KELLY, D. A.: The Functional Morphology of Penile Erection: Tissue Designs That Make the Floppy Stiff

Functional morphology focuses on how biological structures work in the natural world; the specifics of how any structure works depends in part on both the mechanical properties of its materials and how those materials are arranged within the structure. For example, the tissue surrounding the penile vascular spaces of both mammals and turtles is made up primarily of collagen fibers. The tensile properties of collagen let the tissue become stiffer as it extends during penile erection. But the specific organization of the collagen fibers also has a profound effect on the mechanical behavior of the entire penile hydrostatic skeleton. Because the fibers are arranged in an axial orthogonal array, the erect penis is able to resist bending and shape changes during copulation — unlike most biological hydrostats, which are flexible due to their arrangement of inextensible fibers in left- and right- handed helices. In the case of the penis, a low resistance to bending when erect could constitute a mechanical failure of the structure, because its primary function in both mammals and turtles is to enter the female reproductive tract for insemination.