Middle ear pneumatization in nonhuman primates: A comparative analysis.

OBJECTIVES: We test the effects of body mass and phylogeny on middle ear cavity pneumatization, and the role of pneumatization in hearing function, spanning the anatomical, ecological, and behavioral diversity of nonhuman primates. MATERIALS AND METHODS: All cavities were segmented in middle ear scans of 96 specimens, from 12 strepsirrhine and 15 haplorhine extant species. We measured the tympanic cavity (TC) separately, and all other middle ear spaces together (MES), calculating the degree of pneumatization with the surface area-to-volume ratio. We tested body mass effect with linear regression; we evaluated the phylogenetic signal and selection patterns, using a Kappa statistic test, and Ornstein-Uhlenbeck models (OU). We investigated the link between pneumatization and hearing sensitivity using phylogenetic regression. RESULTS: Testing body mass reveals an allometric pattern for both TC and MES dimensions. Degree of pneumatization in MES is dependent on body mass in haplorhines: larger animals have more pneumatized MES. Differences at various taxonomic ranks were observed for MES, while no phylogenetic influence was observed for TC. Infraorder selection patterns are different. Auditory performance is significantly related to degree of pneumatization, indicating that a pneumatized middle ear is associated with better perception of low frequencies. DISCUSSION: Pneumatization in MES is under differential selective pressure, indicating several optima for this trait. Pneumatization in MES probably modifies hearing sensitivity through pressure regulation mechanisms, auditory bulla size reduction, and frequency modulation. This could explain strepsirrhine adaptation to high-frequency perception, while haplorhine auditory perception is adapted to a broader sound range, including high and low frequencies.

Eocene Paleoecology of Adapis parisiensis (Primate, Adapidae): From Inner Ear to Lifestyle.

European adapids, especially Adapis parisiensis, have been extensively studied over the past two centuries, essentially for taxonomic and phylogenetic purposes. Analyses of dental, cranial, and postcranial morphology have led to various hypotheses about the diet, locomotion, and lifestyle of this fossil primate species. As the morphology of Adapis parisiensis is not directly comparable to any extant species, some interpretations are still debated. The inner ear is crucial to several functional parameters, such as auditory acuity, balance, agility, orientation, and head motion speed during locomotion. We examined the inner ear morphology of Adapis parisiensis in order to make some functional inferences, using µCT techniques to access the internal morphology without damaging the fossil specimens. We analyzed the length and shape of the cochlea, the size of the oval window, and the size and orientation of the semicircular canals of seven Adapis parisiensis crania. Results indicate that the species was more sensitive to high frequencies than low frequencies. Results for locomotion style are different, depending on the method used. Results about the coefficient of agility are in good agreement with previous studies, proposing a slow/medium slow locomotion for the fossil species. In contrast, angular velocity magnitude (AVM) results show a great range of variation in the locomotor repertory for Adapis parisiensis, probably indicating that the model used is not adapted to the study of the fossil record. A comparison to measurements of extant strepsirhines leads us to conclude that Adapis parisiensis was probably monogamous and solitary, with a small home range. Anat Rec, 300:1576-1588, 2017. © 2017 Wiley Periodicals, Inc.