Talar trochlear morphology may not be a good skeletal indicator of locomotor behavior in humans and great apes

Shuhei Nozaki,Motoharu Oishi,Naomichi Ogihara

doi:10.1038/s41598-021-03429-y

Abstract

To reconstruct locomotor behaviors of fossil hominins and understand the evolution of bipedal locomotion in the human lineage, it is important to clarify the functional morphology of the talar trochlea in humans and extant great apes. Therefore, the present study aimed to investigate the interspecific-differences of the talar trochlear morphology among humans, chimpanzees, gorillas, and orangutans by means of cone frustum approximation to calculate an apical angle and geometric morphometrics for detailed variability in the shape of the talar trochlea. The apical angles in gorillas and orangutans were significantly greater than those in humans and chimpanzees, but no statistical difference was observed between humans and chimpanzees, indicating that the apical angle did not necessarily correspond with the degree of arboreality in hominoids. The geometric morphometrics revealed clear interspecific differences in the trochlear morphology, but no clear association between the morphological characteristics of the trochlea and locomotor behavior was observed. The morphology of the trochlea may not be a distinct skeletal correlate of locomotor behavior, possibly because the morphology is determined not only by locomotor behavior, but also by other factors such as phylogeny and body size.

Highlights

To reconstruct locomotor behaviors of fossil hominins and understand the evolution of bipedal locomotion in the human lineage, it is important to clarify the functional morphology of the talar trochlea in humans and extant great apes
No studies have attempted to mathematically approximate the talar trochlea by a conical surface to three-dimensionally quantify the apex angle of the cone frustum, the trochlea surface has been previously approximated by a plane8,9, cylinder[9], and paraboloids[10] for morphological analyses of the articular surface. Inman[1] carried out the only study to measure the apex angle of human tali by physically placing a metal rod through the hole of the ankle axis and metal wires on the trochlear surface such that they intersected with the rod; it was not an elaborate approximation of a surface by minimizing the sum of the squared distances between a set of points comprising the talar trochlea and the surface of the cone
The present study demonstrated, for the first time, that the talar trochlea surfaces in humans and great apes can be well approximated by a cone frustum, as suggested by Inman[1] and Latimer et al.[2]

Summary

Introduction

To reconstruct locomotor behaviors of fossil hominins and understand the evolution of bipedal locomotion in the human lineage, it is important to clarify the functional morphology of the talar trochlea in humans and extant great apes. Clarifying the pattern of the morphological variations of the talar trochlea in humans and nonhuman great apes and how it possibly corresponds to the differences in their locomotor behavior is of particular importance in reconstructing locomotor behaviors of fossil hominins and understanding the evolution of bipedal locomotion in the human lineage[2,3,4,5]. If the articular surface of the talar trochlea was three-dimensionally fitted by the surface of the frustum of a cone, the true apical angle of the trochlear surface could be quantified and compared, possibly providing useful morphological correlates of talocrural mobility and locomotor behavior. No studies have attempted to mathematically approximate the talar trochlea by a conical surface to three-dimensionally quantify the apex angle of the cone frustum, the trochlea surface has been previously approximated by a plane8,9, cylinder[9], and paraboloids[10] for morphological analyses of the articular surface. Inman[1] carried out the only study to measure the apex angle of human tali by physically placing a metal rod through the hole of the ankle axis and metal wires on the trochlear surface such that they intersected with the rod; it was not an elaborate approximation of a surface by minimizing the sum of the squared distances between a set of points comprising the talar trochlea and the surface of the cone

Methods

Results

Conclusion