Patterns of strain in the macaque tibia during functional activity

Dynamic pressure patterns in the hands of olive baboons (Papio anubis) during terrestrial locomotion: implications for cercopithecoid primate hand morphology

Habitually terrestrial monkeys adopt digitigrade hand postures at slow speeds to increase effective forelimb length and reduce distal limb joint moments. As these primates move faster, however, their hands transition to a more palmigrade posture, which is likely associated with the inability of wrist and hand joints to resist higher ground reaction forces (GRF) associated with faster speeds. Transitioning to a palmigrade posture may serve to distribute GRFs over a larger surface area (i.e., increased palmar contact), ultimately reducing stresses in fragile hand bones. To test this hypothesis, dynamic palmar pressure data were collected on two adult baboons (Papio anubis) walking, running, and galloping across a runway integrated with a dynamic pressure mat (20 steps each; speed range: 0.46-4.0 m/s). Peak GRF, contact area, peak pressure, and pressure-time integral were quantified in two regions of the hand: fingers and palms (including metacarpal heads). At slower speeds with lower GRFs, the baboons use digitigrade postures resulting in small palmar contact area (largely across the metacarpal heads). At faster speeds with higher GRFs, they used less digitigrade hand postures resulting in increased palmar contact area. Finger contact area did not change across speeds. Despite higher GRFs at faster speeds, metacarpal pressure was moderated across speeds due to increased palmar contact area as animals transitioned from digitigrady to palmigrady. In contrast, the pressure in the fingers increased with faster speeds. Results indicate that the transition from digitigrady to palmigrady distributes increased forces over a larger palmar surface area. Such dynamic changes in palmar pressure likely moderate strain in the gracile bones of the hand, a structure that is integral not only for locomotion, but also feeding and social behaviors in primates.

Allometry of masticatory loading parameters in mammals

Considerable research on the scaling of loading patterns in mammalian locomotor systems has not been accompanied by a similarly comprehensive analysis of the interspecific scaling of loading regimes in the mammalian masticatory complex. To address this deficiency, we analyzed mandibular corpus bone strain in 11 mammalian taxa varying in body size by over 2.5 orders of magnitude, including goats, horses, alpacas, pigs, and seven primate taxa. During alert chewing and biting of hard/tough foods, bone-strain data were collected with rosette gauges placed along the lateral aspect of the mandibular corpus below the molars or premolars. Bone-strain data were used to characterize relevant masticatory loading parameters: peak loading magnitudes, chewing cycle duration, chewing frequency, occlusal duty factor, loading rate, and loading time. Interspecific analyses indicate that much as observed in limb elements, corpus peak-strain magnitudes are similar across mammals of disparate body sizes. Chewing frequency is inversely correlated with body size, much as with locomotor stride frequency. Some of this allometric variation in chewing frequency appears to be due to a negative correlation with loading time, which increases with body size. Similar to the locomotor apparatus, occlusal duty factor, or the duration of the chewing cycle during which the corpus is loaded, does not vary with body size. Peak principal-strain magnitudes are most strongly positively correlated with loading rate and only secondarily with loading, with this complex relationship best described by a multiple regression equation with an interaction term between loading rate and loading time. In addition to informing interpretations of craniomandibular growth, form, function, and allometry, these comparisons provide a skeleton-wide perspective on the patterning of osteogenic stimuli across body sizes.

Radiographs reveal exceptional forelimb strength in the sabertooth cat, Smilodon fatalis

Background

The sabertooth cat, Smilodon fatalis, was an enigmatic predator without a true living analog. Their elongate canine teeth were more vulnerable to fracture than those of modern felids, making it imperative for them to immobilize prey with their forelimbs when making a kill. As a result, their need for heavily muscled forelimbs likely exceeded that of modern felids and thus should be reflected in their skeletons. Previous studies on forelimb bones of S. fatalis found them to be relatively robust but did not quantify their ability to withstand loading.

Methodology/Principal Findings

Using radiographs of the sabertooth cat, Smilodon fatalis, 28 extant felid species, and the larger, extinct American lion Panthera atrox, we measured cross-sectional properties of the humerus and femur to provide the first estimates of limb bone strength in bending and torsion. We found that the humeri of Smilodon were reinforced by cortical thickening to a greater degree than those observed in any living felid, or the much larger P. atrox. The femur of Smilodon also was thickened but not beyond the normal variation found in any other felid measured.

Conclusions/Significance

Based on the cross-sectional properties of its humerus, we interpret that Smilodon was a powerful predator that differed from extant felids in its greater ability to subdue prey using the forelimbs. This enhanced forelimb strength was part of an adaptive complex driven by the need to minimize the struggles of prey in order to protect the elongate canines from fracture and position the bite for a quick kill.

The relationship between bone mechanical properties and ground reaction forces in normal and hypermuscular mice

Understanding the relationship between external load and bone morphology is critical for understanding adaptations to load in extant animals and inferring behavior in extinct forms. Yet, the relationship between bony anatomy and load is poorly understood, with empirical studies often producing conflicting results. It is widely assumed in many ecological and paleontological studies that bone size and strength reflect the forces experienced by the bone in vivo. This study examines that assumption by providing preliminary data on gait mechanics in a hypermuscular myostatin-deficient mouse model with highly mineralized and hypertrophied long bones. A small sample of hypermuscular and wild-type mice was video recorded while walking freely across a force platform. Temporal gait parameters, peak vertical and transverse (mediolateral) ground reaction forces (GRFs), vertical impulse, and loading rates were measured. The only gait parameters that differed between the two groups were the speeds at which the animals traveled and the transverse forces on the hind limb. The myostatin-deficient mice move relatively slowly and experienced the same magnitude of vertical forces on all limbs and transverse forces on the forelimb as the wild-type mice; though the myostatin-deficient mice did experience lower mediolateral forces on their hindlimbs compared with the wild-type mice. These preliminary results call into question the hypothesis that skeletal hypertrophy observed in hypermuscular mice is a result of larger GRFs experienced by the animals' limbs during locomotion. This calls for further analysis and a cautious approach to inferences about locomotor behavior derived from bony morphology in extant and fossil species.

The interplay between speed, kinetics, and hand postures during primate terrestrial locomotion

Nonprimate terrestrial mammals may use digitigrade postures to help moderate distal limb joint moments and metapodial stresses that may arise during high-speed locomotion with high-ground reaction forces (GRF). This study evaluates the relationships between speed, GRFs, and distal forelimb kinematics in order to evaluate if primates also adopt digitigrade hand postures during terrestrial locomotion for these same reasons. Three cercopithecine monkey species (Papio anubis, Macaca mulatta, Erythrocebus patas) were videotaped moving unrestrained along a horizontal runway instrumented with a force platform. Three-dimensional forelimb kinematics and GRFs were measured when the vertical force component reached its peak. Hand posture was measured as the angle between the metacarpal segment and the ground (MGA). As predicted, digitigrade hand postures (larger MGA) are associated with shorter GRF moment arms and lower wrist joint moments. Contrary to expectations, individuals used more palmigrade-like (i.e. less digitigrade) hand postures (smaller MGA) when the forelimb was subjected to higher forces (at faster speeds) resulting in potentially larger wrist joint moments. Accordingly, these primates may not use their ability to alter their hand postures to reduce rising joint moments at faster speeds. Digitigrady at slow speeds may improve the mechanical advantage of antigravity muscles crossing the wrist joint. At faster speeds, greater palmigrady is likely caused by joint collapse, but this posture may be suited to distribute higher GRFs over a larger surface area to lower stresses throughout the hand. Thus, a digitigrade hand posture is not a cursorial (i.e. high speed) adaptation in primates and differs from that of other mammals.

