Behavioral and phylogenetic correlates of limb length proportions in extant apes and monkeys: Implications for interpreting hominin fossils

The body proportions of extant animals help inform inferences about the behaviors of their extinct relatives, but relationships between body proportions, behavior, and phylogeny in extant primates remain unclear. Advances in behavioral data, molecular phylogenies, and multivariate analytical tools make it an opportune time to perform comprehensive comparative analyses of primate traditional limb length proportions (e.g., intermembral, humerofemoral, brachial, and crural indices), body size-adjusted long bone proportions, and principal components. In this study we used a mix of newly-collected and published data to investigate whether and how the limb length proportions of a diverse sample of primates, including monkeys, apes, and modern humans, are influenced by behavior and phylogeny. We reconfirm that the intermembral index, followed by the first principal component of traditional limb length proportions, is the single most effective variable distinguishing hominoids and other anthropoids. Combined limb length proportions and positional behaviors are strongly correlated in extant anthropoid groups, but phylogeny is a better predictor of limb length proportion variation than of behavior. We confirm convergences between members of the Atelidae and extant apes (especially Pan), members of the Hylobatidae and Pongo, and a potential divergence of Presbytis limb proportions from some other cercopithecoids, which correlate with adaptations for forelimb-dominated behaviors in some colobines. Collectively, these results substantiate hypotheses indicating that extinct hominins and other hominoid taxa can be distinguished by analyzing combinations of their limb length proportions at different taxonomic levels. From these results, we hypothesize that fossil skeletons characterized by notably disparate limb length proportions are unlikely to have exhibited similar behavioral patterns.

Carpal kinematics and morphological correlates of wrist ulnar deviation mobility in nonhuman anthropoid primates

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.

Ulnar shape of extant primates: Functional signals and covariation with triquetrum shape

OBJECTIVES

In this study, we investigated the shape differences of the distal ulna in a phylogenetic context among a broad range of primate taxa. Furthermore, we evaluated covariation between ulnar and triquetrum shape and a possible association between ulnar shape and locomotor behavior.

MATERIALS AND METHODS

We applied 3D geometric morphometrics on a large dataset comprising the distal ulna of 124 anthropoid primate specimens belonging to 12 different genera. For each species, a mean shape was calculated using 11 Procrustes-aligned surface landmarks on the distal ulna. These mean shapes are used in a bgPCA, pPCA, and PACA and 3D morphs were used to visualize more subtle differences between taxa. A p2B-PLS analysis was performed to test the covariance between distal ulnar and triquetrum shape.

RESULTS

The results show that more closely related species exhibit a similar distal ulnar shape. Overall, extant hominid ulnae show a shape shift compared to those of extant monkeys and hylobatids. This includes a shortening of the ulnar styloid process and dorspalmarly widening of the ulnar head, shape characteristics that are independent of phylogeny. Within the hominids, Pongo pygmaeus seem to possess the most plesiomorphic distal ulnar shape, while Gorilla and Homo sapiens display the most derived distal ulna. Cercopithecoids, hylobatids, and P. pygmaeus are characterized by a relatively deep ECU groove, which is a shape trait dependent of phylogeny. Although there was no significant covariation between distal ulnar shape and triquetrum shape, the shape differences of the distal ulna between the different primate taxa reveal a possible link with locomotor behavior.

CONCLUSIONS

The comparative analyses of this study reveal different shape trends in a phylogenetic context. Highly arboreal primates, such as hylobatids and Ateles fusciceps, show a distal ulnar morphology that appears to be adapted to tensile and torsional forces. In primates that use their wrist under more compressive conditions, such as quadrupedal cercopithecoids and great apes, the distal ulnar morphology seems to reflect increased compressive forces. In modern humans, the distal ulnar shape can be associated to enhanced manipulative skills and power grips. There was no significant covariation between distal ulnar shape and triquetrum shape, probably due to the variation in the amount of contact between the triquetrum and ulna. In combination with future research on wrist mobility in diverse primate taxa, the results of this study will allow us to establish form-function relationships of the primate wrist and contribute towards an evidence-based interpretation of fossil remains.

Covariation between wrist bone morphology and maximal range of motion during ulnar deviation and supination in extant nonhuman primate taxa

This study investigates the maximal range of motion (ROM) during wrist deviation and forearm rotation for five different primate genera and the possible correlation with the shape of the distal ulna, triquetrum and hamate. A two-block phylogenetic partial least square analysis was performed to test this covariation in a phylogenetic context, using shape coordinates and a matrix of maximal ROM data as input data. The results show that gibbons have the highest ROM for both ulnar deviation and supination, whereas Macaca exhibited the lowest ROM for supination, and Pan had the lowest ROM for ulnar deviation. These results can be attributed to differences in locomotor behaviour, as gibbons need a large wrist mobility in all directions for their highly arboreal lifestyle, whereas Macaca and Pan need a stable wrist during terrestrial locomotion. However, we found no correlation between distal ulna/triquetrum/hamate shape and maximal ROM during ulnar deviation and supination in the different primate taxa. A larger dataset, in combination with behavioural and biomechanical studies, is needed to establish form-function relationships of the primate hand, which will aid the functional interpretation of primate fossil remains.

Functional signals and covariation in triquetrum and hamate shape of extant primates using 3D geometric morphometrics

In this study, we want to investigate the covariation in the shape of two carpal bones, the triquetrum and hamate, and the possible association with locomotor behavior in a broad range of primate taxa. We applied 3D Geometric Morphometrics on a large data set comprising 309 anthropoid primates of 12 different genera. Principal component analyses were performed on the covariance matrix of 18 (triquetrum) and 23 (hamate) Procrustes-aligned surface landmarks. A two-block partial least square analysis was done to test the covariance between triquetrum and hamate shape, without relying on the predictive models implicit in regression analyses. The results show that the carpal shape of quadrupedal anthropoids, which mainly use their wrist under compressive conditions, differs from that of suspensory primates as their wrist is possibly subjected to tensile and torsional forces. Within the hominids, differences in shape also distinguish more terrestrial from more arboreal species. Even within the great apes, we are able to capture shape differences between species of the same genus. In combination with behavioral and biomechanical studies, the results of this research can be used to establish form-function relationships of the primate hand which will aid the functional interpretation of primate fossil remains.

Locomotor correlates of the scapholunar of living and extinct carnivorans

The relationship of carpal morphology to ecology and habitat is under studied in carnivorans and more generally in mammals. Here, we use 3D-scanning techniques to assess the usefulness of a carpal bone, the scapholunar, in carnivorans to reflect ecology and habitat, and to reconstruct the ecology of five extinct carnivorans from two fossil sites: Rancho La Brea and Natural Trap Cave. We 3D-scanned scapholunars and measured articular surface areas and angles between articular facets using GeoMagic and Rhino 3D-software. We analyzed the difference in these metrics using multivariate analysis of variance and discriminant function analysis. Results show that the scapholunar reflects ecological signal, with clear groupings of cursorial carnivorans and grappling/climbing carnivorans; however, phylogenetic signal was also present in the results with hyaenids, canids, and large felids in distinct morphospaces. Extinct species Miracinonyx trumani (American cheetah) and Smilodon fatalis (sabertooth cat) showed surprising results with M. trumani grouping with pantherines instead of Acinonyx or Puma, suggesting it runs but still retains the ability to grapple prey. S. fatalis groups with pantherines, but also shows some unique adaptations, suggesting it had a different range of wrist motion than living cats. Overall, the scapholunar is a good indicator of ecology and functional morphology and can be another tool to use in modern and fossil carnivorans to reconstruct extinct ecologies and locomotor behaviors.

