When Megan Folwell started her PhD at the University of Adelaide in Australia in early 2021, she was looking at reproductive biology and vaginas in snakes. It was then that she noticed a discrepancy.
"'I was thinking 'I hear a lot about hemipenes, the male genitalia, but I really haven't heard anything about any form of clitoris,'" Folwell told DW. "So I started dissecting some of my own specimen."
That research led to the discovery that Folwell and her co-authors published in the journal Proceedings of the Royal Society B: Female snakes of at least nine species have clitorises. And just like male snakes have a double-penis (the hemipenes), female snakes have a pair of clitorises ― the hemiclitores. The organ is separate from the cloaca, the one opening snakes have for reproductive, digestive and urinary purposes, and ranges from less than a millimeter to seven millimeters in size.
"The male genitalia is quite obvious and presents itself," Holwell said. "The hemiclitores is so small and so fragile that it makes sense it could have been missed before."
Scientists don't know yet know why male and female snakes each have a pair of genitalia instead of just one.
The clitoris, a taboo topic
There is another reason, however, that Holwell believes is also responsible for the fact that her study is the first about the genitalia of female snakes.
"There's definitely a taboo about the subject," the PhD candidate said. "It is not always the easiest one to bring up in conversation, and especially when you bring it up initially, it's kind of hard to get the respect that should be around it."
When looking at animals' mating practices, the focus lies often on what the male does, why he does it and what benefits him, Holwell says.
The field of animal sexuality "is usually male-focused, and we don't fully understand what is happening for the females," the researcher added.
Even in humans, female genitalia are under-researched. For decades, researchers believed that the clitoris was just a little pearl, or ball of flesh. By now we know that's not true and the human clitoris is in fact much bigger with wing-like features on the inside.
Hemiclitores not stimulated by the hemipenes
When snakes mate, the process isn't so different from what it is in humans. The penis enters the vagina, or in this case, the cloaca. Male snakes only use one of their penises at a time, but it is thought that they could use the other a short while later, to go another time, if you will.
The hemiclitores is located at the underside of the female snake's tail, away from the cloaca, so it's not involved in this mating process.
"The penetration of the penis shouldn't actually be stimulating the clitoris," Folwell said. "But we think that certain behaviors such as the male wrapping his tail around the female and pulsing could then be potentially stimulating the clitoris."
That's another similarity to humans: Penetration during sex usually does not stimulate the clitoris in women either.
Do animals feel sexual pleasure?
So the hemiclitores is not directly involved in the snakes' reproduction process. What could it be for?
Holwell and her team found nerve bundles in the organ, which indicates that there is "likely a lot of tactile sensitivity to it, which could be used for stimulation during mating," the researcher said.
Why exactly animals developed genitalia that allowed them to potentially feel pleasure during sex is not yet clear.
A January 2022 study, published in the journal Current Biology, found that female dolphins have a clitoris, too. The clitoral erectile tissue in female dolphins has inflatable spaces that resemble those of humans. In humans, blood rushes to these spaces when the clitoris is stimulated, making it swell. Patricia Brennan, the study's lead author and a co-author on Holwell's snake study, said this seems to be the case in dolphins as well.
The clitoral skin in dolphins is three times thinner than other nearby genital tissue, which points to the clitoris being highly sensitive. And the researchers discovered nerve endings in dolphins' clitorises that are also found in the skin of human genitalia and nipples. They are involved in the sexual pleasure response in humans, and the study states "their presence suggests a similar function in the dolphin."
Do snakes, dolphins and potentially other animals, too, have sex for pleasure then?
"We used to think animals didn't feel pain, and obviously that's been disproven," Holwell said.
Cephalopods like octopuses for example feel pain, can remember locations where someone or something caused them pain and avoid these places, a 2022 study published in the journal Science found.
And pain is not the only deep feeling animals experience. Non-human primates have a sense of self, meaning they exhibit cognitive abilities such as logical thinking, and display empathy, according to experts like Karsten Brensing, a German biologist and behavioral scientist.
Looking at what we know about animals' inner lives and taking into account what studies like Brennan's have found, it is not far-fetched to think that animals might also experience pleasure during sex.
"It might be a factor in snakes, too," Holwell said. "But we obviously need to do a lot more work to prove it."