After an exhaustive study of all varieties of locomotion in both living and mechanical systems, two researchers have concluded this month that the same set of engineering principles applies to all systems capable of independent motion. The study, which was carried out by engineering professor Adrian Bejan of Duke University and professor James Marden, a biologist at Penn State University, builds on a 1996 theory proposed by Bejan. Bejan's "constructal theory" states that "For a finite-size system to persist in time [to live], it must evolve in such a way that it provides easier access to the imposed currents that flow through it." For example, the current that flows through an IC package is heat. If the package does not provide an efficient structure for dissipating that heat, the system will overheat and cease to function. In another example, in the circulatory system of the body, blood flow must be optimized by the geometric configuration of blood vessels and capillaries and arteries. The theory aims to be as fundamental as thermodynamics, providing basic design principles rooted in physical laws. Constructal principles apply across virtually any functioning system-biological or man-made. In 2004, Bejan began cooperating with Marden to extend the theory from its origins in comparing airplanes and the flight of birds, moving to comparisons of the gaits of animals and robots. This month Bejan and Marden reported that all forms of locomotion obey the same ratios between the energy destroyed at each step or flap and the energy lost to friction against the ground or air. Even swimmers obey the same body-mass scaling principles as runners or fliers, whether they're living or robotic.