Middle ear pneumatization in nonhuman primates: a comparative analysis.
Résumé
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.