Cochlear Labyrinth Volume and Hearing Abilities in Primates

Abstract

The primate cochlea is a membranous, fluid-filled receptor organ that is specialized for sound detection. Like other parts of the inner ear, the cochlea is contained within the bony labyrinth of the petrous temporal bone. The close anatomical relationship between the bony cochlear labyrinth and the membranous cochlea provides an opportunity to quantify cochlear size using osteological specimens. Although mechanisms of cochlear frequency analysis are well studied, relatively little is known about the functional consequences of interspecific variation in cochlear size. Previous comparative analyses have linked increases in basilar membrane length to decreases in both the high and low frequency limits of hearing in mammals. However, these analyses did not consider the potentially confounding effects of body mass or phylogeny. Here, we present measurements of cochlear labyrinth volume in 33 primate species based on high-resolution computed tomography. These data demonstrate that cochlear labyrinth volume is strongly negatively allometric with respect to body mass. Scaling of cochlear volume in primates is very similar to scaling of basilar membrane length among mammals generally. Furthermore, an analysis of 10 primate taxa with published audiograms reveals that cochlear labyrinth volume is significantly negatively correlated with the high frequency limit of hearing. This result is independent of body mass and phylogeny, suggesting that cochlear size is functionally related to the range of audible frequencies in primates. Although the nature of this functional relationship remains speculative, our findings suggest that some hearing parameters of extinct taxa may be estimated using fossil petrosals. Anat Rec, 292:765-776, 2009. V V C 2009 Wiley-Liss, Inc.Key words: cochlear size; inner ear; basilar membrane; scaling; auditory epitheliumThe mammalian inner ear is a complex series of fluidfilled receptor organs with varying sensory functions (Purves et al., 2008). Three receptor endorgans (the semicircular canals) detect angular accelerations of the head and help to stabilize the eyes and head via the vestibulo-ocular and vestibulo-collic reflexes. Two additional endorgans (the utricle and sacculus) primarily detect linear accelerations of the body and tilting of the head relative to the plane of gravity. The third component of the mammalian inner ear (the cochlea) detects sounds transmitted to the inner ear fluids by the external and middle ears. These six interconnected receptor organs together comprise the ''membranous labyrinth'' of the inner ear.

Social drive and the evolution of primate hearing

“…Two recent studies have identified relationships between high-frequency auditory sensitivity and the morphology of the middle ear ossicles [63] and cochlear labyrinth [62,63]. Such findings, in tandem with the present results, suggest that it is plausible to estimate the group sizes of fossil species, including early human ancestors, on the basis of bony ear morphology.…”

“…However, this audiometric parameter is limited by the selection of test frequencies and complicated by multiple frequencies having similar thresholds. Accordingly, the 60 dB high-frequency limit is a more practical measure of high-frequency auditory sensitivities; and, in primates, it has been linked to morphological variation within the bony ear [62,63]. We found that average foraging group size explained a significant proportion of the variance associated with 60 dB high-frequency limits (figure 1c), again indicating a strong predictive relationship between social complexity and auditory sensitivity.…”

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

“…Also, of note is the fact that the volume of human eye is ~4 mL, while the volume of cochlea is lesser, ~70-80 μL. 188 Overall, it has been explicitly clear that for both eye and ear, the BDNF has a dose-dependent manner of action. Eventually, it has been shown, that increasing the dose may lead to lack of effectiveness of BDNF neuroprotection, 97 which is possibly due to negative feedback-mediated downregulation of TrkB receptors.…”

The labyrinthine morphology of Pronycticebus gaudryi (Primates, Adapiformes)

“…Morphological variation of the bony labyrinth across taxa reflects differences in locomotor behavior (Silcox et al 2009;Spoor and Zonneveld 1998;Spoor et al 2007;Walker et al 2008 ) and hearing performance (Coleman and Boyer 2012;Echteler et al 1994;Gleich et al 2005; Kirk and Gosselin-Ildari 2009;West 1985). This structure is thus highly functionally constrained.…”

“…Nevertheless, a part of labyrinthine morphological variation may still convey phylogenetic information. Basilar membrane length, the number of spiral turns and cochlear volume relate to low frequency and high frequency sensitivity (Coleman and Boyer 2012;Echteler et al 1994;Kirk and Gosselin-Ildari 2009;West 1985). However, other aspects of the morphology of the cochlea, such as its general geometry and orientation relative to the semi-circular canal system are less likely to reflect hearing performance.…”