Bone density spatial patterns in the distal radius reflect habitual hand postures adopted by quadrupedal primates

Comparison of bone density patterns of the subaxial spine between chimpanzees and gorillas - A case study

Case study on the bone density pattern of subaxial vertebral column in African apes.

INTRODUCTION

African apes have been noted to experience fewer back ailments than humans and to have higher vertebral bone density. Yet, research on the subject is quite limited and has usually included only one or few vertebrae. However, to understand vertebral column as whole and how posture and locomotion might have affected it, we need to know how bone density varies between adjacent vertebrae.

MATERIALS AND METHODS

Bone density in the vertebral body was measured for all subaxial vertebrae of five specimens including two Pan troglodytes (1 male and 1 female) and three Gorilla gorilla (2 males and 1 female) using peripheral quantitative computed tomography (pQCT).

RESULTS

The results tentatively indicated differences between species, especially in the trabecular density of the cervical segment and support the need for further studies on this subject.

Cetaceans Humerus Radiodensity by CT: A Useful Technique Differentiating between Species, Ecophysiology, and Age

Cetaceans are mammals that underwent a series of evolutionary adaptations to live in the aquatic environment, including morphological modifications of various anatomical structures of the skeleton and their bone mineral density (BMD); there are few studies on the latter. BMD is related to the radiodensity measured through computed tomography (CT) in Hounsfield units (HU). This work aimed to test and validate the usefulness of studying humeral bone radiodensity by CT of two cetacean species (the Atlantic spotted dolphin and the pygmy sperm whale) with different swimming and diving habits. The radiodensity was analysed at certain levels following a new protocol based on a review of previous studies. Humeral radiodensity values were related to four aspects: species, diving behaviour, swimming activity level, and age. We observed that the consistent differences in the radiodensity of the cortical bone of the distal epiphysis between animals of different life-history categories suggest that this bone portion could be particularly useful for future ontogenetic studies. Hence, this technique may be helpful in studying and comparing species with different ecophysiologies, particularly distinguishing between swimming and diving habits.

The forearm and hand musculature of semi-terrestrial rhesus macaques (Macaca mulatta) and arboreal gibbons (fam.Hylobatidae). Part II. Quantitative analysis

Nonhuman primates have a highly diverse locomotor repertoire defined by an equally diverse hand use. Based on how primates use their hands during locomotion, we can distinguish between terrestrial and arboreal taxa. The 'arboreal' hand is likely adapted towards high wrist mobility and grasping, whereas the 'terrestrial' hand will show adaptations to loading. While the morphology of the forearm and hand bones have been studied extensively, functional adaptations in the forearm and hand musculature to locomotor behaviour have been documented only scarcely. In this paper, we investigate the forelimb musculature of the highly arboreal gibbons (including Hylobates lar,Hylobates pileatus,Nomascus leucogenys,Nomascus concolor and Symphalangus syndactylus) and compare this with the musculature of the semi-terrestrial rhesus macaques (Macaca mulatta). Anatomical data from previous dissections on knuckle-walking bonobos (Pan paniscus) and bipedal humans (Homo sapiens) are also included to further integrate the analyses in the scope of catarrhine hand adaptation. This study indicates that the overall configuration of the arm and hand musculature of these primates is very similar but there are some apparent differences in relative size which can be linked to differences in forelimb function and which might be related to their specific locomotor behaviour. In macaques, there is a large development of wrist deviators, wrist and digital flexors, and m. triceps brachii, as these muscles are important during the different phases of palmi- and digitigrade quadrupedal walking to stabilize the wrist and elbow. In addition, their m. flexor carpi ulnaris is the most important contributor to the total force-generating capacity of the wrist flexors and deviators, and is needed to counteract the adducting torque at the elbow joint during quadrupedal walking. Gibbons show a relatively high force-generating capacity in their forearm rotators, wrist and digital flexors, which are important muscles in brachiation to actively regulate forward movement of the body. The results also stress the importance of the digital flexors in bonobos, during climbing and clambering, and in humans, which is likely linked to our advanced manipulation skills.

