Odontocete mandibles serve multiple functions, including feeding and hearing. One hypothesis is that sound enters the hearing apparatus via the pan bone of the posterior mandibles (Norris, 1968). Another viewpoint, based on computer models, suggests that sound enters primarily through the gular apparatus and the opening created by the absent medial wall of the posterior mandibles. The posterior region of each mandibular ramus has a large, hollow cavity called the mandibular foramen that contains a bulging mandibular fat body (MFB), which terminates on the bony tympanoperiotic complex. The acoustic properties of these fat bodies suggest that they refract sound. This unambiguous link between form and function has catalyzed this current study of mandibular shape. Previous studies have described odontocete mandibles using linear morphometrics and focused on multiple populations within single genera. Geometric Morphometrics (GM) is used to avoid some limitations of linear morphometric studies, using relative 3-D landmark positions instead of lengths. This technique measures shape only, excluding any scaling, rotational, and positional effects. The primary objective of this study is to use GM to quantify mandibular shape across all major lineages of Odontoceti. Eighty-five mandibles, comprised of 40 species, representing all major lineages were included in the shape analysis. Twelve landmarks found on each mandible represent regions of the symphysis and mandibular foramen. The majority of shape variation was found in Jaw Flare and Symphysis Elongation (85.5%). Shape differences in the mandibular foramen also accounts for a portion 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. The second objective of this study is to assess correlates of hearing and mandibular foramen geometry derived from the GM shape analysis, as well as from linear morphometrics. Echolocation peak frequency (EPF) was used as a measure of sound reception. Odontocete anatomy may emphasize frequencies they need to hear and suppress others in a returning echo during echolocation. Frequencies can be characterized by corresponding wavelengths. Surface area between the four landmarks that represent the mandibular foramen was calculated to assess the relationship between EPF and dimensions of a receiving structure (i.e. mandibular foramen and corresponding MFB). A significant relationship between size of the foramen and EPF was found, suggesting that size of the foramen may limit lower frequencies (longer wavelengths) from propagating through the mandibular fat body. The final objective of this study is to examine the amount of phylogenetic signal in the observed morphological variation as well as examine the patterns of mandibular shape evolution. The phylogenetic signal is significant in Jaw Flare and Symphysis Elongation and mandibular foramen shapes. Mandibular foramen shapes, however, may be shaped by other selective pressures. It appears that the shape of the foramen has an optimum shape in the entire toothed whale lineage, perhaps an optimum for sound reception. Phylogenetic relationships may account for mandibular shape on different levels and selective pressures may drive shape variation in less inclusive clades. Convergences among clades is also apparent within Jaw Flare and Symphysis Elongation shapes, indicating a selective force drove two lineages to similar jaw shapes.