Axial orthogonal fiber reinforcement in the penis of the nine-banded armadillo (Dasypus novemcinctus).

Examination of histological sections from flaccid and artificially erected nine-banded armadillo (Dasypus novemcinctus) penises confirms that the mammalian corpus cavernosum is the first known biological hydrostat reinforced by collagen fibers arranged at 0 degree and 90 degrees to its long axis. The morphology of this axial orthogonal fiber array affects the mechanical behavior of mammalian penises during erection and copulation. Specifically, the axial orthogonal array gives the erect penis a reproducible shape, maximum size and resistance to tensile, compressive, and bending forces. These features are more appropriate for the mechanical regime associated with copulation than those found in structures reinforced by crossed-helical fibers, although the axial orthogonal array also gives the corpus cavernosum a tendency to fail by kinking. Crimped collagen fibers in the flaccid array as well as three-dimensional folding of the wall in the flaccid corpus cavernosum allow the structure to expand during erection.