More than 10,000 reptile species in every shape and size scurry and slither across Earth, from garter snakes to crocodiles — and their nether regions are just as wildly varied.

From spiky penises to multiple penises—or none at all—reptiles have some of the most unusual mating gear on Earth. (Learn how lizard genitalia became a black-market craze.)

Most of the current knowledge of reptilian genitalia focuses on male parts, but scientists are turning more and more to the study of their female counterparts—it’s just a bit trickier since female parts are situated inside the animal.

Two Is Better Than One

Snakes and lizards have not just one, but two penises, called hemipenes.

University of Sydney researcher Christopher Friesen says having two hemipenes may benefit males during mating.

View Images A close-up view shows the two hemipenes of a male Cape cobra (Naja nivea) in South Africa. Photograph by Tony Phelps, NaturePL

“In the case of the garter snakes I have studied, being able to mate from on one side of the female or the other might be useful in intense mating competition,” he says.

That’s because the snakes form what Friesen calls “massive mating aggregations” in which from five to 100 males compete for a single female by searching for her within the mating ball. “It might be advantageous to be flexible about which side of the female a male finds himself, because they use the hemipenis closest to the female to mate.” (Read about a pit filled with 75,000 mating garter snakes.)

Too Much Sex Is Killing These Male Snakes Watch thousands of snakes mate—an act so strenuous that it shortens the lives of the males. Video courtesy Christopher R. Friesen, University of Sydney.

And it’s not just males that are doubling up. Female lizards and snakes have two clitorises, or hemiclitores, which were first described in female monitor lizards in 1995 by W. Böhme as “miniaturized mirror images of the hemipenes of the males.”

Hemiclitores and their functions are still “poorly known” but should be considered a defining characteristic of female lizards and snakes, researchers Soledad Valdecantos and Fernando Lobo of the Universidad Nacional de Salta in Argentina reported in a 2014 study.

Or None is Better Than One

On the other hand, the tuatara—a reptile that resembles a lizard—has no penis at all.

View Images Two tuatara, Sphenodon punctatus, at the Wellington Zoo. The males, like many birds, do not have penises at all. Photograph by Joel Sartore/National Geographic Photo Ark

Instead, the male simply mounts the female and places the opening of his cloaca—the cavity where the intestinal, genital, and urinary tracts meet in reptiles—over hers. This allows him to transfer his sperm into her cloaca.

Most birds mate in similar fashion—the majority of male fowl is sans-phallus and instead deliver sperm by rubbing cloaca to cloaca. One notable exception is found in ducks, which have famously long and corkscrew-shaped penises and vaginas.

Fleeting Female Penises

Researchers in Australia made a curious discovery last year while studying embryos of the central bearded dragon, a large lizard that lives in dry environments. It turns out that females temporarily grow the lizard equivalent of a penis while still in the egg.

“The way these females grew hemipenes, the equivalent of a mammalian penis, was decidedly weird,” researcher Vera Weisbecker of the University of Queensland says in a release from Science Daily.

View Images An eastern bearded dragon, Pogona barbata, at the Auckland Zoo. The females of a related species, the central bearded dragon, grow hemipenes. Photograph by Joel Sartore/National Geographic Photo Ark

The females lost their hemipenes as they got close to hatching. This research adds to what Weisbecker calls “scant knowledge” of the genitalia of female reptiles.

Lizards with Spikes

The hemipenes of lizards and snakes sport tiny spikes and hooks.

Scientists have a few ideas about why hemipenes exhibit this sort of ornamentation. According to one hypothesis, male and female genital form has adapted so that mating can occur only between a male and female of the same species. The genitals of males and females of the same species fit together, and the spikes and hooks could help the male keep his hemipenis in place during mating.

View Images A northwestern garter snake, Thamnophis sirtalis, at the Oregon Zoo. Garter snakes, like other snakes and lizards, have two penises called hemipenes. Photograph by Joel Sartore/National Geographic Photo Ark

One study found that those spikes and hooks may also increase the duration of copulation, thereby increasing mating success.

“I would stress that the female genitalia need to be studied in order to fully understand the function of elaboration of the male genitalia,” says Frieson.

Alligators That Are Always Erect

Male American alligators are always prepared. Most—if not all—other animal penises inflate from a flaccid state, but gators keep theirs permanently erect.

And that’s not all—the penis shoots out from inside the animal’s body and then bounces back like a rubber band. Diane Kelly, a researcher and animal phallus expert from the University of Massachusetts, told National Geographic in 2013 that this “permanently erect bungee” penis is “really weird.”

View Images The male saltwater crocodile, like the American alligator, also keep its penis erect but tucked inside its body. Photograph by Joel Sartore/National Geographic Photo Ark

Kelly has a theory about how a permanently erect “almost free-floating” penis with no muscle attached to it is able to extrude from the animal’s cloaca. After seeing this video, she hypothesized that a pair of muscles cradling the penis—the levator cloacae—contract to force the penis out.

So far, the phenomenon has also been observed in Nile and Australian saltwater crocodiles. “We expect that it’s true of all crocodilians,” Kelly says via e-mail.

She’s still working to get live, sexually mature specimens to test her hypothesis. Kelly notes that while the penis itself is permanently erect, the glans—that bulbous structure at the tip—does inflate, and each species has its own look and shape.