Most birds, reptiles, and mammals have penises that become erect by filling with fluid. For example, human penises become erect when engorged with blood. One exception to this rule is the alligator penis, which does not change shape or stiffness before sex. How then, I’m sure you’re wondering, does an alligator get an erection? This paper proposes that specific muscles rotate and pop the perpetually-stiff alligator penis out of the cloaca before sex. (The cloaca is the awesome reptilian orifice used for sex, poop, and pee.) One the deed is done, relaxation of these muscles allows the penis to return inside, safe and sound. Pretty handy!
Penile Anatomy and Hypotheses of Erectile Function in the American Alligator (Alligator mississippiensis): Muscular Eversion and Elastic Retraction.
“The intromittent organs of most amniotes contain variable-volume hydrostatic skeletons that are stored in a flexible state and inflate with fluid before or during copulation. However, the penis in male crocodilians is notable because its shaft does not seem to change either its shape or bending stiffness as blood enters its vascular spaces before copulation. Here I report that crocodilians may have evolved a mechanism for penile shaft erection that does not require inflation and detumescence. Dissections of the cloaca in sexually mature male American alligators (Alligator mississippiensis) show that the cross section of the proximal shaft of the alligator penis contains dense collagenous tissues that do not significantly change shape when fluid is added to the central vascular space. The large amount of collagen in the wall and central space of the alligator penis stiffen the structure so it can be simply everted for copulation and rapidly retracted at its completion. Because no muscles insert directly onto the penis, eversion and retraction must be produced indirectly. My results suggest that the contraction of paired levator cloacae muscles around the anterior end of the cloaca rotates the penis out of the cloacal opening and strains the ligamentum rami that connect the base of the penis to the ischia. When the cloacal muscles relax, the elastic recoil of the ligamentum rami can return the penis to its original position inside the cloaca.”
Photo: flickr/dental ben