Determination of the Optimal Location for Bone Graft Harvest in the Distal Radius

Background: The distal radius is commonly used as a bone graft donor site for surgery in the hand and wrist. The aim of this study was to evaluate the volume and relative density of cancellous bone in the distal radius. Methods: Thirty-four consecutive computed tomographic scans of the wrist in 33 patients without distal radius pathology were included. For each subject, 6 spherical regions of interest (ROIs) were identified within the distal radius. In each ROI, volumetric measurements and mean Hounsfield unit (HFU) values were recorded by 2 observers using a 3-dimensional imaging reconstruction software. Results: Compared with proximal bone, distal bone had larger volume (0.82 vs 0.27 cm³) and greater relative density (178 vs 152 HFU) on average. Among the 6 ROIs, the distal-central region had the largest average volume (1.20 cm³) and the distal-ulnar ROI had the greatest average relative density (193 HFU). Conclusion: Based on these results, we recommend performing cancellous autograft harvest relatively distal and ulnar within the distal radius.

The biomechanics of knuckle-walking: 3-D kinematics of the chimpanzee and macaque wrist, hand and fingers

The origin and evolution of knuckle-walking has long been a key focus in understanding African ape, including human, origins. Yet, despite numerous studies documenting morphological characteristics potentially associated with knuckle-walking, little quantitative three-dimensional (3-D) data exist of forelimb motion during knuckle-walking. Nor do any comparative 3-D data exist for hand postures used during quadrupedalism in monkeys. This lack of data has limited the testability of proposed adaptations for knuckle-walking in African apes. This study presents the first 3-D kinematic data of the wrist, hand and metacarpophalangeal joints during knuckle-walking in chimpanzees and in macaques using digitigrade and palmigrade hand postures. These results clarify the unique characteristics of, and commonalities between, knuckle-walking and digitigrady/palmigrady in multiple planes of motion. Notably, chimpanzees utilized more wrist ulnar deviation than any macaque hand posture. Maximum extension of the chimpanzee wrist was slight (5-20 deg) and generally overlapped with macaque digitigrady. Metacarpophalangeal joint motion displayed distinct differences between digits in both species, likely related to the timing of force application. These data also reveal that maximum metacarpophalangeal extension angles during knuckle-walking (26-59 deg) were generally higher than previously considered. In macaques, maximum metacarpophalangeal extension during digitigrady and palmigrady overlapped for most digits, highlighting additional complexity in the interpretation of skeletal features that may be related to limiting metacarpophalangeal motion. Most importantly, however, these new 3-D data serve as a fundamental dataset with which evaluation of proposed musculoskeletal adaptations for knuckle-walking can be tested.

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.

Unexpected terrestrial hand posture diversity in wild mountain gorillas

OBJECTIVES

Gorillas, along with chimpanzees and bonobos, are ubiquitously described as 'knuckle-walkers.' Consequently, knuckle-walking (KW) has been featured pre-eminently in hypotheses of the pre-bipedal locomotor behavior of hominins and in the evolution of locomotor behavior in apes. However, anecdotal and behavioral accounts suggest that mountain gorillas may utilize a more complex repertoire of hand postures, which could alter current interpretations of African ape locomotion and its role in the emergence of human bipedalism. Here we documented hand postures during terrestrial locomotion in wild mountain gorillas to investigate the frequency with which KW and other hand postures are utilized in the wild.

MATERIALS AND METHODS

Multiple high-speed cameras were used to record bouts of terrestrial locomotion of 77 habituated mountain gorillas at Bwindi Impenetrable National Park (Uganda) and Volcanoes National Park (Rwanda).

RESULTS

We captured high-speed video of hand contacts in 8% of the world's population of mountain gorillas. Our results reveal that nearly 40% of these gorillas used "non-KW" hand postures, and these hand postures constituted 15% of all hand contacts. Some of these "non-KW" hand postures have never been documented in gorillas, yet match hand postures previously identified in orangutans.

DISCUSSION

These results highlight a previously unrecognized level of hand postural diversity in gorillas, and perhaps great apes generally. Although present at lower frequencies than KW, we suggest that the possession of multiple, versatile hand postures present in wild mountain gorillas may represent a shared feature of the African ape and human clade (or even great ape clade) rather than KW per se.

