Positional and Morphological Development of Wild Chimpanzees, Pan Troglodytes.

This project explores the relationship between form and function through an integrated study of chimpanzee osteological and behavioral development. Unlike most mammals, chimpanzees go through several locomotor transitions during development, making them excellent candidates to investigate how changes in behavior influence changes in morphology. The aim of this study was to identify anatomical features that are induced by loading during suspensory and knuckle-walking behavior that can then be used to reconstruct fossil behavior. This task was accomplished by examining the positional behavior of wild chimpanzees at Ngogo, Kibale National Park, Uganda, in tandem with the morphological analysis of wild-caught skeletal specimens from museums. Chimpanzee positional behavior was found to proceed developmentally through a number of distinct stages, each characterized by its own loading regime. Specifically, chimpanzee infants principally used their upper limbs in locomotion and posture with the loading environment changing to hindlimb dominated knuckle-walking as individuals aged. The most dramatic transitions in behavior occurred during juvenility and adolescence. Infants displayed more diversity in their forms of positional behavior than did members of any other age-sex class. Metacarpal morphology and long bone midshaft cross-sectional properties were examined to see if they tracked behavioral changes. The third metacarpal is a weight-bearing conduit in knuckle-walking. Two changes in this bone provide added stability during knuckle-walking: distal ridge formation and increased shaft curvature. Increased curvature and ridge formation occurred at juvenility and adolescence in chimpanzees, the same developmental periods where shifts in behavior occurred. Changes in femoral and humeral bone strength and shape over the course of development were also tracked, since both features are likely influenced by locomotor behavior. A general trend of increased femoral relative to humeral strength was found, in concert with the advent of hindlimb-dominated locomotion. The femur also became more elliptical with age while no such shape change occurred in the humerus. It is proposed that, since the timing of the development of these features occurs together with behavioral transitions, the behaviors themselves are influencing bone development. These features can now be used with confidence to reconstruct locomotion in fossil species.PHDAnthropologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/98074/1/rens_1.pd

Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism

Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees (Pan troglodytes schweinfurthii) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy

Wild chimpanzee behavior suggests that a savanna-mosaic habitat did not support the emergence of hominin terrestrial bipedalism

Bipedalism, a defining feature of the human lineage, is thought to have evolved as forests retreated in the late Miocene-Pliocene. Chimpanzees living in analogous habitats to early hominins offer a unique opportunity to investigate the ecological drivers of bipedalism that cannot be addressed via the fossil record alone. We investigated positional behavior and terrestriality in a savanna-mosaic community of chimpanzees (Pan troglodytes schweinfurthii) in the Issa Valley, Tanzania as the first test in a living ape of the hypothesis that wooded, savanna habitats were a catalyst for terrestrial bipedalism. Contrary to widely accepted hypotheses of increased terrestriality selecting for habitual bipedalism, results indicate that trees remained an essential component of the hominin adaptive niche, with bipedalism evolving in an arboreal context, likely driven by foraging strategy

Skeletal evidence for variable patterns of handedness in chimpanzees, human hunter–gatherers, and recent British populations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98285/1/nyas12067.pd

Metacarpal trabecular bone varies with distinct hand-positions used in hominid locomotion

Trabecular bone remodels during life in response to loading and thus should, at least in part, reflect potential variation in the magnitude, frequency and direction of joint loading across different hominid species. Here we analyse the trabecular structure across all non-pollical metacarpal distal heads (Mc2-5) in extant great apes, expanding on previous volume of interest and whole-epiphysis analyses that have largely focussed on only the first or third metacarpal. Specifically, we employ both a univariate statistical mapping and a multivariate approach to test for both inter-ray and interspecific differences in relative trabecular bone volume fraction (RBV/TV) and degree of anisotropy (DA) in Mc2-5 subchondral trabecular bone. Results demonstrate that while DA values only separate Pongo from African apes (Pan troglodytes, Pan paniscus, Gorilla gorilla), RBV/TV distribution varies with the predicted loading of the metacarpophalangeal (McP) joints during locomotor behaviours in each species. Gorilla exhibits a relatively dorsal distribution of RBV/TV consistent with habitual hyper-extension of the McP joints during knuckle-walking, whereas Pongo has a palmar distribution consistent with flexed McP joints used to grasp arboreal substrates. Both Pan species possess a disto-dorsal distribution of RBV/TV, compatible with multiple hand postures associated with a more varied locomotor regime. Further inter-ray comparisons reveal RBV/TV patterns consistent with varied knuckle-walking postures in Pan species in contrast to higher RBV/TV values toward the midline of the hand in Mc2 and Mc5 of Gorilla, consistent with habitual palm-back knuckle-walking. These patterns of trabecular bone distribution and structure reflect different behavioural signals that could be useful for determining the behaviours of fossil hominins

