Shape analysis of odontocete mandibles: Functional and evolutionary implications

Abstract

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.

Anatomical evidence for low frequency sensitivity in an archaeocete whale: comparison of the inner ear of Zygorhiza kochii with that of crown Mysticeti

“…Furthermore, an enlarged mandibular foramen would allow access for sound waves with larger wavelengths (indicating lower frequencies) and higher amplitudes (Barroso et al. ). This should not imply that the animals were detecting low frequency vibrations, but that those vibrations could be transported to the ear.…”

“…In this sense, the enlarged mandibular foramen would serve as an 'open door' (Cranford et al 2010) that likely facilitates hearing underwater in general for early whales, rather than hearing at specific frequencies. Furthermore, an enlarged mandibular foramen would allow access for sound waves with larger wavelengths (indicating lower frequencies) and higher amplitudes (Barroso et al 2012). This should not imply that the animals were detecting low frequency vibrations, but that those vibrations could be transported to the ear.…”

Nitrogen solubility in odontocete blubber and mandibular fats in relation to lipid composition

“…The higher degree of variation in pygmy sperm whale fats (see above) may have resulted from the heterogeneity of mandibular fat composition combined with the challenges associated with sampling mandibular fats in this particular genus. Pygmy sperm whale heads have very different morphologies from those of other odontocetes (Barroso et al, 2012;Thornton et al, 2015), with exceptionally bulky fat bodies, thin mandibles and high degrees of spatial heterogeneity in lipid composition (Karol et al, 1978;Koopman et al, 2006), potentially making it more difficult to sample homologous sites in these animals. To better understand how these biochemical differences affect sound transmission interspecifically, finer-scale lipid analyses need to be conducted in conjunction with acoustic experiments.…”

Postnatal development of franciscana's (Pontoporia blainvillei) biosonar relevant structures with potential implications for function, life history, and bycatch

“…Odontocete mandibles serve a variety of functions, mainly feeding and hearing (Barroso et al 2012). Today's most discussed hypothesis for the sound perception pathway was proposed by Norris (1968) according to which the sound, that enters the lower jaw of odontocetes closely to the posterior part of the alveolar process, is transmitted via the mandibular pan bone (acoustic window) and then guided by the medial mandibular fat pad through an extended mandibular foramen to the tympanic bulla.…”