Subchondral Bone Radiodensity Patterns in the Glenoid Fossa of Ape and Human Scapulae

Regions of denser subchondral bone deep to a joint's articular surface indicate locations where the joint experiences relatively higher or more frequent compressive trans-articular forces than less dense regions. Human clinically focused studies have hypothesized that regional variation of acquired with computed tomography osteoabsorptiomety (CT-OAM), in the scapular glenoid fossa (GF) is specifically related to forces arising from everyday rotator cuff muscle function. We test this hypothesis by investigating the relationship between rotator cuff function and GF HiRD subchondral bone patterns in a broader comparative context. CT-OAM was used on scapulae of chimpanzees, gibbons and humans to visualize HiRD subchondral bone patterns and assess regional (anterior-posterior; superior-inferior) differences in HiRD concentrations within each group. Like patterns observed in humans, ape GFs show HiRD concentrations in anterior, posterior and superior regions. Gibbons exhibit significantly larger concentrations anteriorly, probably serving as a skeletal correlate of increased subscapularis activity during humeral internal rotation during arm-swinging locomotion. Chimpanzees exhibit relatively larger areas posteriorly (though not statistically significant), conceivably serving as a correlate of increased infraspinatus activity during humeral external rotation and retraction during knuckle-walking. All groups show relatively larger HiRD areas superiorly, likely correlating with forceful humeral abduction (rather than adduction) during routine upper limb use across behaviors. Subchondral bone HiRD patterns in the GF appear to correspond with normal and unbalanced rotator cuff activity and force production not only in humans, but also in other primates, thereby corroborating their value in human clinical studies and functional morphology research. Anat Rec, 301:776-785, 2018. © 2017 Wiley Periodicals, Inc.

Kinematics of the anthropoid os centrale and the functional consequences of scaphoid-centrale fusion in African apes and hominins

In most primates, the os centrale is interposed between the scaphoid, trapezoid, trapezium, and head of the capitate, thus constituting a component of the wrist's midcarpal complex. Scaphoid-centrale fusion is among the clearest morphological synapomorphies of African apes and hominins. Although it might facilitate knuckle-walking by increasing the rigidity and stability of the radial side of the wrist, the exact functional significance of scaphoid-centrale fusion is unclear. If fusion acts to produce a more rigid radial wrist that stabilizes the hand and limits shearing stresses, then in taxa with a free centrale, it should anchor ligaments that check extension and radial deviation, but exhibit motion independent of the scaphoid. Moreover, because the centrale sits between the scaphoid and capitate (a major stabilizing articulation), scaphoid-centrale mobility should correlate with scaphocapitate mobility in extension and radial deviation. To test these hypotheses, the centrale's ligamentous binding was investigated via dissection in Pongo and Papio, and the kinematics of the centrale were quantified in a cadaveric sample of anthropoids (Pongo sp., Ateles geoffroyi, Colobus guereza, Macaca mulatta, and Papio anubis) using a computed-tomography-based method to track wrist-bone motion. Results indicate that the centrale rotates freely relative to the scaphoid in all taxa. However, centrale mobility is only correlated with scaphocapitate mobility during extension in Pongo-possibly due to differences in overall wrist configuration between apes and monkeys. If an extant ape-like wrist characterized early ancestors of African apes and hominins, then scaphoid-centrale fusion would have increased midcarpal rigidity in extension relative to the primitive condition. Although biomechanically consistent with a knuckle-walking hominin ancestor, this assumes that the trait evolved specifically for that biological role, which must be squared with contradictory interpretations of extant and fossil hominoid morphology. Regardless of its original adaptive significance, scaphoid-centrale fusion likely presented a constraint on early hominin midcarpal mobility.

Trabecular and cortical bone structure of the talus and distal tibia in Pan and Homo

OBJECTIVES

Internal bone structure, both cortical and trabecular bone, remodels in response to loading and may provide important information regarding behavior. The foot is well suited to analysis of internal bone structure because it experiences the initial substrate reaction forces, due to its proximity to the substrate. Moreover, as humans and apes differ in loading of the foot, this region is relevant to questions concerning arboreal locomotion and bipedality in the hominoid fossil record.

MATERIALS AND METHODS

We apply a whole-bone/epiphysis approach to analyze trabecular and cortical bone in the distal tibia and talus of Pan troglodytes and Homo sapiens. We quantify bone volume fraction (BV/TV), degree of anisotropy (DA), trabecular thickness (Tb.Th), bone surface to volume ratio (BS/BV), and cortical thickness and investigate the distribution of BV/TV and cortical thickness throughout the bone/epiphysis.

