The juvenile stage of life can be particularly perilous. Immature animals must survive in the same environments as adults despite smaller body size, weaker muscles and other growth-related limitations on physical ability. Because, by definition, juveniles have yet to reproduce, one should expect strong selection for mechanisms that could potentially offset these age-related limits on physical performance, allowing individuals to reach adulthood, reproduce, and thus maintain evolutionary fitness. The proposed research will combine measures of musculoskeletal growth, physical performance, and survivorship in eastern cottontail rabbits (Sylvilagus floridanus) to explicitly test broad-scale hypotheses about the adaptive nature of mammalian growth and development. Eastern cottontails are independent of their mothers by three weeks of age and experience high predation pressure during the first year of life, making them an ideal species in which to address these issues. The fundamental data collected in this study will provide a greater understanding of how natural selection operates on musculoskeletal growth and development in response to predation, providing novel insight into the process of evolution itself. This research is organized into three specific aims. First, researchers will collect detailed measures of musculoskeletal growth in eastern cottontails, including detailed data on muscle and bone strength. Second, the investigators will generate quantitative measures of physical performance ability (i.e., acceleration and sprinting capacities) in juvenile and adult rabbits. Finally, radio-tracking data will be combined with ecological data on home range size, habitat quality and predation risk to empirically document survivorship in juvenile rabbits. These data will allow the researchers to holistically examine associations among musculoskeletal anatomy, physical performance and evolutionary fitness, as required to formally test the adaptive significance of the traits thought to promote juvenile survival. This project will create interdisciplinary collaborations among the three institutions involved, helping to foster a network of research and training in northeastern Ohio. Funding will also provide opportunities for undergraduates to learn in vivo biomechanical and field ecology methods. Such opportunities are rare and constitute a major educational resource for students. Skeletal materials gathered during this project will be donated to local museums and schools, providing valuable public educational and research tools. Finally, the survivorship estimates generated during this research can be used to inform the policy decisions of the Ohio Department of Natural Resources in order to establish acceptable numbers for hunting and trapping throughout Ohio.