Regulation of mechanical signals in bone

OBJECTIVES

Response of the skeleton to application and removal of specific mechanical signals is discussed. Anabolic effects of high-frequency, low-magnitude vibrations, a mechanical intervention with a favorable safety profile, as well as the modulation of bone loss by genetic and epigenetic factors during disuse are highlighted.

METHODS

Review.

RESULTS

Bone responds to a great variety of mechanical signals and both high- and low-magnitude stimuli can be sensed by the skeleton. The ability of physical signals to influence bone morphology is strongly dependent on the signal's magnitude, frequency, and duration. Loading protocols at high signal frequencies (vibrations) allow a dramatic reduction in the magnitude of the signal. In the axial skeleton, these signals can be anabolic and anti-catabolic and increase the structural strength of the tissue. They further have shown potential in maxillofacial applications to accelerate the regeneration of bone within defects. Bone's sensitivity to the application and removal of mechanical signals is heavily under the control of the genome. Bone loss modulated by the removal of weight-bearing from the skeleton is profoundly influenced by factors such as genetics, gender, and baseline morphology.

CONCLUSIONS

Adaptation of bone to functional challenges is complex but it is clear that more is not necessarily better and that even very low-magnitude mechanical signals can be anabolic. The development of effective biomechanical interventions in areas such as orthodontics, craniofacial repair, or osteoporosis will require the identification of the specific components of bone's mechanical environment that are anabolic, catabolic, or anti-catabolic.

Masticatory stress and the mechanics of "wishboning" in colobine jaws

Cercopithecoid monkeys experience relatively high strains along the lingual aspect of the mandibular symphysis because of lateral transverse bending of the mandibular corpora ("wishboning") during mastication. Hylander (Am J Phys Anthropol 64 (1984) 1-46; Am Zool 25 (1985) 315-330) demonstrated that the distribution of strains arising from wishboning loads is comprehensible with reference to the mechanics of curved beams. Theory of curved beams suggests that lingual tensile strains are some multiple of labial compressive strains, yet limitations of experimental methods and uncertainty in estimating parameters needed for theoretical calculations have confounded attempts to characterize the magnitude of this disparity of normal strains. We evaluate the theoretical disparity of normal strains in wishboning in comparison to in vitro strains collected under controlled loads for a sample of mandibles representing two colobine species (N = 6). These data suggest that in colobine monkeys, maximum normal lingual strains should be at least twice maximum labial strains. In addition, we reexamine the distribution of symphyseal stress under an assumption of asymmetric bending, a general approach for calculation of stress appropriate for members that lack a plane of symmetry and are bent along an axis that is not coincident with the member's principal axes. Under asymmetric bending in colobine mandibles, the effect of symphyseal inclination on lingual strain is mitigating at the superior transverse torus and exacerbating at the inferior transverse torus. Relative compliance of colobine mandibular bone further supports the hypothesis that the structural and material properties of the colobine mandibular symphysis do not represent a morphological strategy for minimizing masticatory strain.

Anterior-posterior bending strength at the tibial shaft increases with physical activity in boys: evidence for non-uniform geometric adaptation

We investigated bone structural adaptations to a 16-month school-based physical activity intervention in 202 young boys using a novel analytical method for peripheral quantitative computed tomography scans of the tibial mid-shaft. Our intervention effectively increased bone bending strength in the anterior-posterior plane as estimated with the maximum second moment of area (I(max)).

INTRODUCTION

We previously reported positive effects of a physical activity intervention on peripheral quantitative computed tomography (pQCT)-derived bone strength at the tibial mid-shaft in young boys. The present study further explored structural adaptations to the intervention using a novel method for pQCT analysis.

METHODS

Participants were 202 boys (aged 9-11 years) from 10 schools randomly assigned to control (CON, 63 boys) and intervention (INT, 139 boys) groups. INT boys participated in 60 min/week of classroom physical activity, including a bone-loading program. We used ImageJ to process pQCT images of the tibial mid-shaft and determine the second moments of area (I(max), I(min)) and cortical area (CoA) and thickness (CTh) by quadrant (anterior, medial, lateral, posterior). We defined quadrants according to pixel coordinates about the centroid. We used mixed linear models to compare change in bone outcomes between groups.

RESULTS

The INT boys had a 3% greater gain in I(max) than the CON boys (p = 0.04) and tended to have a greater gain in I(min) ( approximately 2%, NS). Associated with the greater gain in I(max) was a slightly greater (NS) gain (1-1.4%) in CoA and CTh in the anterior, medial, and posterior (but not lateral) quadrants.

CONCLUSION

Our results suggest regional variation in bone adaptation consistent with patterns of bone formation induced by anterior-posterior bending loads.

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.

Role of Nonbehavioral Factors in Adjusting Long Bone Diaphyseal Structure in Free-ranging Pan troglodytes

Limb bones deform during locomotion and can resist the deformations by adjusting their shapes. For example, a tubular-shaped diaphysis best resists variably-oriented deformations. As behavioral profiles change during adulthood, patterns of bone deformation may exhibit age trends. Habitat characteristics, e.g., annual rainfall, tree density, and elevation changes, may influence bone deformations by eliciting individual components of behavioral repertoires and suppressing others, or by influencing movements during particular components. Habituated chimpanzee communities provide a unique opportunity to examine these factors because of the availability of morphological data and behavioral observations from known-age individuals inhabiting natural habitats. We evaluated adult femora and humeri of 18 female and 10 male free-ranging chimpanzees (Pan troglodytes) from communities in Gombe (Tanzania), Mahale Mountains (Tanzania), and Taï Forest (Côte d'Ivoire) National Parks. We compare cross sections at several locations (35%, 50%, 65% diaphyseal lengths). Community comparisons highlight different diaphyseal shapes of Taï females relative to Mahale and Gombe females, particularly in humeral diaphyses. Age trends in diaphyseal shapes are consistent with reduced activity levels in general, not only reduced arboreal activity. Age-related bone loss is apparent among community females, but is less striking among males. Community trends in diaphyseal shape are qualitatively consistent with ranked annual rainfall at localities, tree density, and elevation change or ruggedness of terrain. Habitat characteristics may contribute to variation in diaphyseal shape among chimpanzee communities, much like among modern human groups, but verification awaits further rigorous experimental and comparative analyses.

