Variation and covariation of external shape and cross-sectional geometry in the human metacarpus

OBJECTIVES

Analyses of external bone shape using geometric morphometrics (GM) and cross-sectional geometry (CSG) are frequently employed to investigate bone structural variation and reconstruct activity in the past. However, the association between these methods has not been thoroughly investigated. Here, we analyze whole bone shape and CSG variation of metacarpals 1-5 and test covariation between them.

MATERIALS AND METHODS

We analyzed external metacarpal shape using GM and CSG of the diaphysis at three locations in metacarpals 1-5. The study sample includes three modern human groups: crew from the shipwrecked Mary Rose (n = 35 metacarpals), a Pre-industrial group (n = 50), and a Post-industrial group (n = 31). We tested group differences in metacarpal shape and CSG, as well as correlations between these two aspects of metacarpal bone structure.

RESULTS

GM analysis demonstrated metacarpus external shape variation is predominately related to changes in diaphyseal width and articular surface size. Differences in external shape were found between the non-pollical metacarpals of the Mary Rose and Pre-industrial groups and between the third metacarpals of the Pre- and Post-industrial groups. CSG results suggest the Mary Rose and Post-industrial groups have stronger metacarpals than the Pre-industrial group. Correlating CSG and external shape showed significant relationships between increasing external robusticity and biomechanical strength across non-pollical metacarpals (r: 0.815-0.535; p ≤ 0.05).

DISCUSSION

Differences in metacarpal cortical structure and external shape between human groups suggest differences in the type and frequency of manual activities. Combining these results with studies of entheses and kinematics of the hand will improve reconstructions of manual behavior in the past.

A comparative study of robusticity indices of long bones among hunter-gatherers and early agro-pastoral groups of India

OBJECTIVES

Diaphyseal robusticity and cross-sectional shapes of the bone tissues are influenced by mechanical loading history. It changes according to work demand on the body. It is the objective of this study to identify the shifts in the modes of subsistence, activities and mobility patterns through the comparison of the shape and strength of bone diaphyses among the Mesolithic, Chalcolithic and Harappan populations of India.

MATERIALS AND METHODS

For the analysis, 7 sites including 2 from Mesolithic (8000-4000 bc), 2 from Chalcolithic (2000-700 bc) and 3 from Harappan (3500-1800 bc) contexts are considered. The ratio obtained from the maximum length of the bones to their girth (at 50% or 35% from the distal end) and that from anterior-posterior and medio-lateral diameters were calculated (following Martin and Saller codes, 1957) to understand certain functional adaptations and stress markers on preserved long bones. Independent-sample T-tests and ANOVA were applied to detect mean differences of statistical significance within and between cultures.

RESULTS

The robusticity indices obtained from the humeri increase from the hunting-gathering phase (M/F; 19.39/18.45) to the Chalcolithic phase (M/F; 21.99/19.39) showing a slight drop in the Harappan phase (M/F; 18.58/18.37). The right humeri of Mesolithic females show a directional asymmetry of length (4.08%-5.13%) while that in males reach up to 26.09% indicating right-dominant lateralization. In the Harappan phase, females show a greater value for the right ranging from 0.31% to 3.07%. The femoral robusticity of females increases from 11.42 in hunting-gathering societies to 13.28 in Harappan times while mid-shaft of the Mesolithic population clearly shows greater loading along the anterior-posterior (A-P) plane as the index in most cases exceeds 100. There are significant differences among males and females within each group in terms of the indices but significant differences could be discerned between the Mesolithic, Chalcolithic and Harappan populations in case of tibiae, radii and ulnae only.

CONCLUSION

The occupation patterns certainly changed as humans became a food-producer from a hunter-gatherer. Occupations involving the extensive flexion and extension of upper limbs in the Chalcolithic period increase particularly in the Harappan phase. Sedentism gave rise to roundness of the femur and reduced the magnitude of lateralization. While equal level of mobility can be traced in case of both the sexes before the advent of agriculture, this distinction increases between the males and females in the agro-pastoral phases.

The impact of terrain on lower limb bone structure

OBJECTIVES

Lower limb diaphyseal geometry is often used to evaluate mobility in past populations. Diaphyseal dimensions such as high shape (I_X /I_Y ) indices generally thought to reflect high mobility may also result from walking over rough terrain. This study investigates the possible effects of terrain on lower limb diaphyseal cross-sectional geometric dimensions.

