Research by doctoral candidate Ashley Hammond (University of Missouri-Columbia), under the supervision of Dr. Carol Ward, will identify how hip joint mobility relates to locomotor behavior and anatomical form in living anthropoids (apes and monkeys) in order to reconstruct behavioral capabilities in fossil apes from the evolutionarily critical Miocene time period (ca. 23-5 million years ago). Below-branch suspensory behaviors distinguish modern apes from most monkeys, and the emergence of great ape-like locomotor behaviors has become the defining issue in reconstructing how apes evolved. Behavioral capabilities are notoriously difficult to infer in fossil apes, however, because most fossil apes display unusual combinations of primitive and modern anatomies.
Suspensory behaviors are hypothesized to require high hip joint mobility, providing a method for evaluating suspensory abilities in fossils based on joint function, but the influence of soft tissues on range of motion must be characterized prior to reconstructing joint movement from just fossil bones. This study tests the hypothesis that suspensory anthropoids have larger ranges of hip mobility than non-suspensory anthropoids by measuring passive range of motion on a large sample of living (in vivo) apes and monkeys. The measures collected from live animals are then statistically compared to range of motion estimates from virtual models that rely on bony anatomy to limit joint movement. The validated model of joint movement is then used to assess the influence of anatomical variation on hip joint mobility and applied to all available fossil apes in order to identify whether they were adapted for using suspensory behaviors.
This study establishes a method to test locomotor hypotheses in fossil apes, providing critical data for evolutionary scenarios of human origins. The shape analysis techniques developed here will be useful for all vertebrate morphologists, and the validated virtual models will serve as a comparative model for researchers to approach joint function in any mammalian species. More broadly, this project will identify how specific variation in hip joint anatomical form affects mobility, which can be integrated into clinical models of hip function. This project provides training opportunities for undergraduate students, including minorities, and is supporting the research of a female doctoral student being mentored by a female scientist.
Moving through the trees by climbing, hanging and swinging below the tree branches distinguishes modern apes from most monkeys, and is thought that these suspensory locomotor behaviors represent a defining change in the evolutionary history of apes. However, past work suggests that fossil apes may not have shared this distinctive locomotor behavior. Determining when and where suspensory locomotion originated are key to understanding how apes evolved, and, because we evolved from apes, is also key to reconstructing what kind of ancestor humans evolved from. Unfortunately, the fossil record is incomplete and many fossil species are represented only by a few bones, requiring that paleontologists infer behavior from isolated or fragmentary remains. One bone commonly found is the femur (thigh bone), which forms part of the hip joint. This is important, because it is thought that when apes are moving through the trees below branches, they have to move their feet and legs in many directions to reach the next branch, and so apes should show adaptations of the bones of their hips related to high joint mobility. However, no study has tested whether apes do have a greater range of motion at the hip than non-suspensory monkeys. If the hip can indeed reveal the extent to which fossil apes were adapted to moving below branches, this could be an important tool for learning more about how and why apes evolved. The overarching goal of this study was to evaluate whether adaptations for suspensory behaviors can be detected in fossil apes from the hip joint. Funding from the National Science Foundation was used to (1) test the hypothesis that increased hip joint mobility is related to suspensory behaviors in living species of primates, (2) test whether modeling hip mobility virtually from the bones of the hip can be done accurately, and (3) test locomotor reconstructions of fossil apes based on their simulated hip mobility. First, range of hip mobility was quantified in captive anesthetized primates. This study found that suspensory apes have increased ranges of hip abduction and lateral rotation compared to non-suspensory species of monkey. Second, hip joint abduction was reconstructed in apes and monkeys using digital scans of pelves (hipbones) and femora (thigh bones). As in the living animals, the suspensory species were found to have increased ranges of hip abduction. Third, based on the framework established in the second part of the study, range of hip abduction was simulated in early Miocene ape Proconsul nyanzae (KNM-MW 13142) and later Miocene fossil ape Rudapithecus hungaricus (undescribed fossil materials). Range of abduction in Proconsul nyanzae was consistent with non-suspensory monkeys, which is consistent with its above-branch quadrupedal locomotor reconstruction. Hip abduction in Rudapithecus hungaricus was exclusively in the range of extant suspensory anthropoids and was most similar to the values observed in the lesser apes (gibbons and siamangs) and an unusual type of suspensory monkey from South America (spider monkeys). This study provides the first evidence for suspensory behavior in a fossil ape based on joint function, and demonstrates the forelimb-dominant locomotor behaviors can be inferred using hip joint mobility. Taken together, the findings of this research provide important information for reconstructing the timing and emergence of suspensory behaviors within the crown group of apes. This study will be incorporated into the broader picture of primate evolution and will be an important contribution to our understanding of the evolutionary scenarios for how and why apes and humans evolved such unusual and specialized locomotor adaptations. This award has supported the research of an early career female scientist being mentored by a female scientist, and data generated from this project has provided long-term research opportunities for graduate, undergraduate, and high school students working with the PI and Co-PI. Data from this award has been included in public scientific talks given by the student researcher at various educational institutions in the United States and abroad. This research has also been disseminated to broad audiences through the use of non-technical literature, online demonstrations, and radio interviews.