In vivo strains in the femur of river cooter turtles (Pseudemys concinna) during terrestrial locomotion: tests of force-platform models of loading mechanics

Previous analyses of ground reaction force (GRF) and kinematic data from river cooter turtles (Pseudemys concinna) during terrestrial walking led to three primary conclusions about the mechanics of limb bone loading in this lineage: (1) the femur was loaded in a combination of axial compression, bending and torsion, similar to previously studied non-avian reptiles, (2) femoral shear stresses were high despite the possession of a reduced tail in turtles that does not drag on the ground and (3) stress-based calculations of femoral safety factors indicated high values in bending and torsion, similar to other reptiles and suggesting that substantial 'overbuilding' of limb bones could be an ancestral feature of tetrapods. Because force-platform analyses produce indirect estimates of bone loading, we sought to validate these conclusions by surgically implanting strain gauges on turtle femora to directly measure in vivo strains during terrestrial walking. Strain analyses verified axial compression and bending as well as high torsion in turtle femora, with peak axial strains comparable to those of other non-avian reptiles at similar walking speeds but higher peak shear strains approaching 2000 microepsilon. Planar strain analyses showed patterns of neutral axis (NA) of femoral bending orientations and shifting generally consistent with our previous force-platform analyses of bone stresses, tending to place the anterior and dorsal aspects of the femur in tension and verifying an unexpected pattern from our force studies that differs from patterns in other non-avian reptiles. Calculated femoral safety factors were 3.8 in torsion and ranged from 4.4 to 6.9 in bending. Although these safety factors in bending were lower than values derived from our stress-based calculations, they are similar to strain-based safety factors calculated for other non-avian reptiles in terrestrial locomotion and are still high compared with safety factors calculated for limb bones of birds and mammals. These findings are consistent with conclusions drawn from our previous models of limb bone stresses in turtles and suggest that not only are turtle limb bones 'overbuilt' in terms of resisting the loads that they experience during locomotion but also, across tetrapod lineages, elevated torsion and high limb bone safety factors may be primitive features of limb bone design.

Mechanics of limb bone loading during terrestrial locomotion in river cooter turtles (Pseudemys concinna)

Studies of limb bone loading during terrestrial locomotion have focused primarily on birds and mammals. However, data from a broader functional and phylogenetic range of species are critical for understanding the evolution of limb bone function and design. Turtles are an interesting lineage in this context. Although their slow walking speeds and robust limb bones might lead to low locomotor forces and limb bone stresses similar to other non-avian reptiles, their highly sprawled posture could produce high bending loads, leading to high limb bone stresses similar to those of avian and mammalian species, as well as high torsion. To test between these possibilities, we evaluated stresses experienced by the femur of river cooter turtles (Pseudemys concinna) during terrestrial walking by synchronizing measurements of three-dimensional joint kinematics and ground reaction forces (GRFs) during isolated hindlimb footfalls. Further, we evaluated femoral safety factors for this species by comparing our locomotor stress calculations with the results of mechanical property tests. The net GRF magnitude at peak tensile bone stress averaged 0.35 BW (body weight) and was directed nearly vertically for the middle 40-65% of the contact interval, essentially orthogonal to the femur. Peak bending stresses experienced by the femur were low (tensile: 24.9+/-9.0 MPa; compressive: -31.1+/-9.1 MPa) and comparable to those in other reptiles, yet peak shear stresses were higher than those in other reptiles, averaging 13.7+/-4.2 MPa. Such high torsion is present despite cooters lacking a large tail, a feature that has been hypothesized to contribute to torsion in other reptiles in which the tail is dragged along the ground. Comparison of femoral stresses to measurements of limb bone mechanical properties in cooters indicates safety factors to yield of 13.9 in bending and 6.3 in torsion, considerably higher than values typical for birds and mammals, and closer to the elevated values calculated for other reptile species. Thus, not only do turtle limb bones seem considerably ;over-designed' for resisting the loads that they encounter, but comparisons of bone loading across tetrapod lineages are consistent with the hypothesis that low limb bone loads, elevated torsion and high safety factors may be primitive features of limb bone design.

The relative importance of genetics and phenotypic plasticity in dictating bone morphology and mechanics in aged mice: evidence from an artificial selection experiment

Both genetic and environmental factors are known to influence the structure of bone, contributing to its mechanical behavior during, and adaptive response to, loading. We introduce a novel approach to simultaneously address the genetically mediated, exercise-related effects on bone morphometrics and strength, using mice that had been selectively bred for high levels of voluntary wheel running (16 generations). Female mice from high running and control lines were either allowed (n=12, 12, respectively) or denied (n=11, 12, respectively) access to wheels for 20 months. Femoral shaft, neck, and head were measured with calipers and via micro-computed tomography. Fracture characteristics of the femoral head were assessed in cantilever bending. After adjusting for variation in body mass by two-way analysis of covariance, distal width of the femur increased as a result of selective breeding, and mediolateral femoral diameter was reduced by wheel access. Cross-sectional area of the femoral mid-shaft showed a significant linetype x activity effect, increasing with wheel access in high-running lines but decreasing in control lines. Body mass was significantly positively correlated with many of the morphometric traits studied. Fracture load of the femoral neck was strongly positively predicted by morphometric traits of the femoral neck (r2>0.30), but no significant effects of selective breeding or wheel access were found. The significant correlations of body mass with femoral morphometric traits underscore the importance of controlling for body size when analyzing the response of bone size and shape to experimental treatments. After controlling for body mass, measures of the femoral neck remain significant predictors of femoral neck strength.

Relative strength of the tibia and fibula and locomotor behavior in hominoids

The fibula has rarely been considered in comparative morphological studies, probably due to its relatively minor role in carrying mechanical loads. However, some differences in morphology (and inferred function) of the fibula between humans and apes, and within apes, have been noted and related to differences in positional behavior. Therefore, the study of tibiofibular relations may be useful in characterizing such differences. This study examines cross-sectional geometric (CSG) properties (cortical area and polar section modulus, Z(p)) of the tibia and fibula at mid-diaphysis across a sample (n=87) of humans, chimpanzees, gorillas, orangutans, and gibbons. The fibula is compared against the tibia in the different taxa. The results indicate that the robusticity of the fibula relative to that of the tibia can be explained in terms of differences in positional behavior. In particular, hominoids that are more arboreal (i.e., gibbons, orangutans, and chimpanzees) possess a relatively more robust fibula than do hominoids that are more terrestrial (i.e., gorillas and humans). The difference appears to be a consequence of the more mobile fibula and more adducted position of the hindlimb necessary in an arboreal environment. Apart from providing the first CSG data on the fibula, these results may be helpful in reconstructing the locomotor behavior of fossil hominoids.

Black bear femoral geometry and cortical porosity are not adversely affected by ageing despite annual periods of disuse (hibernation)

Disuse (i.e. inactivity) causes bone loss, and a recovery period that is 2-3 times longer than the inactive period is usually required to recover lost bone. However, black bears experience annual disuse (hibernation) and remobilization periods that are approximately equal in length, yet bears maintain or increase cortical bone material properties and whole bone mechanical properties with age. In this study, we investigated the architectural properties of bear femurs to determine whether cortical structure is preserved with age in bears. We showed that cross-sectional geometric properties increase with age, but porosity and resorption cavity density do not change with age in skeletally immature male and female bears. These findings suggest that structural properties substantially contribute to increasing whole bone strength with age in bears, particularly during skeletal maturation. Porosity was not different between skeletally immature and mature bears, and showed minimal regional variations between anatomical quadrants and radial positions that were similar in pattern and magnitude between skeletally immature and mature bears. We also found gender dimorphisms in bear cortical bone properties: females have smaller, less porous bones than males. Our results provide further support for the idea that black bears possess a biological mechanism to prevent disuse osteoporosis.

Increased non-linear locomotion alters diaphyseal bone shape

Comparative studies of vertebrate morphology that link habitual locomotor activities to bone structural properties are often limited by confounding factors such as genetic variability between groups. Experimental assessment of bone's adaptive response to altered activity patterns typically involves superimposing exercise onto a normal locomotor repertoire, making a distinction between qualitative changes to locomotor repertoires and quantitative increases in activity level difficult. Here, we directly tested the hypothesis that an increase in turning activity, without the application of exercise per se, will alter femoral cross-sectional shape. Thirty day-old female BALB/cByJ mice (n=10 per group) were single-housed for 8 weeks in custom-designed cages that either accentuated linear or turning locomotion or allowed subjects to freely roam standard cages. Consistent with a lack of difference in physical activity levels between groups, there were no significant differences in body mass, femoral length, midshaft cortical area,and individual measures of mediolateral (ML) and anteroposterior (AP) bending rigidity. However, the ratio of ML to AP diaphyseal rigidity, an indicator of cross-sectional shape, was significantly greater (P<0.05) in turning subjects than in linear or control subjects. Considering that across all groups mice were genetically identical and had equivalent levels of bone quantity and physical activity, differences in femoral shape were attributed to qualitative differences in locomotor patterns (i.e. specific locomotor modes). These data indicate that increased turning can alter distribution of bone mass in the femoral diaphysis, and that turning should be considered in efforts to understand form–function relationships in vertebrates.

Functional morphology of the mangabey mandibular corpus: Relationship to dental specializations and feeding behavior

Recent molecular and morphological surveys suggest that mangabeys do not represent a monophyletic group. Specifically, Cercocebus is the sister taxon of Mandrillus, whereas Lophocebus forms an unresolved trichotomy with Papio and Theropithecus. The Cercocebus-Mandrillus clade is characterized by skeletal and dental adaptations related to acquisition and processing of hard-object foods that resist decomposition for months on the forest floor. Although species of both mangabey genera can be described as frugivorous seed predators with a strong reliance on hard-object foods, a growing body of evidence indicates that Cercocebus (terrestrial) and Lophocebus (arboreal) mangabeys differ in the hardness of the seeds they consume and the manner in which seeds are processed. The taxa are also distinguished on the basis of dental morphology. Given the purported differences in feeding behaviors of the two mangabey genera, we consider whether there are predictable biomechanical consequences of these behaviors that are reflected in mandibular corpus dimensions. In addition, we present metric data summarizing functional aspects of mangabey mandibular corpus morphology. Mangabey genera are generally not distinguished by differences in relative corpus size, either in postcanine or symphyseal regions. Distinct symphyseal scaling patterns characterize the Papio-Lophocebus clade and the Mandrillus-Cercocebus clade, while the postcanine corpus scales similarly between them. The hypothesis that preferential use of the incisors vs. premolars to initially process these foods results in distinct stress environments is weakly supported, given circumstantial evidence that the relative importance of bending vs. torsion may differ between Cercocebus and Lophocebus.

Robusticity and sexual dimorphism in the postcranium of modern hunter-gatherers from Australia

Throughout much of prehistory, humans practiced a hunting and gathering subsistence strategy. Elevated postcranial robusticity and sexually dimorphic mobility patterns are presumed consequences of this strategy, in which males are attributed greater robusticity and mobility than females. Much of the basis for these trends originates from populations where skeletal correlates of activity patterns are known (e.g., cross-sectional geometric properties of long bones), but in which activity patterns are inferred using evidence such as archaeological records (e.g., Pleistocene Europe). Australian hunter-gatherers provide an opportunity to critically assess these ideas since ethnographic documentation of their activity patterns is available. We address the following questions: do skeletal indicators of Australian hunter-gatherers express elevated postcranial robusticity and sexually dimorphic mobility relative to populations from similar latitudes, and do ethnographic accounts support these findings. Using computed tomography, cross-sectional images were obtained from 149 skeletal elements including humeri, radii, ulnae, femora, and tibiae. Cross-sectional geometric properties were calculated from image data and standardized for body size. Australian hunter-gatherers often have reduced robusticity at femoral and humeral midshafts relative to forager (Khoi-San), agricultural/industrialized (Zulu), and industrialized (African American) groups. Australian hunter-gatherers display more sexual dimorphism in upper limb robusticity than lower limb robusticity. Attributing specific behavioral causes to upper limb sexual dimorphism is premature, although ethnographic accounts support sex-specific differences in tool use. Virtually absent sexual dimorphism in lower limb robusticity is consistent with ethnographic accounts of equivalently high mobility among females and males. Thus, elevated postcranial robusticity and sexually dimorphic mobility do not always characterize hunter-gatherers.

Ontogenetic relationships between in vivo strain environment, bone histomorphometry and growth in the goat radius

Vertebrate long bone form, at both the gross and the microstructural level, is the result of many interrelated influences. One factor that is considered to have a significant effect on bone form is the mechanical environment experienced by the bone during growth. The work presented here examines the possible relationships between in vivo bone strains, bone geometry and histomorphology in the radii of three age/size groups of domestic goats. In vivo bone strain data were collected from the radii of galloping goats, and the regional cortical distribution of peak axial strain magnitudes, radial and circumferential strain gradients, and longitudinal strain rates related to regional patterns in cortical growth, porosity, remodelling and collagen fibre orientation. Although porosity and remodelling decreased and increased with age, respectively, these features showed no significant regional differences and did not correspond to regional patterns in the mechanical environment. Thicker regions of the radius's cortex were significantly related to high strain levels and higher rates of periosteal, but not endosteal, growth. However, cortical growth and strain environment were not significantly related. Collagen fibre orientation varied regionally, with a higher percentage of transverse fibres in the caudal region of the radius and primarily longitudinal fibres elsewhere, and, although consistent through growth, also did not generally correspond to regional strain patterns. Although strain magnitudes increased during ontogeny and regional strain patterns were variable over the course of a stride, mean regional strain patterns were generally consistent with growth, suggesting that regional growth patterns and histomorphology, in combination with external loads, may play some role in producing a relatively 'predictable' strain environment within the radius. It is further hypothesized that the absence of correlation between regional histomorphometric patterns and the measured strain environments is the result of the variable mechanical environment. However, the potential effects of other physiological and mechanical factors, such as skeletal metabolism and adjacent muscle insertions, that can influence the gross and microstructural morphology of the radius during ontogeny, cannot be ignored.

Regional variation in the postcranial robusticity of late Upper Paleolithic humans

Early modern humans from the European Upper Paleolithic (UP) demonstrate trends in postcranial biomechanical features that coincide with the last glacial maximum (LGM). These features have been interpreted as evidence that ecological changes of the LGM played a critical role in cultural and biological adaptation in European UP populations. In areas outside of Europe, similar environmental changes occurred with the LGM. This analysis introduces postcranial material from the Late Upper Paleolithic (LUP) of North Africa and Southeast Asia and tests two related hypotheses: 1) LUP samples across the Old World had similar patterns of postcranial robusticity and 2) relative to an available Early Upper Paleolithic (EUP) sample, regional LUP samples demonstrate similar trends in robusticity that may be attributable to climatic effects of the LGM. Cross-sectional geometric data of the humeri and femora were obtained for 26 EUP and 100 LUP humans from Europe, Africa, and Asia. Despite regional differences, LUP samples are similar relative to the EUP sample. In the humerus, bilateral asymmetry decreases in all LUP samples relative to the EUP sample. In the femur, LUP samples demonstrate increasingly circular femoral midshaft sections, reflecting reduced anteroposterior bending strength relative to the EUP sample. These patterns suggest changes in subsistence behavior and mobility after the LGM across the Old World that are most consistent with reduced mobility and broad-spectrum resource exploitation.

