KEMP, T/J; BACHUS, K/N; NAIRN, J/A; CARRIER, D/R*; University of Utah; University of Utah; University of Utah; University of Utah: Mechanical Properties of Limb Bones in Dogs Specialized for Running Versus Fighting

Anatomical and physiological specialization for fighting has been suggested to conflict with specialization for rapid and/or economical running. Such a tradeoff may exist in the evolution of limb bones. Limb bones of animals specialized for running are expected to be lightweight and composed of stiff tissue to transmit forces economically, whereas limb bones of animals specialized for fighting are expected to be composed of tissue with higher work of fracture to resist failure (Currey, 1979). To address this, we compared the mechanical properties of the limb bones of two breeds of the domestic dog that have undergone strong selection for fighting, Pit Bulls, and for high-speed running, Greyhounds. Bones were obtained from donated cadavers. The bones were loaded to failure in three-point static bending. Because the bone specimens had different lengths, the bending tests were analyzed using a corrected beam theory that accounts for the effects of shear stress on deformation. The work required to fracture the bones was the most dramatic difference observed between the breeds. For the humerus, radius, femur and tibia, 1.9- to 2.6-fold more energy was required to break the bones of the Pit Bulls than to break those of the Greyhounds. The breeds also differed in the elastic modulus of their bone tissue. The stiffness of the tissue was 1.5- to 2.4-fold greater in the four proximal limb bones of the Greyhounds than in the Pit Bulls. Significant differences were not observed in the long bones of the feet, metacarpals and metatarsals. Nevertheless, the differences observed in the four proximal bones are consistent with specialization of the limb skeleton for high-speed running in Greyhounds and for fighting in Pit Bulls.