Snakes have clitoris after all, study finds

RResearchers pay a lot of attention to penises, historically speaking. This is especially true in snakes, because scientists know a lot about the male reproductive organs of animals, but relatively little on female organs, especially the clitoris. This led some scientists to conclude that the structure had little to do with mating in snake species, and some doubted it existed.

Now, a study published today (December 13) in Proceedings of the Royal Society B sets the record straight: the clitoris of the snake does not only exist, it can also play an important role in reproduction. Researchers from the University of Adelaide have, for the first time, described the striking diversity of snake hemiclitores in nine snake species. In doing so, they provided evidence that the structure is widespread among snakes and likely serves a vital function. This study adds to the growing pile of evidence that the clitoris is probably no less important than the penis when it comes to copulation in the animal kingdom.

“The study is really interesting,” says Soledad Valdecantos, a biologist at the National University of Salta in Argentina who was not involved in the study, in an interview in Spanish. She says she wasn’t surprised to learn that snakes have a clitoris, because “the presence of the hemiclitoris in reptiles is very common.” However, she adds that “[i]It’s super important because it’s the first study where we see the internal morphology.

Since the field began in the 1800s, researchers have spent two centuries studying and documenting the remarkable diversity of snake and lizard penises, actually called “hemipenis” (singular: hemipenis) due to the fact that they have two penises that are connected in a branched structure. .

See “Evolution of the penis”

By contrast, the topic of female reproductive organs, even in snakes, has long been considered ‘taboo’, says University of Adelaide graduate student Megan Folwell, co-author of the study. Female hemiclitores in reptiles, which are thought to derive from the same ancestral organ as animal hemipenes and also occur in a paired structure, were only described in lizards in 1995, and since then there only a few studies have been done on the subject, she says. Researchers’ focus on hemipenes shaped their conclusions about snake sex and reproduction. Scientists have speculated that the clitoris was vestigial or even lost in some species of scaly reptiles.

“It’s good that the research community is starting to look at the other side of the story,” Folwell says.

Folwell, graduate student in biology Catherine Sanderat the University of Adelaide, was studying the morphology of snake hemipenes when she realized how little biologists knew about hemiclitores. She also noted inconsistencies in the existing scientific literature on what constituted hemiclitores, with some studies appearing to mislabel the organs. In some studies, the scent glands, also located on the ventral side of the snake near the tip of the tail (just below the hemipenes in males), had been labeled as the hemiclitores. In others, hemipenis-like structures in hermaphrodite snakes had also been dubbed hemiclitores. In a exam published earlier this year, Folwell and his co-authors concluded that, based on these interpretations, hardly anyone had ever really done a real comparative study of sex organs in snakes.

So, to clear up this confusion, Folwell and a team of researchers took a deeper look. They started by dissecting the death vipers (Acanthophis antarcticus) that they already had in the lab. The dissection involved three steps: opening the abdomen, carefully dissecting the scent glands, and cutting a thin layer of muscle. Folwell says the first time she did this dissection, she saw the structures she identified as hemiclitores, clear as day.

“At this point, I [was] very ecstatic. It’s unlike anything anyone has ever seen or described,” she says.

She then studied the structure of the organ. She stained putative hemiclitores with dyes that selectively highlight vasculature, nerves, and erectile tissue, doing the same with hemipenes from snakes of the same age. In females, the structure was highly vascularized, which is consistent with findings in other reptiles and mammals. And like the hemipenes, the hemilitores were highly innervated. Unlike hemipenes, however, hemilitores do not contain muscle fibers and instead are composed solely of collagen.

Folwell also created a 3D model of the organ using DiceCT, a form of iodine-based tomography that allowed him to view thin cross-sections of snake tissue. This allowed him to see in detail the shape and size of the entire structure.

She and her team completed this procedure with nine species of snakes from four families, finding that there was enormous variation in organ size, shape and tissue composition. “It was fantastic,” Folwell says, adding that the variety indicates selective pressures likely shaped the snakes’ genitals.

Although snake hemiclitores probably have a function, that function remains unclear, for now. Folwell has a few hypotheses, though: In many other animals, the clitoris “sends a signal to the brain to start preparing the body for copulation,” Folwell says, and the same could be true in snakes. Many snake hemipenes look like “medieval contraptions,” covered in everything from spikes to suckers to hooks. Therefore, hemiclitoral stimulation could encourage vaginal lubrication and relaxation, Folwell says, to prevent trauma or damage after the mating process.

The study may also have implications for how snakes choose their mates. In many species of snakes, scientists believe that snake mating occurs primarily through coercion. But this study suggests it could be happening through “seduction” instead, says Folwell.

Next, Folwell says she would like to know more about the neural pathways that involve hemiclitores. She also plans to better understand the role hemiclitores play in snake behavior, particularly during copulation, and if and how they are stimulated by mating behaviors such as vibration and tail curling.

The results show that clitorises are even more widespread among amniotes – the group of vertebrates that includes mammals, reptiles and birds – than previously thought, indicating a single ancestral origin for the structure. And all of these clitoris require further investigation, as researchers are only just beginning to understand their functions, Folwell says. Such insights and an evolving context could illuminate disparate fields, from animal behavior at human sexual health.

See “Why the female orgasm evolved”

“There’s no doubt that we want to see more acceptance and excitement around female genitalia and female reproductive research,” Folwell says. Until then, she’ll enjoy the little shock people get when she tells them what she’s up to. “It’s a fun party gadget,” she says.