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Mountain gorillas are particularly inbred compared to other gorillas and even the most inbred human populations. As mountain gorilla skeletal material accumulated during the 1970s, researchers noted their pronounced facial asymmetry and hypothesized that it reflects a population-wide chewing side preference. However, asymmetry has also been linked to environmental and genetic stress in experimental models. Here, we examine facial asymmetry in 114 crania from three Gorilla subspecies using 3D geometric morphometrics. We measure fluctuating asymmetry (FA), defined as random deviations from perfect symmetry, and population-specific patterns of directional asymmetry (DA). Mountain gorillas, with a current population size of about 1000 individuals, have the highest degree of facial FA (explaining 17% of total facial shape variation), followed by Grauer gorillas (9%) and western lowland gorillas (6%), despite the latter experiencing the greatest ecological and dietary variability. DA, while significant in all three taxa, explains relatively less shape variation than FA does. Facial asymmetry correlates neither with tooth wear asymmetry nor increases with age in a mountain gorilla subsample, undermining the hypothesis that facial asymmetry is driven by chewing side preference. An examination of temporal trends shows that stress-induced developmental instability has increased over the last 100 years in these endangered apes.

The process of making stone tools, specifically knapping, is a hominin behaviour that typically involves using the upper limb to manipulate a stone hammer and apply concentrated percussive force to another stone, causing fracture and detachment of stone chips with sharp edges. To understand the emergence and subsequent evolution of tool-related behaviours in hominins, the connections between the mechanics of stone knapping, including the delivery of percussive forces, and biomechanics and hominin anatomy, especially in the upper limb, are required. However, there is an absence of direct experimental means to measure the actual forces generated and applied to produce flakes during knapping. Our study introduces a novel solution to this problem in the form of an ergonomic hand-held synthetic hammerstone that can record the percussive forces that occur during knapping experiments. This hammerstone is composed of a deformable pneumatic 3D-printed chamber encased within a 3D-printed grip and a stone-milled striker. During knapping, hammer impact causes the pneumatic chamber to deform, which leads to a change in pressure that is measured by a sensor. Comparisons of recorded pressure data against corresponding force values measured using a force plate show that the synthetic hammer quantifies percussion forces with relatively high accuracy. The performance of this hammerstone was further validated by conducting anvil-supported knapping experiments on glass that resulted in a root mean square error of under 6%, while recording forces up to 730 N with successful flake detachments. These validation results indicate that accuracy was not sensitive to variations up to 15° from the vertical in the hammer striking angle. Our approach allows future studies to directly examine the role of percussive force during the stone knapping process and its relationship with both anatomical and technological changes during human evolution. [ABSTRACT FROM AUTHOR]

Objectives: Gorillas are diverse behaviorally and ecologically with higher frequencies of arboreality and frugivory observed in populations living at lower altitudes compared to those living at higher altitudes. Previous studies have shown that the more arboreal western lowland gorilla has hallucial tarsometatarsal and talocrural joint morphologies associated with enhanced hallucial abduction and foot inversion capabilities whereas the more terrestrial eastern gorilla does not. Here we examine how known ecological and behavioral differences among gorillas relate to variation in calcaneal and cuboid morphology. Specifically, we test whether gorillas that climb more frequently (i.e., western lowland and lowland grauer gorillas) have cuboid‐metatarsal and calcaneocuboid morphologies that facilitate increased foot mobility compared to those that climb less often (i.e., mountain and highland grauer gorillas). Methods: We quantify the relative areas of articular and nonarticular surfaces, angles between articular surfaces, articular surface curvatures, the relative length and depth of the calcaneocuboid joint, and the relative size of the calcaneal tuber using three‐dimensional models of gorilla calcanei and cuboids (n = 90 and 81, respectively). Results: Our results, based on univariate and multivariate analyses, show that western and eastern gorillas follow our predicted arboreal‐terrestrial morphocline in terms of their calcaneal and cuboid morphology; however, highland and lowland grauer gorilla populations do not. Conclusions: This study has implications for understanding gorilla foot function and the evolutionary processes responsible for producing the morphological patterns among living gorilla populations. Future studies of gorilla morphological diversity may provide a useful comparative model for studies of locomotor evolution in extinct clades. [ABSTRACT FROM AUTHOR]

