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First published online July 20, 2007
Journal of Experimental Biology 210, 2676-2690 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.004580

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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{at}cornell.edu)

Accepted 27 March 2007

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

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