In vivo bone strain and bone functional adaptation

Mechanistic interpretations of bone cross-sectional shapes are based on the paradigm of shape optimization such that bone offers maximum mechanical resistance with a minimum of material. Recent in vivo strain studies (Demes et al., Am J Phys Anthropol 106 (1998) 87-100, Am J Phys Anthropol 116 (2001) 257-265; Lieberman et al., Am J Phys Anthropol 123 (2004) 156-171) have questioned these interpretations by demonstrating that long bones diaphyses are not necessarily bent in planes in which they offer maximum resistance to bending. Potential limitations of these in vivo studies have been pointed out by Ruff et al. (Am J Phys Anthropol 129 (2006) 484-498). It is demonstrated here that two loading scenarios, asymmetric bending and buckling, would indeed not lead to correct predictions of loads from strain. It is also shown that buckling is of limited relevance for many primate long bones. This challenges a widely held view that circular bone cross sections make loading directions unpredictable for bones which is based on a buckling load model. Asymmetric bending is a potentially confounding factor for bones with directional differences in principal area moments (I(max) > I(min)). Mathematical corrections are available and should be applied to determine the bending axis in such cases. It is concluded that loads can be reliably extrapolated from strains. More strain studies are needed to improve our understanding of the relationships between activities, bone loading regimes associated with them, and the cross-sectional geometry of bones.

Who's afraid of the big bad Wolff?: "Wolff's law" and bone functional adaptation

"Wolff's law" is a concept that has sometimes been misrepresented, and frequently misunderstood, in the anthropological literature. Although it was originally formulated in a strict mathematical sense that has since been discredited, the more general concept of "bone functional adaptation" to mechanical loading (a designation that should probably replace "Wolff's law") is supported by much experimental and observational data. Objections raised to earlier studies of bone functional adaptation have largely been addressed by more recent and better-controlled studies. While the bone morphological response to mechanical strains is reduced in adults relative to juveniles, claims that adult morphology reflects only juvenile loadings are greatly exaggerated. Similarly, while there are important genetic influences on bone development and on the nature of bone's response to mechanical loading, variations in loadings themselves are equally if not more important in determining variations in morphology, especially in comparisons between closely related individuals or species. The correspondence between bone strain patterns and bone structure is variable, depending on skeletal location and the general mechanical environment (e.g., distal vs. proximal limb elements, cursorial vs. noncursorial animals), so that mechanical/behavioral inferences based on structure alone should be limited to corresponding skeletal regions and animals with similar basic mechanical designs. Within such comparisons, traditional geometric parameters (such as second moments of area and section moduli) still give the best available estimates of in vivo mechanical competence. Thus, when employed with appropriate caution, these features may be used to reconstruct mechanical loadings and behavioral differences within and between past populations.

Habitual use of the primate forelimb is reflected in the material properties of subchondral bone in the distal radius

Bone mineral density is directly proportional to compressive strength, which affords an opportunity to estimate in vivo joint load history from the subchondral cortical plate of articular surfaces in isolated skeletal elements. Subchondral bone experiencing greater compressive loads should be of relatively greater density than subchondral bone experiencing less compressive loading. Distribution of the densest areas, either concentrated or diffuse, also may be influenced by the extent of habitual compressive loading. We evaluated subchondral bone in the distal radius of several primates whose locomotion could be characterized in one of three general ways (quadrupedal, suspensory or bipedal), each exemplifying a different manner of habitual forelimb loading (i.e. compression, tension or non-weight-bearing, respectively). We employed computed tomography osteoabsorptiometry (CT-OAM) to acquire optical densities from which false-colour maps were constructed. The false-colour maps were used to evaluate patterns in subchondral density (i.e. apparent density). Suspensory apes and bipedal humans had both smaller percentage areas and less well-defined concentrations of regions of high apparent density relative to quadrupedal primates. Quadrupedal primates exhibited a positive allometric effect of articular surface size on high-density area, whereas suspensory primates exhibited an isometric effect and bipedal humans exhibited no significant relationship between the two. A significant difference between groups characterized by predominantly compressive forelimb loading regimes vs. tensile or non-weight-bearing regimes indicates that subchondral apparent density in the distal radial articular surface distinguishes modes of habitually supporting of body mass.

Locomotor behavior and long bone morphology in individual free-ranging chimpanzees

We combine structural limb data and behavioral data for free-ranging chimpanzees from Taï (Ivory Coast) and Mahale National Parks (Tanzania) to begin to consider the relationship between individual variation in locomotor activity and morphology. Femoral and humeral cross sections of ten individuals were acquired via computed tomography. Locomotor profiles of seven individuals were constructed from 3387 instantaneous time-point observations (87.4 hours). Within the limited number of suitable chimpanzees, individual variation in locomotor profiles displayed neither clear nor consistent trends with diaphyseal cross-sectional shapes. The percentages of specific locomotor modes did not relate well to diaphyseal shapes since neither infrequent nor frequent locomotor modes varied consistently with shapes. The percentage of arboreal locomotion, rather than estimated body mass, apparently had comparatively greater biological relevance to variation in diaphyseal shape. The mechanical consequences of locomotor modes on femoral and humeral diaphyseal shapes (e.g., orientation of bending strains) may overlap between naturalistic modes more than currently is recognized. Alternatively, diaphyseal shape may be unresponsive to mechanical demands of these specific locomotor modes. More data are needed in order to discern between these possibilities. Increasing the sample to include additional free-ranging chimpanzees, or primates in general, as well as devoting more attention to the mechanics of a greater variety of naturalistic locomotor modes would be fruitful to understanding the behavioral basis of diaphyseal shapes.

Body size, body proportions, and mobility in the Tyrolean "Iceman"

Body mass and structural properties of the femoral and tibial midshafts of the "Iceman," a late Neolithic (5,200 BP) mummy found in the Tyrolean Alps, are determined from computed tomographic scans of his body, and compared with those of a sample of 139 males spanning the European early Upper Paleolithic through the Bronze Age. Two methods, based on femoral head breadth and estimated stature/bi-iliac (pelvic) breath, yield identical body-mass estimates of 61 kg for the Iceman. In combination with his estimated stature of 158 cm, this indicates a short but relatively wide or stocky body compared to our total sample. His femur is about average in strength compared to our late Neolithic (Eneolithic) males, but his tibia is well above average. His femur also shows adaptations for his relatively broad body (mediolateral strengthening), while his tibia shows adaptations for high mobility over rough terrain (anteroposterior strengthening). In many respects, his tibia more closely resembles those of European Mesolithic rather than Neolithic males, which may reflect a more mobile lifestyle than was characteristic of most Neolithic males, perhaps related to a pastoral subsistence strategy. There are indications that mobility in general declined between the European Mesolithic and late Neolithic, and that body size and shape may have become more variable throughout the continent following the Upper Paleolithic.