RESULTS

We find that Pan has a greater BV/TV, a lower BS/BV and thicker cortices than Homo in both the talus and distal tibia. The trabecular structure of the talus is more divergent than the tibia, having thicker, less uniformly aligned trabeculae in Pan compared to Homo. Differences in dorsiflexion at the talocrural joint and in degree of mobility at the talonavicular joint are reflected in the distribution of cortical and trabecular bone.

DISCUSSION

Overall, quantified trabecular parameters represent overall differences in bone strength between the two species, however, DA may be directly related to joint loading. Cortical and trabecular bone distributions correlate with habitual joint positions adopted by each species, and thus have potential for interpreting joint position in fossil hominoids.

Trabecular bone structural variation throughout the human lower limb

Trabecular bone is responsive to mechanical loading, and thus may be a useful tool for interpreting past behaviour from fossil morphology. However, the ability to meaningfully interpret variation in archaeological and hominin trabecular morphology depends on the extent to which trabecular bone properties are integrated throughout the postcranium or are locally variable in response to joint specific loading. We investigate both of these factors by comparing trabecular bone throughout the lower limb between a group of highly mobile foragers and two groups of sedentary agriculturalists. Trabecular bone structure is quantified in four volumes of interest placed within the proximal and distal joints of the femur and tibia. We determine how trabecular structures correspond to inferred behavioural differences between populations and whether the patterns are consistent throughout the limb. A significant correlation was found between inferred mobility level and trabecular bone structure in all volumes of interest along the lower limb. The greater terrestrial mobility of foragers is associated with higher bone volume fraction, and thicker and fewer trabeculae (lower connectivity density). In all populations, bone volume fraction decreases while anisotropy increases proximodistally throughout the lower limb. This observation mirrors reductions in cortical bone mass resulting from proximodistal limb tapering. The reduction in strength associated with reduced bone volume fraction may be compensated for by the increased anisotropy in the distal tibia. A similar pattern of trabecular structure is found throughout the lower limb in all populations, upon which a signal of terrestrial mobility appears to be superimposed. These results support the validity of using lower limb trabecular bone microstructure to reconstruct terrestrial mobility levels from the archaeological and fossil records. The results further indicate that care should be taken to appreciate variation resulting from differences in habitual activity when inferring behaviour from the trabecular structure of hominin fossils through comparisons with modern humans.

Trabecular architecture of the manual elements reflects locomotor patterns in primates

The morphology of trabecular bone has proven sensitive to loading patterns in the long bones and metacarpal heads of primates. It is expected that we should also see differences in the manual digits of primates that practice different methods of locomotion. Primate proximal and middle phalanges are load-bearing elements that are held in different postures and experience different mechanical strains during suspension, quadrupedalism, and knuckle walking. Micro CT scans of the middle phalanx, proximal phalanx and the metacarpal head of the third ray were used to examine the pattern of trabecular orientation in Pan, Gorilla, Pongo, Hylobates and Macaca. Several zones, i.e., the proximal ends of both phalanges and the metacarpal heads, were capable of distinguishing between knuckle-walking, quadrupedal, and suspensory primates. Orientation and shape seem to be the primary distinguishing factors but differences in bone volume, isotropy index, and degree of anisotropy were seen across included taxa. Suspensory primates show primarily proximodistal alignment in all zones, and quadrupeds more palmar-dorsal orientation in several zones. Knuckle walkers are characterized by having proximodistal alignment in the proximal ends of the phalanges and a palmar-dorsal alignment in the distal ends and metacarpal heads. These structural differences may be used to infer locmotor propensities of extinct primate taxa.

Joint loads in marsupial ankles reflect habitual bipedalism versus quadrupedalism

