The nonlinear, compliant, and inhomogeneous materials found in even the simplest animals provide them with a sophistication and robustness that human made devices cannot match. Natural biomaterials produce compliant and viscoelastic structures that can be stronger than steel, more elastic than synthetic rubber, and can bend without breaking. This is in contrast to the stiff, homogeneous, and isotropic design of most engineered structures. The main goal of our research is to understand how the mechanical properties of the structures used for animal movement, feeding, and reproduction arise from their component parts. We apply engineering techniques to an array of animals and behaviors to extract general rules regarding how body structure affects mechanical function in nature. Understanding the principles of biological material design across multiple levels of organization can provide insights into how these materials allow for and even enhance whole body performance. Such principles can then be used to inspire and improve the design of robots and medical prosthetics.