Regional distinctions in cortical bone mineral density measured by pQCT can predict alterations in material property at the tibial diaphysis of the Cynomolgus monkey

We examined whether regional differences in cortical bone mineral density (Ct.BMD) measured by peripheral quantitative computed tomography is related to the heterogeneity of bone tissue and whether regional Ct.BMD is a better indicator of changes in bone material properties. Bilateral tibiae were obtained from 17 female adult Cynomolgus monkeys (Macaca fascicularis; mean age 16.8 years). After determining that Ct.BMD was similar between the right and left tibiae, the left tibiae were used for bone histomorphometry and the right for a three-point bending test. The Ct.BMD in the posterior quadrant was significantly higher than that in the anterior quadrant. In the bone histomorphometric analysis, all parameters (i.e., average osteonal area, average osteonal bone area, osteon population density, percent osteonal area [%On.Ar], percent osteonal bone area [%On.B.Ar], percent osteonal area of initial remodeling [%Il.On.Ar], percent osteonal area of secondary remodeling [%Sd.On.Ar], porosity, and percent osteoid area in the posterior region) were significantly lower than those in the anterior region. The results indicated that in the same cross-section, bone tissue structure was heterogeneous. Both total- and posterior-Ct.BMD were positively correlated with breaking stress and negatively correlated with toughness, whereas anterior-Ct.BMD was positively correlated with elastic modulus. Backward stepwise multiple regression analyses indicated that posterior-Ct.BMD and total-Ct.BMD were the best variables for predicting breaking stress and toughness, respectively, when age is taken into account. The %On.Ar, %On.B.Ar, and %Il.On.Ar in the posterior region were negatively correlated with elastic modulus. The %On.Ar, %On.B.Ar, and %Sd.On.Ar in the posterior region were positively correlated with toughness. These findings indicated that regional Ct.BMD measurement is useful to assess changes in the material properties of bone associated with the degree of mineralization. In particular, anterior-, posterior-, and total-Ct.BMD can be used separately to predict changes in the material properties of the tibial diaphysis.

In Vivo Bone Strain and Ontogenetic Growth Patterns in Relation to Life‐History Strategies and Performance in Two Vertebrate Taxa: Goats and Emu

This study examined ontogenetic patterns of limb loading, bone strains, and relative changes in bone geometry to explore the relationship between in vivo mechanics and size-related changes in the limb skeleton of two vertebrate taxa. Despite maintaining similar relative limb loads during ontogeny, bone strain magnitudes in the goat radius and emu tibiotarsus generally increased. However, while the strain increases in the emu tibiotarsus were mostly insignificant, strains within the radii of adult goats were two to four times greater than in young goats. The disparity between ontogenetic strain increases in these taxa resulted from differences in ontogenetic scaling patterns of the cross-sectional bone geometry. While the cross-sectional and second moments of area scaled with negative allometry in the goat radius, these measures were not significantly different from isometry in the emu tibiotarsus. Although the juveniles of both taxa exhibited lower strains and higher safety factors than the adults, the radii of the young goats were more robust relative to the adult goats than were the tibiotarsi of the young compared with adult emu. Differences in ontogenetic growth and strain patterns in the limb bones examined likely result from different threat avoidance strategies and selection pressures in the juveniles of these two taxa.

The cross-sectional geometry of the hand and foot bones of the hominoidea and its relationship to locomotor behavior

Cheiridia are valuable indicators of positional behavior, as they directly contact the substrate, but systematic comparison of the structural properties of both metacarpals and metatarsals has never been carried out. Differences in locomotor behavior among the great apes (knuckle-walking vs. quadrumanous climbing) can produce biomechanical differences that may be elucidated by the parallel study of cross-sectional characteristics of metacarpals and metatarsals. The aim of this work is to study the cross-sectional geometric properties of these bones and their correlation with locomotor behavior in large-bodied hominoids. The comparisons between bending moments of metacarpals and metatarsals of the same ray furnished interesting results. Metacarpals III and especially IV of the knuckle-walking African apes were relatively stronger than those of humans and orangutans, and metatarsal V of humans was relatively stronger than those of the great apes. Interestingly, the relative robusticity of the metacarpal IV of the quadrumanous orangutan was between that of the African apes and that of humans. The main conclusions of the study are: 1) cross-sectional dimensions of metacarpals and metatarsals are influenced by locomotor modes in great apes and humans; 2) interlimb comparisons of cross-sectional properties of metacarpals and metatarsals are good indicators of locomotor modes in great apes and humans; and 3) the results of this study are in accord with those of previous analyses of plantar pressure and morphofunctional traits of the same bones, and with behavioral studies. These results provide a data base from which it will be possible to compare the morphology of the fossils in order to gain insight into the locomotor repertoires of extinct taxa.

Cross-sectional geometry of the human forefoot

The human forefoot presents an interesting biomechanical problem of clinical importance because loads are distributed unequally across multiple bones. The fact that long bones typically have a several-fold safety factor relative to peak loads suggests that metatarsal strengths should be related to their peak loads. This study is the first to systematically examine the cross-sectional geometric properties of the human forefoot and their relationship to external loads during walking and running. We report midlength cross-sectional geometric properties (CA, Ix, Iy, Imax, Imin, J, Zx, and Zy) of metatarsals (1-5) and the hallucial proximal phalanx of a shod industrial population (n = 40) obtained using computed tomography. We then examine the relationship between these measures of shaft strength and published plantar pressure data sets recorded during the following functional activities: standing, at the push-off stage of the walking cycle, the full walking cycle, and running. Cross-sectional geometric properties of the first ray are greater than those of other rays, even when scaled to bone length. This pattern corresponds to the high pressures recorded for the first ray during most activities. The relationships between cross-sectional geometric properties of the lateral metatarsals and peak plantar pressure data are more complex. Metatarsals 2-4 are weakest in most cross-sectional geometric properties. However, metatarsal 2, and to a lesser extent metatarsal 3, experience relatively high peak pressures. On average, geometric measures of axial and bending strengths (adjusted relative to body size) are lower in females than males, and in European Americans than in African Americans, which corresponds to the respective rates of general metatarsal stress fracture in these groups. The discrepancy between strength and plantar pressure values in metatarsals 2 and 3 is consistent with the high incidence of stress fractures in these bones and underscores the importance of soft tissues, such as the plantar fascia and flexor musculature, in moderating metatarsal shaft strain.

