It is generally accepted that the trochlear surface of humans and great apes can be approximated by a cone frustum1,2, but no previous studies have demonstrated this, to the best of our knowledge. The present study demonstrated, for the first time, that the talar trochlea surfaces in humans and great apes can actually be well approximated by a cone frustum, as suggested by Inman1 and Latimer et al.2. However, in gorillas, if the whole region of the talar trochlea was approximated by the cone, the apex of the cone was found to be located on the lateral side of the trochlea, owing to the fact that the curvature radii of the medial rims of the gorilla trochlea were larger than those of the lateral rims because the surface of the posteromedial portion of the trochlea was flattened, as shown in Fig. 2, as previously reported12,24. To bring the apex of the cone on the medial side of the trochlea as in other species, only the conical portion of the trochlea should be used to approximate the trochlear surface in gorillas.

Although the calculated apical angles of the approximated cones were significantly correlated with the talar angles (P=0.0003) conventionally used to estimate the apical angles of the cones3,7 (Fig.7), the present study demonstrated that the correlation between these angles was weak (R=0.379). Geometrically, the talar angle should be half of the apical angle because the cone apex is at the intersection between the line passing through the supratalar surface and the talocrural rotational axis corresponding to the cone axis, and the talar angle is the angle between the two lines on the coronal plane. However, this geometrical relationship was not clearly observed in our study (Fig.7B). These discrepancies indicate that the talar angle could not precisely estimate the apical angle of the cone frustum fitted to the trochlea. This is because the apical angle of the cone is a 3D quantity but the talar angle estimated the apical angle only two-dimensionally on the coronal plane, and the angle projected on the transverse plane was not incorporated. Therefore, the talar angle cannot be used as a synonym of the apical angle of the cone frustum approximating the trochlea but is only a 2D angle of the trochlear rotation axis estimated based on the two inferior-most points of the tibial and fibular facets with respect to the superior surface of the talar trochlea.

The present study demonstrated statistically significant interspecific differences in the apical angle. The apical angles of the humans and chimpanzees were significantly smaller than those of gorillas and orangutans, but no statistical difference was detected between humans and chimpanzees, as well as between gorillas and orangutans (Fig.5). This result contradicted the findings of Latimer et al.2 and DeSilva3, who reported that the talar angle of humans was smaller than that of chimpanzees and gorillas. In the present study, the human talar angle was confirmed to be significantly smaller than that of gorillas (P<0.0001), but not that of chimpanzees (P=0.149). These findings suggest that the talar angle may not be as different as once thought between humans and chimpanzees. Humans engage in habitual bipedalism. Gorillas engage in knuckle-walking and are regarded as the most terrestrial of the great apes, although western lowland gorillas are known to climb on trees for feeding to some extent25,26. Chimpanzees also engage in knuckle-walking and travel between feeding trees mainly on the ground27, but they frequently engage in vertical climbing and suspensory locomotion as well28,29. Orangutans are fundamentally quadrumanous climbers in the rain forest canopy, and they seldom walk on the ground30,31,32. Therefore, there is a distinctive difference in the degree of arboreality among species. However, the present study suggested that the apical (or talar) angle is not clearly associated with the degree of arboreality in humans and great apes.

Our geometric morphometric analysis clearly extracted and visualized interspecific differences in the shape of the talar trochlea among humans and great apes, which were not clearly observed in the comparisons of the apical and talar angles. Chimpanzees, along with macaques, possessed a longer and highly curved talar trochlea (Fig.9A). The longer and curved trochlea possibly allows greater sagittal rotation of the tibia on the trochlea surface at the ankle joint, possibly increasing the mobility of the ankle plantar and dorsiflexion. The greater mobility in dorsiflexion of the ankle joint has been suggested to facilitate vertical climbing3,33,34. It was also found that chimpanzees and gorillas possessed more trapezoidal trochleae than humans (Fig.9A) as reported by previous studies5,12,24. Because the anterior region of the superior surface of the talar trochlea contacts the tibial plafond during dorsiflexion35, the relatively wider anterior part of the trochlea may increase the contact area of the ankle joint during ankle dorsiflexion, possibly to adapt to greater weight bearing when the ankle is in a dorsiflexed posture. Conversely, the human (and macaque) trochleae did not exhibit such a feature, but the trochlea was more rectangular than those of the other species, possibly to adapt to increased contact force during plantarflexion, particularly in the late stance phase of human walking36. The gorilla trochlea differed from that of the other four species in having a less-curved posteromedial trochlea, more medially projected medial malleolar extension onto the talar neck, and deeper central groove. The former two features may be related to the reduced range of the talocrural joint in gorillas to accommodate their large body mass37,38. The enhanced central groove provides increased stability of the talocrural joint in the mediolateral direction. The orangutan trochlea is unique in having a wider posterior margin of the trochlea and a more dorsally turned anteromedial and posterolateral trochlear surface, indicating that the trochlear surface is relatively flatter. The functional significance of this morphological feature is obscure, but it might be related to the fact that the orangutan foot functions as a suspensory supporting organ for hook-like digital gripping without involvement of the hallux, although in the chimpanzee and gorillas, the foot may be adapted to hallux-assisted power gripping39. However, to make further inferences about the form-function relationship of the talar trochlea, data on actual foot use in African great apes and orangutans during terrestrial and arboreal locomotion are lacking and should be investigated in future studies.

The present study demonstrated that the talar trochlea was clearly different in shape between humans and great apes. However, the present study also found that the talar shape was not clearly associated with the differences in locomotor behavior and the degree of arboreality among the species. For example, the apical or talar angle of the trochlea is believed to be correlated with the degree of foot inversion facilitating vertical climbing by positioning the foot sole against the tree substrate2,3, but the apical angle of the trochlea was not substantially different between humans and chimpanzees (Fig.5). In addition, the scatter diagram (Fig.8A) demonstrated that the trochlea is more similar in shape between humans and orangutans, which differ substantially in locomotor behavior. The morphology of the talar trochlea may not be a distinct skeletal correlate of locomotor behavior possibly because the talar morphology is determined not only by locomotor behavior, but also by other factors such as phylogeny and body size. This is consistent with Sorrentino et al.5 indicating that the morphology of the hominin trochlea is not unequivocally linked to locomotor behavior. Therefore, caution needs to be exercised in assessing the morphological affinities of fossil hominid tali to reconstruct their locomotor behavior.

The present study has some limitations. First, the apical angle of the cone may be affected by the manual extraction of the trochlear surface. However, we defined the extracted region as objectively as possible; hence, this effect was confirmed to be relatively minor. Second, the present study included both wild and captive specimens in non-human species. However, we confirmed that the use of captive specimens has only minor effect on our results (Supplementary Information). Third, the present study did not investigate morphological variations in the distal tibia40 and fibula41, which are also important determinants of the mobility of the talocrural joint.

In conclusion, we demonstrated that the trochlea of the talus can be approximated by a conical surface in humans and great apes. However, it was found that the calculated apical angle did not clearly correspond to the degree of arboreality. Our detailed trochlear shape analysis using geometric morphometrics successfully extracted interspecific differences in the morphology of the trochlea; however, no clear association was observed between the morphology and locomotor behavior. The morphology of the talar trochlea may not be a distinct skeletal correlate of locomotor behavior.

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Talar trochlear morphology may not be a good skeletal indicator of locomotor behavior in humans and great apes | Scientific Reports - Nature.com