Adaptations of the cetacean hyolingual apparatus for aquatic feeding and thermoregulation.

Foraging methods vary considerably among semiaquatic and fully aquatic mammals. Semiaquatic animals often find food in water yet consume it on land, but as truly obligate aquatic mammals, cetaceans (whales, dolphins, and porpoises) must acquire and ingest food underwater. It is hypothesized that differences in foraging methods are reflected in cetacean hyolingual apparatus anatomy. This study compares the musculoskeletal anatomy of the hyolingual apparatus in 91 cetacean specimens, including 8 mysticetes (baleen whales) in two species and 91 odontocetes (toothed whales) in 11 species. Results reveal specific adaptations for aquatic life. Intrinsic fibers are sparser and extrinsic musculature comprises a significantly greater proportion of the cetacean tongue relative to terrestrial mammals and other aquatic mammals such as pinnipeds and sirenians. Relative sizes and connections of cetacean tongue muscles to the hyoid apparatus relate to differences in feeding methods used by cetaceans, specifically filtering, suction, and raptorial prehension. In odontocetes and eschrichtiids (gray whales), increased tongue musculature and enlarged hyoids allow grasping and/or lingual depression to generate intraoral suction for prey ingestion. In balaenopterids (rorqual whales), loose and flaccid tongues enable great distention of the oral cavity for prey engulfing. In balaenids (right and bowhead whales), large but stiffer tongues direct intraoral water flow for continuous filtration feeding. Balaenid and eschrichtiid (and possibly balaenopterid) mysticete tongues possess vascular retial adaptations for thermoregulation and large amounts of submucosal adipose tissue for nutritional storage. All cetacean tongues also function in prey transport and swallowing. These hyolingual musculoskeletal differences are unique cetacean anatomical adaptations for foraging entirely in an aquatic environment.