Investigating the form-function interface in African apes: Relationships between principal moments of area and positional behaviors in femoral and humeral diaphyses

Investigations of cross-sectional geometry in nonhuman primate limb bones typically attribute shape ratios to qualitative behavioral characterizations, e.g., leaper, slow climber, brachiator, or terrestrial vs. arboreal quadruped. Quantitative positional behavioral data, however, have yet to be used in a rigorous evaluation of such shape-behavior connections. African apes represent an ideal population for such an investigation because their relatedness minimizes phylogenetic inertia, they exhibit diverse behavioral repertoires, and their locomotor behaviors are known from multiple studies. Cross-sectional data from femoral and humeral diaphyses were collected for 222 wild-shot specimens, encompassing Pan paniscus and all commonly recognized African ape subspecies. Digital representations of diaphyseal cross sections were acquired via computed tomography at three locations per diaphysis. Locomotor behaviors were pooled broadly into arboreal and terrestrial categories, then partitioned into quadrupedal walking, quadrumanous climbing, scrambling, and suspensory categories. Sex-specific taxonomic differences in ratios of principal moments of area (PMA) were statistically significant more often in the femoral diaphysis than the humeral diaphysis. While it appears difficult to relate a measure of shape (e.g., PMA ratio) to individual locomotor modes, general locomotor differences (e.g., percentage arboreal vs. terrestrial locomotion) are discerned more easily. As percentage of arboreal locomotion for a group increases, average cross sections appear more circular. Associations between PMA ratio and specific locomotor behaviors are less straightforward. Individual behaviors that integrate eccentric limb positions (e.g., arboreal scrambling) may not engender more circular cross sections than behaviors that incorporate repetitive sagittal movements (e.g., quadrupedal walking) in a straightforward manner.

Curvature, length, and cross-sectional geometry of the femur and humerus in anthropoid primates

The aims of this study were to describe the curvature of anthropoid limb bones quantitatively, to determine how limb bone curvature scales with body mass, and to discuss how bone curvature influences static measures of bone strength. Femora and humeri in six anthropoid genera of Old World monkeys, New World monkeys, and gibbons were used. Bone length, curvature, and cross-sectional properties were incorporated into the analysis. These variables were obtained by a new method using three-dimensional morphological data reconstructed from consecutive CT images. This method revealed the patterns of curvature of anthropoid limb bones. Log-transformed scaling analyses of the characters revealed that bone length and especially bone curvature strongly reflected taxonomic/locomotor differences. As compared with Old World monkeys, New World monkeys and gibbons in particular have a proportionally long and less curved femur and humerus relative to body mass. It is also revealed that the section modulus relative to body mass varies less between taxonomic/locomotor groups in anthropoids. Calculation of theoretical bending strengths implied that Old World monkeys achieve near-constant bending strength in accordance with the tendency observed in general terrestrial mammals. Relatively shorter bone length and larger A-P curvature of Old World monkeys largely contribute to this uniformity. Bending strengths in New World monkeys and gibbons were, however, a little lower under lateral loading and extremely stronger and more variable under axial loading as compared with Old World monkeys, due to their relative elongated and weakly curved femora and humeri. These results suggest that arboreal locomotion, including quadrupedalism and suspension, requires functional demands quite dissimilar to those required in terrestrial quadrupedalism.

Physiological strains remodel extracellular matrix and cell-cell adhesion in osteoblastic cells cultured on alumina-coated titanium alloy

The effects of mechanical strains on cellular activities were assessed in an in vitro model using human osteoblastic MG-63 cells grown on titanium alloy discs coated with porous alumina and exposed to chronic intermittent loading. Strain was applied with a Dynacell device for three 15-min sequences per day for several days with a magnitude of 600 microepsilon strain and a frequency of 0.25 Hz. We have previously demonstrated that this regimen increased alkaline phosphatase activity in confluent cultures on ceramic coated titanium (alumina and hydroxyapatite) (Biomaterials 24 (2003) 3139). In this study, we analysed the production of bone matrix proteins. Osteocalcin secretion quantified by ELISA between day 5 and 11 was not affected by mechanical strain. Strain had even no quantifiable effect on collagen production from day 1 to 5 as measured by carboxy terminal collagen type I propeptide release. On the other hand, stress stimulation resulted in increased expression of fibronectin (FN) measured by Western blot after 1 day stretching. This upregulation of FN production was followed by reorganisation of the FN network after 5 days stretching observed by immunostaining. The receptors for collagen and FN, alpha2beta1, alpha5beta1 and beta1 integrins were not quantitatively affected by the strains as measured by flow cytometry. A modification of cell morphology was seen after 5 days of loading that appeared to increase cell spreading, implying consequences on intercellular contacts. For this reason, N, C11 and E-adherins were examined. We noted a selective effect characterised by increased expression of N-cadherin using both RT-PCR and Western blot analyses. We concluded that reinforcement of cell-cell adhesion and remodelling of the FN network are important adaptive responses to physiological strains for human osteoblasts grown on alumina-coated biomaterials.

Predicting long bone loading from cross-sectional geometry

Long bone loading histories are commonly evaluated using a beam model by calculating cross-sectional second moments of areas (SMAs). Without in vivo strain data, SMA analyses commonly make two explicit or implicit assumptions. First, while it has long been known that axial compression superimposed on bending shifts neutral axes away from cross-sectional area centroids, most analyses assume that cross-sectional properties calculated through the area centroid approximate cross-sectional strength. Second, the orientation of maximum bending rigidity is often assumed to reflect the orientation of peak or habitual bending forces the bone experiences. These assumptions are tested in sheep in which rosette strain gauges mounted at three locations around the tibia and metatarsal midshafts measured in vivo strains during treadmill running at 1.5 m/sec. Calculated normal strain distributions confirm that the neutral axis of bending does not run through the midshaft centroid. In these animals, orientations of the principal centroidal axes around which maximum SMAs (Imax) are calculated are not in the same planes in which the bones experienced bending. Cross-sectional properties calculated using centroidal axes have substantial differences in magnitude (up to 55%) but high correlations in pattern compared to cross-sectional properties calculated around experimentally determined neutral axes. Thus interindividual comparisons of cross-sectional properties calculated from centroidal axes may be useful in terms of pattern, but are subject to high errors in terms of absolute values. In addition, cross-sectional properties do not necessarily provide reliable data on the orientations of loads to which bones are subjected.

The aging of Wolff's ?law?: Ontogeny and responses to mechanical loading in cortical bone

The premise that bones grow and remodel throughout life to adapt to their mechanical environment is often called Wolff's law. Wolff's law, however, is not always true, and in fact comprises a variety of different processes that are best considered separately. Here we review the molecular and physiological mechanisms by which bone senses, transduces, and responds to mechanical loads, and the effects of aging processes on the relationship (if any) between cortical bone form and mechanical function. Experimental and comparative evidence suggests that cortical bone is primarily responsive to strain prior to sexual maturity, both in terms of the rate of new bone growth (modeling) as well as rates of turnover (Haversian remodeling). Rates of modeling and Haversian remodeling, however, vary greatly at different skeletal sites. In addition, there is no simple relationship between the orientation of loads in long bone diaphyses and their cross-sectional geometry. In combination, these data caution against assuming without testing adaptationist views about form-function relationships in order to infer adult activity patterns from skeletal features such as cross-sectional geometry, cortical bones density, and musculo-skeletal stress markers. Efforts to infer function from shape in the human skeleton should be based on biomechanical and developmental models that are experimentally tested and validated.

