Understanding male and female differences (dimorphism) in body size is critical for determining the biological meaning of skeletal variation in living and extinct species. Furthermore, dimorphism provides some of the only evidence for reconstructing behavior in extinct species. Postcrania offer critical information about body size and size dimorphism in extinct species. However, given the irregular form of most postcranial bones and fragmentary nature of the fossil record, the types of data that can be gathered and analyzed have been limited. This study will employ new laser-scanning technology to capture postcranial bone form in 3-D point clouds, and develop techniques for assessing surface form using POLYWORKS analytical software. Data will be gathered for a comparative sample of 27 primates, with a focus on measuring true joint surface areas, bone-cross sectional form, and volumes. Further geometric measurements will be developed, with an emphasis on identifying male and female differences in bone curvature, joint surface orientations and topography, and the size of muscle attachment areas. Analyses will quantify patterns of sex differences within and across species, evaluate the relationship between skeletal and body size dimorphism, and develop methods for reconstructing size dimorphism from skeletal remains. Results from this project will significantly improve our ability to characterize and evaluate aspects of bone form that have been inaccessible using traditional techniques. The comparative data base will allow a more refined understanding of the evolution of size dimorphism and postcranial morphology in extinct species. The broader impacts of this research include opportunities to train students in cutting edge technology for comparative studies of skeletal form, and the development of methodologies which offer enormous potential for future biomechanical and comparative studies in primates and humans in biological and clinical fields. Finally, scanned data will be made available to other researchers, offering opportunities for further analysis and potentially reducing wear and tear on irreplaceable museum collections by permanently recording data in a non-invasive manner.
It is well known that on average men are larger, stronger and heavier than women. However, compared other primates (monkeys and apes), our closest living relatives, the size differences between men and women are relatively modest. Among primates large size differences between males and females are usually associated with intense, aggressive competition among males for access to mating partners. In spite of the fact that the majority of human societies allow polygyny, most modern humans are effectively monogamous. Though men are highly competitive and even violent, and matings outside of monogamous unions certainly occur, the relatively modest size differences between the sexes is consistent with a lack of pervasive polygyny and aggressive male fighting to exclude other men from access to mates. Interestingly, the fossil record of hominins, the closest relatives and likely ancestors of modern humans, strongly suggests that our ancestors showed very large size differences between males and females, with some being gorilla-like in this regard. This implies that the modern human mating system is derived from an intensely polygynyous mating system, where males compete and fight to exclude other males from access to mates. This observation is important, because it implies that what makes us human in our social and mating behavior, especially with regard to male violence, involved a fundamental change from a very intensely polygynous system, to one where pair-bonds between males and females are the general norm. Because of this, there is a strong interest in understanding how size-dimorphic our ancestors were, and how the modern pattern of sexual size difference arose. Unfortunately, assessing size differences in the fossil record is complicated by the fact that most fossils are fragmentary, and that many of the bones that are best for inferring body size do not preserve the landmarks that allow us to measure them. Furthermore, even though the limb bones are known to be associated with body size, it is not known how well measurements of various limb bones reflect body size differences between the sexes. This project sought to document patterns of male and female size differences in living primates using new 3D laser scanning technology, to allow us to measure functionally and anatomically important features of bones (joint surface areas, bone shaft cross sectional shapes and sizes) that previously were difficult or impossible to measure. The new technology also allowed us apply new methods for assessing size differences between the sexes in fossil samples. We laser scanned over 2000 individual limb bones of primates. Through collaboration with Carol Ward at the University of Missouri, Columbia, we also secured supplemental funding that allowed us to scan all available fossil hominin limb bones from East Africa and South Africa. We have been able to demonstrate that 3D cad software can be used to accurately estimate joint sizes from fragmentary fossil remains, and so have increased to available sample of size estimates of fossil hominins by 42% simply by applying our methods to previously unmeasurable specimens. We have demonstrated that sex differences in the limb bones of primates are not uniform in expression, and especially that joint surfaces yield far more stable estimates of size dimorphism than estimates derived from other portions of the bones. We have also developed techniques for comparing the size and shape of disparate, fragmentary elements of long bones that preserve no landmarks. Results from this project have had applications far beyond the assessment of sex differences in limb bone size and fossils. Data and techniques developed for this project have formed the basis for one dissertation evaluating the functional anatomy of hip joints in primates, and another looking at limb function in crocodyles. The methods developed for this project are being applied to studies of other fossils, including new ape and human ancestors. Methods and data derived from this project have also been fundamental to a collaboration with orthopedic physicians studying the use of 3D cad software to design and implement custom hip joint replacements to improve patient outcomes. Finally, the data derived from this project will ultimately help museums and educators by providing public access to visualize specimens housed in museum collections, and will assist researchers around the world by providing access to high-resolution 3D models of limb bones that can be used in research without the cost and expense of visiting museums. In the process of this grant, we have provided training and experience in laser scanning and analytical 3D cad software to 28 undergraduate and graduate students.