Joint surfaces of limb bones are loaded in compression by reaction forces generated from body weight and musculotendon complexes bridging them. In general, joints of eutherian mammals have regions of high radiodensity subchondral bone that are better at resisting compressive forces than low radiodensity subchondral bone. Identifying similar form-function relationships between subchondral radiodensity distribution and joint load distribution within the marsupial postcranium, in addition to providing a richer understanding of marsupial functional morphology, can serve as a phylogenetic control in evaluating analogous relationships within eutherian mammals. Where commonalities are established across phylogenetic borders, unifying principles in mammalian physiology, morphology, and behavior can be identified. Here, we assess subchondral radiodensity patterns in distal tibiae of several marsupial taxa characterized by different habitual activities (e.g., locomotion). Computed tomography scanning, maximum intensity projection maps, and pixel counting were used to quantify radiodensity in 41 distal tibiae of bipedal (5 species), arboreal quadrupedal (4 species), and terrestrial quadrupedal (5 species) marsupials. Bipeds (Macropus and Wallabia) exhibit more expansive areas of high radiodensity in the distal tibia than arboreal (Dendrolagus, Phascolarctos, and Trichosurus) or terrestrial quadrupeds (Sarcophilus, Thylacinus, Lasiorhinus, and Vombatus), which may reflect the former carrying body weight only through the hind limbs. Arboreal quadrupeds exhibit smallest areas of high radiodensity, though they differ non-significantly from terrestrial quadrupeds. This could indicate slightly more compliant gaits by arboreal quadrupeds compared to terrestrial quadrupeds. The observed radiodensity patterns in marsupial tibiae, though their statistical differences disappear when controlling for phylogeny, corroborate previously documented patterns in primates and xenarthrans, potentially reflecting inferred limb use during habitual activities such as locomotion. Despite the complex nature of factors contributing to joint loads, broad observance of these patterns across joints and across a variety of taxa suggests that subchondral radiodensity can be used as a unifying form-function principle within Mammalia.

Carpal kinematics in quadrupedal monkeys: towards a better understanding of wrist morphology and function

The purpose of this study is to provide new data on carpal kinematics in primates in order to deepen our understanding of the relationships between wrist morphology and function. To that end, we provide preliminary data on carpal kinematics in seven species of quadrupedal monkeys that have not been previously investigated in this regard (cercopithecoids, n = 4; ceboids, n = 3). We radiographed wrists from cadavers at their maximum radial and ulnar deviations, as well as at maximum flexion and extension. We took angular measurements to quantify the contribution of the mobility of the two main wrist joints (antebrachiocarpal and midcarpal) with respect to total wrist mobility. We also recorded qualitative observations. Our quantitative results show few clear differences among quadrupedal monkeys for radioulnar deviation and flexion-extension: all the primates studied exhibit a greater midcarpal mobility (approximately 54-83% of the total range of motion) than antebrachiocarpal mobility; however, we identified two patterns of carpal kinematics that show the functional impact of previously recognised morphological variations in quadrupedal monkeys. Firstly, qualitative results show that the partition that divides the proximal joint of the wrist in ceboids results in less mobility and more stability of the ulnar part of the wrist than is seen in cercopithecoids. Secondly, we show that the olive baboon specimen (Papio anubis) is characterised by limited antebrachiocarpal mobility for extension; this effect is likely the result of a radial process that projects on the scaphoid notch, as well as an intraarticular meniscus. Because of these close relationships between carpal kinematics and morphology in quadrupedal monkeys, we hypothesise that, to some extent, these functional tendencies are related to their locomotor hand postures.

Metacarpal head biomechanics: a comparative backscattered electron image analysis of trabecular bone mineral density in Pan troglodytes, Pongo pygmaeus, and Homo sapiens

Great apes and humans use their hands in fundamentally different ways, but little is known about joint biomechanics and internal bone variation. This study examines the distribution of mineral density in the third metacarpal heads in three hominoid species that differ in their habitual joint postures and loading histories. We test the hypothesis that micro-architectural properties relating to bone mineral density reflect habitual joint use. The third metacarpal heads of Pan troglodytes, Pongo pygmaeus, and Homo sapiens were sectioned in a sagittal plane and imaged using backscattered electron microscopy (BSE-SEM). For each individual, 72 areas of subarticular cortical (subchondral) and trabecular bone were sampled from within 12 consecutive regions of the BSE-SEM images. In each area, gray levels (representing relative mineralization density) were quantified. Results show that chimpanzee, orangutan, and human metacarpal III heads have different gray level distributions. Weighted mean gray levels (WMGLs) in the chimpanzee showed a distinct pattern in which the 'knuckle-walking' regions (dorsal) and 'climbing' regions (palmar) are less mineralized, interpreted to reflect elevated remodeling rates, than the distal regions. Pongo pygmaeus exhibited the lowest WMGLs in the distal region, suggesting elevated remodeling rates in this region, which is loaded during hook grip hand postures associated with suspension and climbing. Differences among regions within metacarpal heads of the chimpanzee and orangutan specimens are significant (Kruskal-Wallis, p < 0.001). In humans, whose hands are used for manipulation as opposed to locomotion, mineralization density is much more uniform throughout the metacarpal head. WMGLs were significantly (p < 0.05) lower in subchondral compared to trabecular regions in all samples except humans. This micro-architectural approach offers a means of investigating joint loading patterns in primates and shows significant differences in metacarpal joint biomechanics among great apes and humans.

