Primate auditory diversity and its influence on hearing performance

Abstract

The auditory region contains numerous structures that have proven useful for phylogenetic classification at various taxonomic levels. However, little work has been done in primates relating differences in morphology to variations in hearing performance. This study documents anatomical and physiological distinctions within primates and begins to address the functional and evolutionary consequences of these and other auditory features. The dimensions of the outer ear (pinna) were measured in cadaveric specimens representing nearly every primate family and used to calculate a shape ratio (height/width). It was found that nonanthropoids have a significantly higher ratio than anthropoids, although the actual height was not found to differ. This indicates that most nonanthropoids have ears that are tall and narrow, whereas monkeys and apes are characterized by ears with more equal height and width dimensions. Eardrum area, stapedial footplate area, and ossicular lever arm lengths were measured in dried specimens to calculate an impedance transformer ratio. A distinction was found between anthropoids and strepsirrhines, with the latter group having a transformer ratio indicative of a higher percentage of acoustic energy transmission through the middle ear. Audiogram data were gathered from the literature to analyze hearing sensitivity and it was found that platyrrhines illustrate more low-frequency sensitivity than like-sized lorisoids. The effects of intraspecific variation on the audiogram results were also examined and were found to produce similar results as the analysis using species mean threshold values. Lastly, correlations between morphological and audiogram variables were examined. Several measures of hearing sensitivity were found to be correlated with pinna shape but correlations with middle ear transmission properties were weaker. In addition to using traditional statistical techniques, phylogenetic corrective methods were applied to address the problem of statistical nonindependence of the data and the results of both analyses are compared. These findings are discussed with respect to how sensory adaptations and phylogenetic history may be related to the current radiation of primates. Key words: primate hearing; outer ears; middle ears; audiogram; phylogenyPrimates express numerous modifications on the fundamental mammalian middle ear morphology and these differences have proven useful for phylogenetic classification at various taxonomic levels (MacPhee and Cartmill, 1986). Among extant strepsirrhines, most Malagasy primates are characterized by a free-floating tympanic ring surrounded by a single relatively large tympanic cavity while lorisoids have a tympanic ring that is fused to the lateral wall of a smaller tympanic cavity. Lorises appear to maintain a substantial effective cavity volume by having additional pneumatic spaces off of the epitympanic recess. Extant haplorhines are similar to lorises in possessing epitympanic sinuses (except tarsiers) (MacPhee and Cartmill, 1986) and a fused tym...

What Do Primates Hear? A Meta-analysis of All Known Nonhuman Primate Behavioral Audiograms

“…The first is a narrow allometric comparison presented by Coleman and Ross (2004) that contrasted the audiograms of lorisoids with those of platyrrhines. The mean body mass for the species in the 2 groups -the same ones considered here-was nonsignificantly different, although there was a significant difference in low-frequency sensitivity.…”

Early hominin auditory ossicles from South Africa

“…Discovery of additional fossil ear ossicles from even earlier hominin taxa would make it possible to identify more precisely when this human-like malleus first appeared in the hominin evolutionary lineage. Although the precise reasons behind these changes in malleus proportions are currently unclear, the functional length of the malleus (manubrium length) shows a strong correlation with the area of the tympanic membrane across haplorhines, including humans (27,28). The tight developmental and functional relationship with the tympanic membrane (28)(29)(30) suggests the shortened manubrium in modern humans may be related with the smaller dimensions of the tympanic membrane compared with the African and Asian great apes (28,31,32).…”

Social drive and the evolution of primate hearing

“…Such digital morphology aids percussive foraging or tap-scanning [70 -73], acoustic behaviours that contribute to the detection of embedded beetle larvae and to the material assessment of foods [74]. Thus, aye-ayes are highly specialized foragers; indeed, their large pinnae [63,75] and expanded frontal cortex [76] appear to support their dependence on asocial acoustic cues. Accordingly, the good high-frequency auditory sensitivity of aye-ayes is perhaps best interpreted as a foraging adaptation.…”