Objectives: This research examines whether the distribution of trabecular bone in the proximal capitates of extant hominids, as well as several fossil hominin taxa, is associated with the oblique path of the midcarpal joint known as the dart‐thrower's motion (DTM). Materials and Methods: We analyzed proximal capitates from extant (Pongo n = 12; Gorilla n = 11; Pan n = 10; fossil and recent Homo sapiens n = 29) and extinct (Australopithecus sediba n = 2; Homo naledi n = 1; Homo floresiensis n = 2; Neandertals n = 3) hominids using a new canonical holistic morphometric analysis, which quantifies and visualizes the distribution of trabecular bone using relative bone volume as a fraction of total volume (rBV/TV). Results: Homo sapiens and Neandertals had a continuous band of high rBV/TV that extended across the scaphoid, lunate, and hamate subarticular regions, but other fossil hominins and extant great apes did not. A. sediba expressed a distinct combination of human‐like and Pan‐like rBV/TV distribution. Both H. floresiensis and H. naledi had high rBV/TV on the ulnar‐side of the capitate but low rBV/TV on the radial‐side. Conclusion: The proximal capitates of H. sapiens and Neandertals share a distinctive distribution of trabecular bone that suggests that these two species of Homo regularly load(ed) their midcarpal joints along the full extent of the oblique path of the DTM. The observed pattern in A. sediba suggests that human‐like stress at the capito‐scaphoid articular surface was combined with Pan‐like wrist postures, whereas the patterns in H. floresiensis and H. naledi suggest their midcarpal joints were loaded differently from that of H. sapiens and Neandertals. [ABSTRACT FROM AUTHOR]

Objectives: Primates employ wrist ulnar deviation during a variety of locomotor and manipulative behaviors. Extant hominoids share a derived condition in which the ulnar styloid process has limited articulation or is completely separated from the proximal carpals, which is often hypothesized to increase ulnar deviation range of motion. Acute angulation of the hamate's triquetral facet is also hypothesized to facilitate ulnar deviation mobility and mechanics. In this study, we test these longstanding ideas. Methods: Three‐dimensional (3D) carpal kinematics were examined using a cadaveric sample of Pan troglodytes, Pongo sp., and five monkey species. Ulnar styloid projection and orientation of the hamate's triquetral facet were quantified using 3D models. Results: Although carpal rotation patterns in Pan and Pongo were uniquely similar in some respects, P. troglodytes exhibited overall kinematic similarity with large terrestrial cercopithecoids (Papio and Mandrillus). Pongo, Macaca, and Ateles had high wrist ulnar deviation ranges of motion, but Pongo did this via a unique mechanism. In Pongo, the triquetrum functions as a distal carpal rather than part of the proximal row. Ulnar styloid projection and wrist ulnar deviation range of motion were not correlated but ulnar deviation range of motion and the triquetrohamate facet orientation were correlated. Conclusions: Increased ulnar deviation mobility is not the function of ulnar styloid withdrawal in hominoids. Instead, this feature probably reduces stress on the ulnar side wrist or is a byproduct of adaptations that increase supination. Orientation of the hamate's triquetral facet offers some potential to reconstruct ulnar deviation mobility in extinct primates. [ABSTRACT FROM AUTHOR]

Objectives: Bonobos and chimpanzees do not differ from one another in overall frequencies of arboreality versus terrestriality as much as once thought. Thus, at a broad level, one would predict that there is little difference in foot morphology among Pan taxa. However, behavioral data suggest that bonobos more often use smaller diameter substrates (<10 cm) when climbing whereas western chimpanzees frequently climb larger diameter (>15 cm) substrates. This study tests the hypothesis that if Pan medial cuneiform and talus morphology reflects these substrate preferences, then the morphology of these bones should favor hallucial grasping in bonobos and an inverted foot set in western chimpanzees. Materials and Methods: Three‐dimensional geometric morphometric (3DGM) methods were used to explore shape variation in 126 talus and 127 medial cuneiform 3D surface models acquired from 108 chimpanzees (24 western, four Nigeria‐Cameroon, 33 central, 32 eastern, and 15 captive unknowns) and 22 bonobos Results: The shapes of the talus and medial cuneiform in Pan covary as a functional unit emphasizing hallucial grasping with a less inverted foot set in bonobos and a more inverted foot set with a less abducted hallucial set in western chimpanzees. Other chimpanzee subspecies fall between these two extremes. Discussion: Bonobo and western chimpanzee medial cuneiform and talus shapes are consistent with their differing preferences for using smaller and larger diameter substrates, respectively, when vertically climbing. These results suggest that even among closely related taxa, foot, hand, and other postcranial anatomy may be fine‐tuned for specific locomotor behaviors or preferences. [ABSTRACT FROM AUTHOR]