Metacarpal trabecular architecture variation in the chimpanzee (Pan troglodytes): Evidence for locomotion and tool-use?

Trabecular architecture was assessed by 3D micro-computed tomography from spherical volumes of interest located within the head and base of metacarpals (MC) 1, 2, and 5 from n = 19 adult common chimpanzees. Two subspecies, West African Pan troglodytes verus from the Taï Forest, Côte d'Ivoire (n = 12) and Central African P. t. troglodytes from Cameroon (n = 7), were studied. For the combined sample, the metacarpal head is distinguished by greater bone volume fraction across all metacarpals, though the MC 1 is distinctive in having thicker, more plate-like trabeculae. The architecture in the MC 2 and MC 5 can be related to strains associated with terrestrial knuckle-walking. In particular, the relatively robust MC 5 head architecture may result from functional loading incurred during braking and use of a palm-in hand posture. Examining differences between samples, we found that the Cameroon chimpanzees possess a more robust architecture across all metacarpals in the form of greater bone volume fraction, higher connectivity, and somewhat more plate-like structure. These differences are not explicable in terms of population distinctions in body size or daily travel distance, but possibly reflect a combination of more terrestrial knuckle-walking in the Cameroon sample and more diverse hand postures and precision handling required of nut-cracking in West African chimpanzees.

Trabecular bone structure in the humeral and femoral heads of anthropoid primates

The functional significance of three-dimensional trabecular bone architecture in the primate postcranial skeleton has received significant interest over the last decade. Some previous work has produced promising results, finding significant relationships between femoral head trabecular bone structure and hypothesized locomotor loading in leaping and nonleaping strepsirrhines. Conversely, most studies of anthropoid femoral head bone structure have found broad similarity across taxonomic and locomotor groups. The goal of this study is to expand on past analyses of anthropoid trabecular bone structure by assessing the effects of differential limb usage on the trabecular bone architecture of the forelimb and hindlimb across taxa characterized by diverse locomotor behaviors, including brachiation, quadrupedalism, and climbing. High-resolution x-ray computed tomography scans were collected from the proximal humerus and proximal femur of 55 individuals from five anthropoid primate species, including Symphalangus syndactylus, Papio sp., Presbytis rubicunda, Alouatta caraya, and Pan troglodytes. Trabecular bone structural features including bone volume fraction, anisotropy, trabecular thickness, and trabecular number were quantified in large volumes positioned in the center of the humeral or femoral head. Femoral head trabecular bone volume is consistently and significantly higher than trabecular bone volume in the humerus in all taxa independent of locomotor behavior. Humeral trabecular bone is more isotropic than femoral trabecular bone in all species sampled, possibly reflecting the emphasis on a mobile shoulder joint and manipulative forelimb. The results indicate broad similarity in trabecular bone structure in these bones across anthropoids.

Studying primate carpal kinematics in three dimensions using a computed-tomography-based markerless registration method

The functional morphology of the wrist pertains to a number of important questions in primate evolutionary biology, including that of hominins. Reconstructing locomotor and manipulative capabilities of the wrist in extinct species requires a detailed understanding of wrist biomechanics in extant primates and the relationship between carpal form and function. The kinematics of carpal movement, and the role individual joints play in providing mobility and stability of the wrist, is central to such efforts. However, there have been few detailed biomechanical studies of the nonhuman primate wrist. This is largely because of the complexity of wrist morphology and the considerable technical challenges involved in tracking the movements of the many small bones that compose the carpus. The purpose of this article is to introduce and outline a method adapted from human clinical studies of three-dimensional (3D) carpal kinematics for use in a comparative context. The method employs computed tomography of primate cadaver forelimbs in increments throughout the wrist's range of motion, coupled with markerless registration of 3D polygon models based on inertial properties of each bone. The 3D kinematic principles involved in extracting motion axis parameters that describe bone movement are reviewed. In addition, a set of anatomically based coordinate systems embedded in the radius, capitate, hamate, lunate, and scaphoid is presented for the benefit of other primate functional morphologists interested in studying carpal kinematics. Finally, a brief demonstration of how the application of these methods can elucidate the mechanics of the wrist in primates illustrates the closer-packing of carpals in chimpanzees than in orangutans, which may help to stabilize the midcarpus and produce a more rigid wrist beneficial for efficient hand posturing during knuckle-walking locomotion.

