Shape analysis of odontocete mandibles: functional and evolutionary implications

Odontocete mandibles serve multiple functions, including feeding and hearing. We consider that these two major functions have their primary influence in different parts of the mandibles: the anterior feeding component and the posterior sound reception component, though these divisions are not mutually exclusive. One hypothesis is that sound enters the hearing apparatus via the pan bone of the posterior mandibles (Norris, Evolution and Environment,1968, pp 297-324). Another viewpoint, based on finite element models, suggests that sound enters primarily through the gular region and the opening created by the absent medial lamina of the posterior mandibles. This unambiguous link between form and function has catalyzed this study, which uses Geometric Morphometrics to quantify mandibular shape across all major lineages of Odontoceti. The majority of shape variation was found in the anterior (feeding) region: Jaw Flare (45.0%) and Symphysis Elongation (35.5%). Shape differences in the mandibular foramen, within the posterior (sound reception) region, also accounted for a small portion of the total variation (10.9%). The mandibles are an integral component of the sound reception apparatus in toothed whales and the geometry of the mandibular foramen likely plays a role in hearing. Furthermore, model goodness-of-fit tests indicate that mandibular foramina shapes, which appear conserved, evolved under a selective regime, possibly driven by sound reception requirements across Odontoceti.