In the video above, Brandon Moore from Louisiana Tech University is dissecting a freshly dead male alligator. You’re looking at the creature’s underside, near its hip area. As the sequence begins, Moore’s scalpel touches the alligator’s pelvic nerve. The metal makes the nerve fire and… well… just watch the video.

That’s the alligator’s penis—ten centimetres long, ghostly white, and surprisingly quick-moving. It flips out in an instant and just hangs there for a few seconds to greet a startled Moore, before twanging back inside just as quickly. I love that Moore back-steps and freezes. There’s no audio. I wish there was audio.

The video was a revelation to Diane Kelly from the University of Massachusetts, and the key to interpreting the utterly bizarre penis of the American alligator. Here are the highlights: it’s permanently erect; it shoots out like toothpaste from a tube; and it bounces back because it basically has a rubber band attached to it. “It is really weird,” says Kelly. “Really weird.”

View Images American alligator Photography by Sarah Towne

That’s not something she’d say lightly. Animal penises are her speciality. She has studied and dissected the male organs of mammals and turtles to better understand their anatomy and their evolutionary history. Meanwhile, her close colleague Patricia Brennan—“the other penis lady”—had done a lot of work on birds, snakes and lizards (remember the infamous duck penis video?). The crocodilians—crocodiles, alligators and their kin—seemed like the obvious group to target next.

Here’s what we knew about their penises before Kelly started. The males have a single large phallus stashed within the cloaca. That’s the joint opening you can see in the video above, which also gives way to the urinary and digestive tracts. Sperm travels along a deep groove running the length of the penis, which is surrounded by dense tissue. Okay, but how does a crocodilian inflate its penis or eject it from the cloaca?

To find out, Kelly needed to get her hands on some crocodilians. Fortunately, she didn’t have to work with living ones. In September, she headed down to the Rockerfeller Wildlife Refuge in Louisiana, which has an annual cull to control the local population of American alligators. They let her work with four of the animals that had been shot, including a huge four-metre-long bull. “It was really scary, even though it was dead!” says Kelly.

Through dissection, she discovered that the alligator’s penis doesn’t inflate at all. It’s permanently erect. The whole structure is filled with dense layers of a stiff protein called collagen. Even the central bit, where blood would normally flow down, is just collagen, collagen and more collagen.

That’s bizarre. Whenever a reptile, bird or mammal has a penis (and some don’t), it always inflates in some way. Maybe blood pumps into it, as in humans, other mammals, and turtles. Maybe it explosively turns inside-out like in ducks and some lizards or snakes. Either way, some shape-changing occurs. But not in alligators—when Kelly tried to artificially inflate the penis by pumping saline, it didn’t change in either length or diameter.

So, if the penis doesn’t inflate, how does the male alligator extrude it from the cloaca? Muscles would be the obvious answer, but continuing the theme of “really weird”, the penis has no muscles attached to it. It’s almost free-floating. One pair of muscles—the levator cloacae—cradle the penis like a sling, but doesn’t actually connect.

This stumped Kelly until she saw Moore’s video. She now thinks that when the levator cloacae contract, they force the penis out by squeezing the cloacal chamber. “This squishes the whole penis and pops it out of the vent,” she says. Indeed, when Kelly yanked on the levator cloacae by hand, the penis of her dead alligator leapt into the outside world.

I said the penis is almost free-floating. It’s got a tendon that connects to its middle, which helps to swivel it forwards when it pops out. Its base is also attached to the hip bone by a pair of large ligaments, that are “the consistency of the big rubber bands the post office uses,” says Kelly. When the penis comes out, these rubber bands are stretched. As soon as the levator cloacae relax, the elastic ligaments yank the penis back in.

View Images Alligator penis. Photograph by Diane Kelly

This is just a hypothesis. Kelly now wants to do some tests of the muscles, tendons and ligaments to check that the “ones I think are everting the penis are actually the ones that are doing this”. Then, it’s time to look at living crocodilians to see how they actually use their penises, and how they might benefit from a permanently erect organ.

I leave you with this—an autographed cocktail napkin on which Kelly sketched out the alligators’ genital tract for me at a bar in Raleigh, North Carolina. She would like me to note that is it not to scale.

View Images Cocktail napkin with schematic of alligator gentalia (Not to scale). Drawing by Diane Kelly; photograph by Ed Yong

Reference: Kelly. 2013. Penile Anatomy and Hypotheses of Erectile Function in the American Alligator (Alligator mississippiensis): Muscular Eversion and Elastic Retraction. Anatomical Record