MATERIALS

The sample (N = 3,195) comprises adult skeletons from Europe, Africa, North America, and Asia, spanning from around 30,000 BP to mid-twentieth century.

METHODS

Femoral and tibial shape and bending/torsional strength dimensions were gathered either as part of a previous project or were generously provided by researchers. Local terrain for each site was quantified with ArcGIS mapping software using geographic coordinates and USGS elevation data, and characterized as flat, hilly, or mountainous.

RESULTS

Analysis of variance shows significant differences (p < .05) in midshaft femoral and tibial shape ratio and relative bending/torsional strength among the three terrain categories, with more AP oriented diaphyseal shapes and greater relative strength in hilly and mountainous groups, even after correcting for the effect of subsistence. As expected, the impact of terrain is much more marked for hunter-gatherers and agriculturalists than for more mechanized recent populations. Interestingly, the effect of terrain is confounded in higher latitude individuals that exhibit increased ML bending strength, probably reflecting larger body breadth.

DISCUSSION

This study underscores the mechanical significance of traveling over rough terrain and highlights the complex interactions of mobility, terrain, and body shape that contribute to shaping lower limb bone diaphyseal structure.

Changes in shape and cross-sectional geometry in the tibia of mice selectively bred for increases in relative bone length

Limb bone size and shape in terrestrial mammals scales predictably with body mass. Weight-bearing limb bones in these species have geometries that enable them to withstand deformations due to loading, both within and between species. Departures from the expected scaling of bone size and shape to body mass occur in mammals that have become specialized for different types of locomotion. For example, mammals adapted for frequent running and jumping behaviors have hind limb bones that are long in relation to body mass, but with narrower cross-sections than predicted for their length. The Longshanks mouse was recently established, a selectively bred line of mice with ~12-13% longer tibiae relative to body mass. This increased limb length resembles superficially the derived limb proportions of rodents adapted for hopping and jumping. Here, 3D geometric morphometrics and analyses of bone cross-sectional geometry were combined to determine whether selection for increased relative tibia length in Longshanks mice has altered the scaling relationship of size and shape, and/or bone robusticity, relative to the tibiae of random-bred control mice from the same genetic background. The results suggest that the Longshanks tibia is not a geometrically scaled version of the control tibiae. Instead, the Longshanks tibia has become narrower in cross-section in relation to its increased length, leading to a decrease in overall bending strength when compared with control tibiae. These changes in bone shape and robusticity resemble the derived morphology of mammals adapted for running and jumping, with important implications for the material properties and strength of bone in these mammals.

Limb bone morphology, bone strength, and cursoriality in lagomorphs

The primary aim of this study is to broadly evaluate the relationship between cursoriality (i.e. anatomical and physiological specialization for running) and limb bone morphology in lagomorphs. Relative to most previous studies of cursoriality, our focus on a size-restricted, taxonomically narrow group of mammals permits us to evaluate the degree to which 'cursorial specialization' affects locomotor anatomy independently of broader allometric and phylogenetic trends that might obscure such a relationship. We collected linear morphometrics and μCT data on 737 limb bones covering three lagomorph species that differ in degree of cursoriality: pikas (Ochotona princeps, non-cursorial), jackrabbits (Lepus californicus, highly cursorial), and rabbits (Sylvilagus bachmani, level of cursoriality intermediate between pikas and jackrabbits). We evaluated two hypotheses: cursoriality should be associated with (i) lower limb joint mechanical advantage (i.e. high 'displacement advantage', permitting more cursorial species to cycle their limbs more quickly) and (ii) longer, more gracile limb bones, particularly at the distal segments (as a means of decreasing rotational inertia). As predicted, highly cursorial jackrabbits are typically marked by the lowest mechanical advantage and the longest distal segments, non-cursorial pikas display the highest mechanical advantage and the shortest distal segments, and rabbits generally display intermediate values for these variables. Variation in long bone robusticity followed a proximodistal gradient. Whereas proximal limb bone robusticity declined with cursoriality, distal limb bone robusticity generally remained constant across the three species. The association between long, structurally gracile limb bones and decreased maximal bending strength suggests that the more cursorial lagomorphs compromise proximal limb bone integrity to improve locomotor economy. In contrast, the integrity of distal limb bones is maintained with increasing cursoriality, suggesting that the safety factor takes priority over locomotor economy in those regions of the postcranial skeleton that experience higher loading during locomotion. Overall, these findings support the hypothesis that cursoriality is associated with a common suite of morphological adaptations across a range of body sizes and radiations.