Insights into the evolution of human bipedalism from experimental studies of humans and other primates

An understanding of the evolution of human bipedalism can provide valuable insights into the biomechanical and physiological characteristics of locomotion in modern humans. The walking gaits of humans, other bipeds and most quadrupedal mammals can best be described by using an inverted-pendulum model, in which there is minimal change in flexion of the limb joints during stance phase. As a result, it seems logical that the evolution of bipedalism in humans involved a simple transition from a relatively stiff-legged quadrupedalism in a terrestrial ancestor to relatively stiff-legged bipedalism in early humans. However, experimental studies of locomotion in humans and nonhuman primates have shown that the evolution of bipedalism involved a much more complex series of transitions, originating with a relatively compliant form of quadrupedalism. These studies show that relatively compliant walking gaits allow primates to achieve fast walking speeds using long strides, low stride frequencies, relatively low peak vertical forces, and relatively high impact shock attenuation ratios. A relatively compliant, ape-like bipedal walking style is consistent with the anatomy of early hominids and may have been an effective gait for a small biped with relatively small and less stabilized joints, which had not yet completely forsaken arboreal locomotion. Laboratory-based studies of primates also suggest that human bipedalism arose not from a terrestrial ancestor but rather from a climbing, arboreal forerunner. Experimental data, in conjunction with anatomical data on early human ancestors, show clearly that a relatively stiff modern human gait and associated physiological and anatomical adaptations are not primitive retentions from a primate ancestor, but are instead recently acquired characters of our genus.

Interpreting the posture and locomotion of Australopithecus afarensis: where do we stand?

Reconstructing the transition to bipedality is key to understanding early hominin evolution. Because it is the best-known early hominin species, Australopithecus afarensis forms a baseline for interpreting locomotion in all early hominins. While most researchers agree that A. afarensis individuals were habitual bipeds, they disagree over the importance of arboreality for them. There are two main reasons for the disagreement. First, there are divergent perspectives on how to interpret primitive characters. Primitive traits may be retained by stabilizing selection, pleiotropy, or other ontogenetic mechanisms. Alternately, they could be in the process of being reduced, or they simply could be selectively neutral. Second, researchers are asking fundamentally different questions about the fossils. Some are interested in reconstructing the history of selection that shaped A. afarensis, while others are interested in reconstructing A. afarensis behavior. By explicitly outlining whether we are interested in reconstructing selective history or behavior, we can develop testable hypotheses to govern our investigations of the fossils. To infer the selective history that shaped a taxon, we must first consider character polarity. Derived traits that enhance a particular function, are found to be associated with that function in extant homologs, and that epigenetically sensitive data indicate were actually being used for that function, can be interpreted as adaptations. The null hypothesis to explain the retention of primitive traits is that of selective neutrality, or nonaptation. Disproving this requires demonstration of active stabilizing or negative selection (disaptation). Stabilizing selection can be inferred when primitive traits compromise a derived function clearly of adaptive value. Prolonged stasis, continued use of the trait for a particular function, or no change in variability in the trait are evidence that can support a hypothesis of adaptation for primitive traits, but still do not falsify the null hypothesis. Disaptation, or negative selection, should result in a trait being reduced or lost. To infer the behaviors of a fossil species, we must first determine its adaptations, use this to make hypotheses about its behavior, and test these hypotheses using epigenetically sensitive traits that are modified by an individual's activity pattern. When the A. afarensis data are evaluated using this framework, it is clear that these hominins had undergone selection for habitual bipedality, but the null hypothesis of nonaptation to explain the retention of primitive, ape-like characters cannot be falsified at present. The apparent stasis in Australopithecus postcranial form is currently the strongest evidence for stabilizing selection maintaining its primitive features. Evidence from features affected by individual behaviors during ontogeny shows that A. afarensis individuals were habitually traveling bipedally, but evidence presented for arboreal behavior so far is not conclusive. By clearly identifying the questions we are asking about early hominin fossils, refining our knowledge about character polarities, and elucidating the factors influencing morphology, we will be able to progress in our understanding of the posture and locomotion of A. afarensis and all early hominins.

Variations in cortical material properties throughout the human dentate mandible

Material properties and their variations in individual bone organs are important for understanding bone adaptation and quality at a tissue level, and are essential for accurate mechanical models. Yet material property variations have received little systematic study. Like all other material property studies in individual bone organs, studies of the human mandible are limited by a low number of both specimens and sampled regions. The aims of this study were to determine: 1) regional variability in mandibular material properties, 2) the effect of this variability on the modeling of mandibular function, and 3) the relationship of this variability to mandibular structure and function. We removed 31 samples on both facial and lingual cortices of 10 fresh adult dentate mandibles, measured cortical thickness and density, determined the directions of maximum stiffness with a pulse transmission ultrasonic technique, and calculated elastic properties from measured ultrasonic velocities. Results showed that each of these elastic properties in the dentate human mandible demonstrates unique regional variation. The direction of maximum stiffness was near parallel to the occlusal plane within the corpus. On the facial ramus, the direction of maximum stiffness was more vertically oriented. Several sites in the mandible did not show a consistent direction of maximum stiffness among specimens, although all specimens exhibited significant orthotropy. Mandibular cortical thickness varied significantly (P < 0.001) between sites, and decreased from 3.7 mm (SD = 0.9) anteriorly to 1.4 mm posteriorly (SD = 0.1). The cortical plate was also significantly thicker (P < 0.003) on the facial side than on the lingual side. Bone was 50-100% stiffer in the longitudinal direction (E(3), 20-30 GPa) than in the circumferential or tangential directions (E(2) or E(1); P < 0.001). The results suggest that material properties and directional variations have an important impact on mandibular mechanics. The accuracy of stresses calculated from strains and average material properties varies regionally, depending on variations in the direction of maximum stiffness and anisotropy. Stresses in some parts of the mandible can be more accurately calculated than in other regions. Limited evidence suggests that the orientations and anisotropies of cortical elastic properties correspond with features of cortical bone microstructure, although the relationship with functional stresses and strains is not clear.

Long bone articular and diaphyseal structure in old world monkeys and apes. I: locomotor effects

The relationship between locomotor behavior and long bone structural proportions is examined in 179 individuals and 13 species of hominoids and cercopithecoids. Articular surface areas, estimated from linear caliper measurements, and diaphyseal section moduli (strengths), determined from CT scans, were obtained for the femur, tibia, humerus, radius, and ulna. Both within-bone (articular to shaft) and between-bone (forelimb to hindlimb) proportions were calculated and compared between taxa. It was hypothesized that: 1) species emphasizing slow, cautious movement and/or more varied limb positioning (i.e., greater joint excursion) would exhibit larger articular to cross-sectional shaft proportions, and 2) species with more forelimb suspensory behavior would have relatively stronger/larger forelimbs, while those with more leaping would have relatively stronger/larger hindlimbs. The results of the analysis generally confirm both hypotheses. Several partial exceptions can be explained on the basis of more detailed structural-functional considerations. Associations between locomotion and structural proportions can be demonstrated both across major groupings (hominoids and cercopithecoids) and between relatively closely related taxa, e.g., mountain and lowland gorillas, siamangs and gibbons, and Trachypithecus and other colobines. Furthermore, structure and function do not always covary with taxonomy. For example, compared to cercopithecoids, mountain gorillas have relatively larger joints, like other hominoids, but do not have relatively stronger forelimbs, unlike other hominoids. This is consistent with a locomotor repertoire emphasizing relatively slow movement but with very little forelimb suspension. Proportions of Proconsul nyanzae, Proconsul heseloni, Morotopithecus bishopi, and Theropithecus oswaldi are compared with modern distributions to illustrate the application of the techniques to fossil taxa.