2676 Introduction Scaling, or the relative change in organismal dimensions, affects many aspects of vertebrate biology. Whether concerning metabolic rates (Taylor et al., 1970; Full and Tu, 1991; Schmidt- Nielsen, 1997), prey capture and food processing mechanisms (Richard and Wainwright, 1995; Herring et al., 2005; Herrel and O’Reilly, 2006), muscle–tendon mechanics (Biewener, 1989; Biewener, 2005; Pollock and Shadwick, 1994), or skeletal support during locomotion (Alexander et al., 1979a; Biewener, 1982; Biewener, 1983; Olmos et al., 1996; Cubo and Casinos, 1998), differences in physiological and biomechanical performance that result from changes in body size are critical to an organism’s success. Although most studies have examined scale effects across an inter-specific size range, changes in size during ontogenetic growth can have important implications for performance in growing animals as well. Specifically, ontogenetic scaling of the musculoskeletal anatomy of the limbs has important consequences for locomotor mechanics and life history strategies. Two studies showed how musculoskeletal allometry and changing material properties of the limb bones through growth can affect locomotor performance at different ontogenetic stages (Carrier, 1983; Carrier and Leon, 1990). In the California gull Larus californicus (Carrier and Leon, 1990), ontogenetic scaling of the musculoskeletal system and increased elastic moduli in the bones of fledgling birds demonstrated the coordinated development of the anatomy required for powered flight with the achievement of adult wing bone length and mineralization. In the highly precocial jack rabbit Lepus californicus, Carrier described another ontogenetic strategy, in which metatarsal length scaled with positive allometry, while muscle mass and mechanical advantage of the gastrocnemius at the ankle and the second moment of area of the metatarsals (a measure of their bending resistance) scaled with negative allometry (Carrier, 1983). From these scaling patterns, Carrier concluded that, for their mass, younger rabbits could generate relatively larger propulsive forces at the ankle without an increased fracture risk of the metatarsals, which were composed of relatively weaker bone tissue than in adult rabbits. These concerted scaling patterns allowed the musculoskeletal system of the young hares to be capable of producing large enough forces to achieve adult escape velocities early in ontogenetic growth. Similar ontogenetic Most studies examining changes in mechanical performance in animals across size have typically focused on inter-specific comparisons across large size ranges. Scale effects, however, can also have important consequences in vertebrates as they increase in size and mass during ontogeny. The goal of this study was to examine how growth and development in the emu (Dromaius novaehollandiae) hindlimb skeleton reflects the demands placed upon it by ontogenetic changes in locomotor mechanics and body mass. Bone strain patterns in the femur and tibiotarsus (TBT) were related to ontogenetic changes in limb kinematics, ground reaction forces, and ontogenetic scaling patterns of the cross-sectional bone geometry, curvature and mineral ash content over a 4.4- fold increase in leg length and 65-fold increase in mass. Although the distribution of principal and axial strains remained similar in both bones over the ontogenetic size range examined, principal strains on the cranial femur and caudal femur and TBT increased significantly during growth. The ontogenetic increase in principal strains in these bones was likely caused by isometry or only slight positive allometry in bone cross-sectional geometry during growth, while relative limb loading remained similar. The growth-related increase in bone strain magnitude was likely mitigated by increased bone mineralization and decreased curvature. Throughout most of ontogeny, shear strains dominated loading in both bones. This was reflected in the nearly circular cross-sectional geometry of the femur and TBT, suggesting selection for resistance to high torsional loads, as opposed to the more eccentric cross- sectional geometries often associated with the bending common to tetrapods with parasagittal limb orientations, for which in vivo bone strains have typically been measured to date. Key words: bone, scaling, ontogeny, growth, bone strain, bone geometry, torsion, emu. Summary The Journal of Experimental Biology 210, 2676-2690 Published by The Company of Biologists 2007 doi:10.1242/jeb.004580 Skeletal strain patterns and growth in the emu hindlimb during ontogeny Russell P. Main* and Andrew A. Biewener Concord Field Station, Department of Organismic and Evolutionary Biology, Harvard University, 100 Old Causeway Road, Bedford, MA 01730, USA *Author for correspondence at present address: Sibley School of Mechanical and Aerospace Engineering, 187 Grumman Hall, Cornell University, Ithaca, NY 14853, USA (e-mail: rpm74@cornell.edu) Accepted 27 March 2007 THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY