Correlations between auditory structures and hearing sensitivity in non‐human primates

Abstract

Primates show distinctions in hearing sensitivity and auditory morphology that generally follow phylogenetic patterns. However, few previous studies have attempted to investigate how differences in primate hearing are directly related to differences in ear morphology. This research helps fill this void by exploring the form-to-function relationships of the auditory system in a phylogenetically broad sample of non-human primates. Numerous structures from the outer, middle, and inner ears were measured in taxa with known hearing capabilities. The structures investigated include the overall size and shape of the pinna, the areas of the tympanic membrane and stapedial footplate, the masses and lever arm lengths of the ossicles, the volumes of the middle ear cavities, and the length of the cochlea. The results demonstrate that a variety of auditory structures show significant correlations with certain aspects of hearing (particularly low-frequency sensitivity). Although the majority of these relationships agree with expectations from auditory theory, some traditional (and possibly outdated) ideas were not supported. For example, the common misconception that higher middle ear transformer ratios (e.g., impedance transformer ratio) result in increased hearing sensitivity was not supported. Although simple correlations between form and function do not necessarily imply causality, the relationships defined in this study not only increase our understanding of auditory patterns in extant taxa but also lay the foundation to begin investigating the hearing in fossil primates.

Morphology and function of Neandertal and modern human ear ossicles

“…The malleus, incus, and stapes form the ossicular chain that connects the tympanic membrane to the oval window of the inner ear. These bones play an important role in audition by amplifying and regulating the sound waves transmitted to the cochlea (11,(13)(14)(15). In particular, the middle ear acts as a transformer that matches the impedances between the air and the perilymph of the cochlea (16), participating in the tuning of the sensitivity levels.…”

“…In primates, such hearing variability is assumed to be partly related to forms of vocalization and habitat acoustics (8)(9)(10). Diverse hearing capabilities are also related to the morphology of the diminutive middle ear ossicles housed in the tympanic cavity (11,12). The malleus, incus, and stapes form the ossicular chain that connects the tympanic membrane to the oval window of the inner ear.…”

Reassessment of the La Ferrassie 3 Neandertal ossicular chain

“…It is not oossible to confirm based on LF3 whether Neandertals had an absolutely longer malleus and more open angle between the manubrium and corpus, as suggested by Heim (1982 higher lever ratios (Coleman and Colbert. 2010), including chimpanzees (Martinez et al.…”

“…This approach has been used previously in other studies [Martinez et al. 2004;Coleman and Colbert. 2010).…”

“…Anatomical differences in the ear ossicles across ~r i m a t e s have been shown to be correlated with asoects of their hearing sensitivitv (Coleman and Ross. 2004;Coleman and Colbert. 2010), and fossil hominin ear ossicles have been included in models that reconstruct the auditorv caoacities in hominin .…”