Comparative 3D quantitative analyses of trapeziometacarpal joint surface curvatures among living catarrhines and fossil hominins

Comparisons of joint surface curvature at the base of the thumb have long been made to discern differences among living and fossil primates in functional capabilities of the hand. However, the complex shape of this joint makes it difficult to quantify differences among taxa. The purpose of this study is to determine whether significant differences in curvature exist among selected catarrhine genera and to compare these genera with hominin fossils in trapeziometacarpal curvature. Two 3D approaches are used to quantify curvatures of the trapezial and metacarpal joint surfaces: (1) stereophotogrammetry with nonuniform rational B-spline (NURBS) calculation of joint curvature to compare modern humans with captive chimpanzees and (2) laser scanning with a quadric-based calculation of curvature to compare modern humans and wild-caught Pan, Gorilla, Pongo, and Papio. Both approaches show that Homo has significantly lower curvature of the joint surfaces than does Pan. The second approach shows that Gorilla has significantly more curvature than modern humans, while Pongo overlaps with humans and African apes. The surfaces in Papio are more cylindrical and flatter than in Homo. Australopithecus afarensis resembles African apes more than modern humans in curvatures, whereas the Homo habilis trapezial metacarpal surface is flatter than in all genera except Papio. Neandertals fall at one end of the modern human range of variation, with smaller dorsovolar curvature. Modern human topography appears to be derived relative to great apes and Australopithecus and contributes to the distinctive human morphology that facilitates forceful precision and power gripping, fundamental to human manipulative activities.

Body size and joint posture in primates

Body mass has been shown in experimental and comparative morphological studies to have a significant effect on joint posture in major limb joints. The generalizability of experimental studies is limited by their use of small sample sizes and limited size ranges. In contrast, while comparative morphological studies often have increased sample sizes, the connection between joint posture and morphological variables is often indirect. The current study infers joint postures for a large sample of primates using an experimentally validated method, and tests whether larger primates use more extended joint postures than smaller species. Postures are inferred through the analysis of patterns of subchondral bone apparent density on the medial femoral condyle. Femora from 94 adult wild-shot individuals of 28 species were included. Apparent density measurements were obtained from CT scans using AMIRA software, and the angular position of the anterior-most extent of the region of maximum apparent density on the medial femoral condyle was recorded. In general, the hypothesis that larger-bodied primates use more extended knee posture was supported, but it should be noted that considerable variation exists, particularly at small body sizes. This indicates that smaller species are less constrained by their body size, and their patterns of apparent density are consistent with a wide range of knee postures. The size-related increase in inferred joint posture was observed in most major groups of primates, and this observation attests to the generalizability of Biewener's model that relates body size and joint posture.

Apparent density patterns in subchondral bone of the sloth and anteater forelimb

Vertebrate morphologists often are interested in inferring limb-loading patterns in animals characterized by different locomotor repertoires. Because bone apparent density (i.e. mass per unit volume of bone inclusive of porosities) is a determinant of compressive strength, and thus indicative of compressive loading, recent comparative studies in primates have proposed a structure-function relationship between apparent density of subchondral bone and locomotor behaviours that vary in compressive loading. If such patterns are found in other mammals, then these relationships would be strengthened further. Here, we examine the distal radius of suspensory sloths that generally load their forelimbs (FLs) in tension and of quadrupedal anteaters that generally load their FLs in compression. Computed tomography osteoabsorptiometry was used to visualize the patterns in subchondral apparent density. Suspensory sloths exhibit relatively smaller areas of high apparent density than quadrupedal anteaters. This locomotor-based pattern is analogous to the pattern observed in suspensory and quadrupedal primates. Similarity between xenarthran and primate trends suggests broad-scale applicability for analysing subchondral bone apparent density and supports the idea that bone functionally alters its material properties in response to locomotor behaviours.