Systemic patterns of trabecular bone across the human and chimpanzee skeleton

Aspects of trabecular bone architecture are thought to reflect regional loading of the skeleton, and thus differ between primate taxa with different locomotor and postural modes. However, there are several systemic factors that affect bone structure that could contribute to, or be the primary factor determining, interspecific differences in bone structure. These systemic factors include differences in genetic regulation, sensitivity to loading, hormone levels, diet, and/or activity levels. Improved understanding of inter/intraspecific variability, and variability across the skeleton of an individual, is required to properly interpret potential functional signals present within trabecular structure. Using a whole-region method of analysis, we investigated trabecular structure throughout the skeleton of humans and chimpanzees. Trabecular bone volume fraction (BV/TV), degree of anisotropy (DA) and trabecular thickness (Tb.Th) were quantified from high resolution micro-computed tomographic scans of the humeral and femoral head, third metacarpal and third metatarsal head, distal tibia, talus and first thoracic vertebra. We find that BV/TV is, in most anatomical sites, significantly higher in chimpanzees than in humans, suggesting a systemic difference in trabecular structure unrelated to local loading regime. Differences in BV/TV between the forelimb and hindlimb do not clearly reflect differences in locomotor loading in the study taxa. There are no clear systemic differences between the taxa in DA and, as such, this parameter may reflect function and relate to differences in joint loading. This systemic approach reveals both the pattern of variability across the skeleton and between taxa, and helps identify those features of trabecular structure that may relate to joint function

Ontogeny and variability of trabecular bone in the chimpanzee humerus, femur and tibia

Although adult skeletal morphological variation is best understood within the framework of age-related processes, relatively little research has been directed towards the structure of and variation in trabecular bone during ontogeny. We report here new quantitative and structural data on trabecular bone microarchitecture in the proximal tibia during growth and development, as demonstrated in a sub adult archaeological skeletal sample from the Late Prehistoric Ohio Valley. These data characterize the temporal sequence and variation in trabecular bone structure and structural parameters during ontogeny as related to the acquisition of normal functional activities and changing body mass. The skeletal sample from the Fort Ancient Period site of SunWatch Village is composed of 33 sub adult and three young adult proximal tibiae. Non-destructive micro CT scanning of the proximal metaphyseal and epiphyseal tibia captures the micro architectural trabecular structure, allowing quantitative structural analyses measuring bone volume fraction, degree of anisotropy, trabecular thickness, and trabecular number. The micro CT resolution effects on structural parameters were analysed. Bone volume fraction and degree of anisotropy are highest at birth, decreasing to low values at 1 year of age, and then gradually increasing to the adult range around 6-8 years of age. Trabecular number is highest at birth and lowest at skeletal maturity; trabecular thickness is lowest at birth and highest at skeletal maturity. The results of this study highlight the dynamic sequential relationships between growth/development, general functional activities, and trabecular distribution and architecture, providing a reference for comparative studies

A novel experimental design for the measurement of metacarpal bone loading and deformation and fingertip force

Background: Musculoskeletal and finite element modelling are often used to predict joint loading and bone strength within the human hand, but there is a lack of in vitro evidence of the force and strain experienced by hand bones. Methods: This study presents a novel experimental setup that allows the positioning of a cadaveric digit in a variety of postures with the measurement of force and strain experienced by the third metacarpal. The setup allows for the measurement of fingertip force as well. We tested this experimental setup using three cadaveric human third digits in which the flexor tendons were loaded in two tendon pathways: (1) parallel to the metacarpal bone shaft, with bowstringing; (2) a semi-physiological condition in which the tendons were positioned closer to the bone shaft. Results: There is substantial variation in metacarpal net force, metacarpal strain and fingertip force between the two tendon pathways. The net force acting on the metacarpal bone is oriented palmarly in the parallel tendon condition, causing tension along the dorsum of the metacarpal shaft, while the force increases and is oriented dorsally in the semi-physiological condition, causing compression of the dorsal metacarpal shaft. Fingertip force is also greater in the semi-physiological condition, implying a more efficient grip function. Inter-individual variation is observed in the radioulnar orientation of the force experienced by the metacarpal bone, the fingertip force, and the strain patterns on the metacarpal shaft. Conclusion: This study demonstrates a new method for measuring force and strain experienced by the metacarpal, and fingertip force in cadaveric digits that can, in turn, inform computation models. Inter-individual variation in loads experienced by the third digit suggest that there are differences in joint contact and/or internal bone structure across individuals that are important to consider in clinical and evolutionary contexts

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

$\textbf{Objectives:}$ 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. $\textbf{Materials and methods:}$ We apply a whole-bone/epiphysis approach to analyze trabecular and cortical bone in the distal tibia and talus of $\textit{Pan troglodytes}$ and $\textit{Homo sapiens}$. 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. $\textbf{Results:}$ We find that $\textit{Pan}$ has a greater BV/TV, a lower BS/BV and thicker cortices than $\textit{Homo}$ 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 $\textit{Pan}$ compared to $\textit{Homo}$. 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. $\textbf{Discussion:}$ 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.This research was supported by The Max Planck Society and the European Research Council Starting Grant #336301