A functional comparison of the hyolingual complex in pygmy and dwarf sperm whales (Kogia breviceps and K-sima), and bottlenose dolphins (Tursiops truncatus)

The function of the hyolingual complex in three odontocete species was investigated to compare adaptations of divergent feeding strategies, suction and ram feeding. Pygmy and dwarf sperm whales, members of the genus Kogia (or kogiids), are known to be strong suction feeders. We tested the hypotheses that kogiids would have a larger, more robust hyolingual complex, and that hyolingual muscles would have a greater maximum theoretical muscle tension compared with ram-based feeders such as bottlenose dolphins (Tursiops truncatus). A robustness index and surface area was calculated for bony hyoid elements in kogiids and bottlenose dolphins. The anatomy, muscle architecture, pinnation, two-dimensional angle of attachment and maximum theoretical muscle tension were measured in each hyolingual muscle. A functional model incorporating vector analyses of hyolingual musculature was created for kogiids and bottlenose dolphins to assess differences in function of their hyolingual complexes. Kogiid hyoid surface areas were significantly greater (P = 0.01) than in bottlenose dolphins. Most maximum theoretical muscle tensions of hyolingual complexes were not significantly different within or between species. The data suggest that associated orofacial and tongue morphology, particularly the relationship of hyoid shape and tongue retractor muscles, greatly influences suction capability in odontocetes. Kogiids demonstrated adaptations that occlude lateral gape, including a novel vertical ridge on each side of the mandible, and a shortened mandible that is capable of a large gape, and gape angle. These adaptations presumably assist in maintaining negative intraoral pressure generated by the depression and retraction of the relatively short and wide kogiid tongue. The tongues of kogiids should be capable of generating greater intraoral volume changes compared with the long, narrow tongue of bottlenose dolphins.