KELLY, D.A.; Univ. of Massachusetts, Amherst: Penile inflation in the red-eared slider (Trachemys scripta).

Turtle penises contain a single erectile structure that inflates with blood during erection, making the penis both larger and more resistant to bending. Like other hydrostats, the turtle erectile structure is reinforced by collagen fibers in its wall. Like the mammalian corpus cavernosum, those fibers are arranged at 0&deg and 90&deg to the penile long axis. But in turtles the wall of the erectile structure surrounds two distinct tissues: the soft and highly vascularized corpus spongiosum and the more collagenous corpus fibrosum. It is not known how these tissue differences affect the mechanical behavior of the penis during erection. Artificial inflation of penile tissue from the red-eared slider (Trachemys scripta) shows that the erectile tissue expands to strains of 48% longitudinally, 10% dorsoventrally and 75% laterally, with most expansion occurring in the corpus spongiosum. Dorsoventral expansion of the erectile structure is limited by a three-dimensional network of collagen fibers inside the corpus spongiosum; the large difference in dorsoventral and lateral expansion produces a highly asymmetrical erect cross section. Because tissue expansion increases the erectile structure�s second moment of area, the erect penis has a larger lateral second moment of area and is harder to bend laterally. Smaller length changes in the corpus fibrosum during inflation cause the penis to bend and open the distal end of the sperm duct.