Ontogenetic changes in limb bone structural proportions in mountain gorillas (Gorilla beringei beringei)

Cross-sectional structural variation relative to midshaft along hominine diaphyses. II. The hind limb

OBJECTIVES

In comparative analyses of hominine hind limb diaphyseal structure, homologous cross sections are located according to half bone length (midshaft). Here, we address three questions. First, how accurately must midshaft be defined to yield comparable data? Second, does variation in midshaft location due to different ways of measuring length fall within error ranges such that data gathered using different metrics are comparable? Third, do error ranges and length metric effects differ between elements or taxa such that certain bones or species are more prone to issues of comparability?

MATERIALS AND METHODS

Femora and tibiae of Homo, Pan, and Gorilla were CT-scanned longitudinally and error ranges for multiple structural parameters (CSA, J, Imax /Imin ) were calculated around midshafts.

RESULTS

Distances proximally and distally from midshaft where structural values differ significantly from midshaft values vary between bones, species, and structural traits. Femoral error ranges are typically larger than tibial ranges. In the femur, error ranges are generally largest for chimpanzees and smallest for gorillas. A similar taxonomic pattern is not evident in the tibia. No structural trait consistently displays larger or smaller error ranges across both elements and all species. Variation in midshaft locations stemming from different length definitions is small and falls within observed error ranges defined by any one metric.

DISCUSSION

Incorporating fragmentary specimens (e.g., fossils) for which midshaft location is unknown in comparisons of diaphyseal structure necessitates evaluation on a case-by-case basis, with thought to element, taxon, and structural traits of interest. Midshaft data recorded from distinct length measurements are generally comparable.

Wing bone laminarity is not an adaptation for torsional resistance in bats

Torsional loading is a common feature of skeletal biomechanics during vertebrate flight. The importance of resisting torsional loads is best illustrated by the convergence of wing bone structure (e.g., long with thin walls) across extant bats and birds. Whether or not such a convergence occurs at the microstructural level is less clear. In volant birds, the humeri and ulnae often contain abundant laminar bony tissue in which primary circumferential vascular canals course concentrically about the long axis of the bone. These circumferential canals and the matrix surrounding them presumably function to resist the tissue-level shear stress caused by flight-induced torsion. Here, we assess whether or not laminar bone is a general adaptive feature in extant flying vertebrates using a histological analysis of bat bones. We sampled the humeri from six adult taxa representing a broad phylogenetic and body size range (6–1,000 g). Transverse thick sections were prepared from the midshaft of each humerus. Bone tissue was classified based on the predominant orientation of primary vascular canals. Our results show that humeri from bats across a wide phylogenetic and body size range do not contain any laminar bone. Instead, humeri are essentially avascular in bats below about 100 g and are poorly vascularized with occasional longitudinal to slightly radial canals in large bats. In contrast, humeri from birds across a comparable size range (40–1,000 g) are highly vascularized with a wide range in bone laminarity. Phylogenetically-informed scaling analyses reveal that the difference in vascularity between birds and bats is best explained by higher somatic relative growth rates in birds. The presence of wing bone laminarity in birds and its absence in bats suggests that laminar bone is not a necessary biomechanical feature in flying vertebrates and may be apomorphic to birds.

The influence of life history and sexual dimorphism on entheseal changes in modern humans and African great apes

Entheseal changes have been widely studied with regard to their correlation to biomechanical stress and their usefulness for biocultural reconstructions. However, anthropological and medical studies have demonstrated the marked influence of both age and sex on the development of these features. Studies of entheseal changes are mostly aimed in testing functional hypotheses and are mostly focused on modern humans, with few data available for non-human primates. The lack of comparative studies on the effect of age and sex on entheseal changes represent a gap in our understanding of the evolutionary basis of both development and degeneration of the human musculoskeletal system. The aim of the present work is to compare age trajectories and patterns of sexual dimorphism in entheseal changes between modern humans and African great apes. To this end we analyzed 23 postcranial entheses in a human contemporary identified skeletal collection (N = 484) and compared the results with those obtained from the analysis of Pan (N = 50) and Gorilla (N = 47) skeletal specimens. Results highlight taxon-specific age trajectories possibly linked to differences in life history schedules and phyletic relationships. Robusticity trajectories separate Pan and modern humans from Gorilla, whereas enthesopathic patterns are unique in modern humans and possibly linked to their extended potential lifespan. Comparisons between sexes evidence a decreasing dimorphism in robusticity from Gorilla, to modern humans to Pan, which is likely linked to the role played by size, lifespan and physical activity on robusticity development. The present study confirms previous hypotheses on the possible relevance of EC in the study of life history, pointing moreover to their usefulness in evolutionary studies.