Interplay between evolutionary history, morphological constraints and functional adaptations in the primate cochlea

Area of Science:

• Evolutionary biology
• Comparative anatomy
• Paleontology

Background:

• The evolution of the mammalian cochlea and its functional significance are not fully understood.
• The cochlea's intricate structure is crucial for hearing across mammals.

Purpose of the Study:

• To investigate cochlear morphology variation across 101 Euarchonta species (extant and fossil).
• To analyze evolutionary rates, ancestral states, and functional implications of cochlear shape.
• To understand the relationship between cochlear coiling, hearing, and cranial morphology.

Main Methods:

• Micro-computed tomography (micro-CT) for detailed imaging.
• Three-dimensional geometric morphometrics for shape analysis.
• Phylogenetic comparative analyses and ancestral state reconstructions.

Main Results:

• Significant cochlear shape variation observed across Euarchonta, driven by evolutionary history, constraints, and function.
• Heterogeneous evolutionary rates detected, with high rates in lineages like tarsiers and Cercopithecus, suggesting adaptive selection.
• Lemuriforms retain ancestral conical cochleae; lorisiforms show derived cylindrical, coiled shapes. Tarsier cochleae are highly coiled due to cranial and functional demands.
• Anthropoid platyrrhines fall within catarrhine variation; cercopithecins show increased coiling, while colobines and hominoids have less coiled shapes.
• Body size has minimal impact, but its interaction with cochlear length predicts turns, supporting coiling for a longer basilar membrane in a compact space.

Conclusions:

• Cochlear shape evolution in Euarchonta is complex, involving diverse evolutionary rates and pressures.
• Cochlear coiling is associated with enhanced high-frequency hearing and constrained by cranial morphology.
• The study provides insights into the evolutionary pathways of hearing structures in mammals.