Are there morpho-acoustic patterns of adaptation in nonhuman primate ears? Testing the role of ecology and habitat in shaping ear morphology and function.

Analysis of the variation in the bony structures of the inner and middle ear provides critical insights into functional morphology, as well as adaptive morphology across primates. In this study, we investigated whether ear morphology patterns are related to the ecological characteristics of species and their habitats to test two acoustic hypotheses: the Acoustic Adaptation Hypothesis (AAH), originally proposed for vocalizations, and the Sensory Drive (SD) hypothesis (i.e., environment-driven signal evolution) across the entire order Primates. This study examines the morphological variation of the middle and inner ear in 201 specimens representing 57 primate species, at a macroevolutionary scale. We analyzed four morphological variables indicative of auditory function, alongside five ecological traits and habitat composition derived from 19 climatic categories. Our results reveal at least three distinct morpho-acoustic patterns associated with different middle ear impedance mechanisms. We also found a signal of covariation between ear morphology and ecological traits in one of the identified morpho-acoustic groups that includes Platyrrhines and Strepsirrhines and are known to experience high levels of interspecific competition and ecological constraints. While some associations with climatic habitat did not reach statistical significance, their near-threshold p-values suggest potentially meaningful trends that deserve further investigation with expanded datasets or alternative ecological descriptors. These findings suggest that while the Acoustic Adaptation Hypothesis (AAH) may offer a useful framework for understanding certain aspects of primate auditory evolution, it does not fully account for the observed variation in morpho-acoustic specializations. In contrast, the Sensory Drive framework, particularly when incorporating the role of interspecific competition, may provide a more comprehensive explanation for the coupling between sensory morphology and ecological traits. This study highlights the different evolutionary drivers of the studied auditory structures, therefore shaping nonhuman primate bioacoustics.