Anatomy of the hyoid apparatus in Odontoceti (toothed whales): specializations of their skeleton and musculature compared with those of terrestrial mammals.

Abstract

The hyoid apparatus of odontocetes (toothed whales) serves as a major attachment point for many of the muscles and ligaments subserving breathing, swallowing, and sound production. This study examines the hyoid apparatus in 48 specimens of ten odontocete genera (Phocoena, Lagenorhynchus, Stenella, Delphinus, Tursiops, Grampus, Globicephala, Mesoplodon, Physeter, and Kogia) collected post mortem from beach strandings. The odontocete hyoid apparatus, as that of their closest terrestrial relatives, the artiodactyls, is divisible into a basal portion (basihyal, paired thyrohyals) and a suspensory portion (paired ceratohyals, epihyals, stylohyals, and tympanohyals) connecting the basal portion to the skull base. Unlike other terrestrial mammals, the basal portion lies inferior to the laryngeal aditus, is flattened dorso-ventrally, and is relatively large, thus providing a broad surface area for muscle attachments. The suspensory elements are not as flattened and are joined by synovial joints (except for epihyal-stylohyal fusion). Muscular specializations include enlargement of those which retract the hyoid apparatus (e.g., sternohyoid) or control the tongue (e.g., styloglossus, hyoglossus). These muscles may be particularly important in a specialized prey capture behavior called suction feeding. In addition, the hyoid apparatus has a tilted placement, which allows asymmetrical enlargement of the piriform sinuses. Asymmetry is also seen in the muscular attachments between the larynx and the hyoid apparatus. The most pronounced differences from the basic pattern are observed in two families: Physeteridae and Ziphiidae. The derived position and shape of the odontocete hyoid apparatus may have evolved to subserve several specialized upper respiratory/digestive tract functions, such as simultaneous feeding (suction and swallowing) and sound production.