa shows tubercles and grooves which channel food particles toward a central groove. Food is then returned to the toothrows by the tongue for further mastication or passed along a posteriorly deepening groove for bolus formation prior to swallowing.