MAIN, R.P.*; BIEWENER, A.A.; Harvard University; Harvard University: Ontogenetic patterns of bone strain in the emu femur: Do bone strains remain similar as animals grow?

Vertebrates vary in size, both through ontogeny and across phylogenetically different taxa. As animals grow through ontogeny, not only do the physical demands placed upon the skeleton change, but depending upon the ontogenetic scaling patterns of the shapes and sizes of the limb elements, a bone�s ability to withstand the mechanical demands placed upon it may change as well. This could affect the bone�s safety factor and possibly result in different behaviors in the animal during growth, in response to certain selective pressures. Using emu (Dromaius novaehollandiae), this study�s aim was to determine how limb loading and bone geometry relate to the magnitudes and distribution of bone strains in the femur through ontogeny. This was done through the examination of ground reaction forces, in vivo bone strain measurements, and midshaft cross-sectional bone geometry. Although relative limb loads did not change with growth, bone strains in the femur increased for both walking and running gaits with an ontogenetic increase in mass. Although strain magnitudes changed, the femur was, however, consistently loaded primarily in torsion throughout growth. These results are consistent with the ontogenetic scaling patterns of the cross-sectional geometry of the femur. The cross-sectional area (negative allometry), second moments of area (isometry), and polar moment of inertia (isometry) all scale significantly below the relationships expected if the femur were to maintain strain similarity through ontogeny given similar relative limb loads. Thus, as a result of the femur�s decreased structural ability to resist mechanical loads during growth, younger, smaller birds have lower femoral strains than older, larger birds.