Phylogenetic Perspectives on Catarrhine Talo-Crural Joint Phenotypic Plasticity

Previous investigations of the primate talo-crural joint (TCJ; specifically on the talus and distal tibia) have demonstrated that substrate preference significantly influences morphology, but this association is not necessarily found in subadults. This has been interpreted as the result of a plastic, behaviorally induced response of bone due to substrate use. In this investigation, we use geometric morphometric and phylogenetic comparative methods to investigate ontogenetic phenotypic plasticity in the catarrhine TCJ. Osteological specimens from four African hominoid and four cercopithecid species, divided into subadult and adult developmental stages based on molar eruption, formed the study group. We tested for phylogenetic signal in the shape of both the talar and tibial articular surfaces, at both developmental stages. We then used phylomorphospaces to examine the evolution of shape differences at each developmental stage for each element, and to determine if substrate usage is associated with shape in this phylogenetic context. A significant phylogenetic signal was found for both articular surfaces in subadults, but not adults. In phylomorphospace, both talar and tibial articular morphologies show an association with substrate preference in adults, but not in subadults. Our results provide confirmation of the significant effect of habitual substrate usage and the consequences of bone remodeling during ontogeny on the shape and presentation of the TCJ. These results also suggest caution when using adult talo-tibial shapes to evaluate phylogenetic relationships as TCJ morphology can be considered as a palimpsest, with substrate usage overwriting phylogenetic information in adult specimens. Anat Rec, 302:1977-1984, 2019. © 2019 American Association for Anatomy.

Evaluation of Articular Surface Similarity of Hemi-Hamate Grafts and Proximal Middle Phalanx Morphology: A 3D Geometric Morphometric Approach

PURPOSE

Hemi-hamate arthroplasty has been described as a viable treatment option for unstable proximal interphalangeal joint fracture-dislocations. The procedure uses a dorsal distal hamate osteochondral graft to recreate the injured volar middle phalanx (MP) proximal base. The purpose of this study was to evaluate the similarity in shape of these articular surfaces using quantitative 3-dimensional methods.

METHODS

Three-dimensional virtual renderings were created from laser scans of the articular surfaces of the dorsal distal hamate and the volar MP bases of the index, middle, ring, and little fingers from cadaveric hands of 25 individuals. Three-dimensional landmarks were obtained from the articular surfaces of each bone and subjected to established geometric morphometric analytical approaches to quantify shape. For each individual, bone shapes were evaluated for covariation using 2-block partial least-squares and principal component analyses.

RESULTS

No statistically significant covariation was found between the dorsal distal hamate and volar MP bases of the middle, ring, or little digits. Whereas the volar MP bases demonstrated relative morphologic uniformity among the 4 digits both within and between individuals, the dorsal distal hamates exhibited notable variation in articular surface morphology.

CONCLUSIONS

Despite the early to midterm clinical success of hemi-hamate arthroplasty, there is no statistically significant, uniform similarity in shape between the articular surfaces of the dorsal distal hamate and the volar MP base. In addition, there is wide variation in the articular morphology of the hamate among individuals.

CLINICAL RELEVANCE

The lack of uniform similarity in shape between the dorsal distal hamate and the volar MP base may result in unpredictable outcomes in HHA. It is recommended that the variation in hamate morphology be considered while reconstructing the injured volar MP base in the procedure.

Does the shape of forelimb long bones co-vary with grasping behaviour in strepsirrhine primates?

Fine prehensile activities are often thought to have been associated with the evolution of the human hand. However, there has been no holistic approach establishing the link between the morphology of the forelimb and grasping ability in living primates. The present study investigated the possible relationships between grasping behaviour and the morphology of the forelimb in strepsirrhines in a phylogenetic context. To do so, grasping behaviour during feeding and the shape of the long bones of the forelimb were analysed for 22 species of strepsirrhines. The data obtained show that there is a phylogenetic signal in forelimb morphology in primates in relation to grasping behaviour, but also that there is a marked co-evolution between grasping behaviour and the shape of the humerus and radius. This latter finding suggests a functional association between grasping and forelimb shape, which in turn suggests that bone shape constrains or facilitates behaviour. This result may permit future inferences to be made regarding this behaviour in extinct species and deserves further examination in more detail.

Trabecular bone patterning across the human hand

Hand bone morphology is regularly used to link particular hominin species with behaviors relevant to cognitive/technological progress. Debates about the functional significance of differing hominin hand bone morphologies tend to rely on establishing phylogenetic relationships and/or inferring behavior from epigenetic variation arising from mechanical loading and adaptive bone modeling. Most research focuses on variation in cortical bone structure, but additional information about hand function may be provided through the analysis of internal trabecular structure. While primate hand bone trabecular structure is known to vary in ways that are consistent with expected joint loading differences during manipulation and locomotion, no study exists that has documented this variation across the numerous bones of the hand. We quantify the trabecular structure in 22 bones of the human hand (early/extant modern Homo sapiens) and compare structural variation between two groups associated with post-agricultural/industrial (post-Neolithic) and foraging/hunter-gatherer (forager) subsistence strategies. We (1) establish trabecular bone volume fraction (BV/TV), modulus (E), degree of anisotropy (DA), mean trabecular thickness (Tb.Th) and spacing (Tb.Sp); (2) visualize the average distribution of site-specific BV/TV for each bone; and (3) examine if the variation in trabecular structure is consistent with expected joint loading differences among the regions of the hand and between the groups. Results indicate similar distributions of trabecular bone in both groups, with those of the forager sample presenting higher BV/TV, E, and lower DA, suggesting greater and more variable loading during manipulation. We find indications of higher loading along the ulnar side of the forager sample hand, with high site-specific BV/TV distributions among the carpals that are suggestive of high loading while the wrist moves through the 'dart-thrower's' motion. These results support the use of trabecular structure to infer behavior and have direct implications for refining our understanding of human hand evolution and fossil hominin hand use.

Evaluating causes of error in landmark-based data collection using scanners

In this study, we assess the precision, accuracy, and repeatability of craniodental landmarks (Types I, II, and III, plus curves of semilandmarks) on a single macaque cranium digitally reconstructed with three different surface scanners and a microCT scanner. Nine researchers with varying degrees of osteological and geometric morphometric knowledge landmarked ten iterations of each scan (40 total) to test the effects of scan quality, researcher experience, and landmark type on levels of intra- and interobserver error. Two researchers additionally landmarked ten specimens from seven different macaque species using the same landmark protocol to test the effects of the previously listed variables relative to species-level morphological differences (i.e., observer variance versus real biological variance). Error rates within and among researchers by scan type were calculated to determine whether or not data collected by different individuals or on different digitally rendered crania are consistent enough to be used in a single dataset. Results indicate that scan type does not impact rate of intra- or interobserver error. Interobserver error is far greater than intraobserver error among all individuals, and is similar in variance to that found among different macaque species. Additionally, experience with osteology and morphometrics both positively contribute to precision in multiple landmarking sessions, even where less experienced researchers have been trained in point acquisition. Individual training increases precision (although not necessarily accuracy), and is highly recommended in any situation where multiple researchers will be collecting data for a single project.

Form and function of the human and chimpanzee forefoot: implications for early hominin bipedalism

During bipedal walking, modern humans dorsiflex their forefoot at the metatarsophalangeal joints (MTPJs) prior to push off, which tightens the plantar soft tissues to convert the foot into a stiff propulsive lever. Particular features of metatarsal head morphology such as “dorsal doming” are thought to facilitate this stiffening mechanism. In contrast, chimpanzees are believed to possess MTPJ morphology that precludes high dorsiflexion excursions during terrestrial locomotion. The morphological affinity of the metatarsal heads has been used to reconstruct locomotor behavior in fossil hominins, but few studies have provided detailed empirical data to validate the assumed link between morphology and function at the MTPJs. Using three-dimensional kinematic and morphometric analyses, we show that humans push off with greater peak dorsiflexion angles at all MTPJs than do chimpanzees during bipedal and quadrupedal walking, with the greatest disparity occurring at MTPJ 1. Among MTPJs 2–5, both species exhibit decreasing peak angles from medial to lateral. This kinematic pattern is mirrored in the morphometric analyses of metatarsal head shape. Analyses of Australopithecus afarensis metatarsals reveal morphology intermediate between humans and chimpanzees, suggesting that this species used different bipedal push-off kinematics than modern humans, perhaps resulting in a less efficient form of bipedalism.