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, ...
