The animal kingdom's clitoris club officially welcomed its newest members today: snakes.
That's according to researchers from Australia and the US, who examined nine species of snake and found each had a distinct clitoris.
These findings suggest the genitalia of female snakes might be as diverse as their male counterparts', which can sometimes be used to tell species apart, says study co-author Jenna Crowe-Riddell, an evolutionary biologist at La Trobe University.
"The Australian death adder's [clitoris] seems to be fairly large, while vipers from America have quite big, almost muscular ones," Dr Crowe-Riddell said.
"Then other snakes again [have a clitoris that's] thin and stretched out."
The snake clitorises have been unveiled in the Proceedings of the Royal Society B: Biological Sciences.
South Australian Museum and Finders University evolutionary biologist Mike Lee, who was not involved in the study, says the work shows we still have much to learn about the basic anatomy of even well-studied animals around us.
"It also illustrates the gender bias that has permeated science … where there's been an unbalanced focus on looking at the male side of evolution rather than the female side."
Snake sex secrets
Under their scaly skin, male snakes collectively hide a formidable assortment of penises.
Snake penises come in pairs — called "hemipenes" — and are often adorned with barbs and spines.
But the female side of the snake genitalia equation has been somewhat overlooked, despite us now knowing many species, including all mammals, have a clitoris.
(As an aside: it was only in 1998 that Australian urologist Helen O'Connell and her team at the Royal Melbourne Hospital chronicled the full anatomy of the human clitoris. Before that, textbooks minimised the clitoris or overlooked it altogether.)
So Megan Folwell from the University of Adelaide and lead author of today's study decided to investigate the different shapes and sizes of snake vaginas.
In doing so, she found a triangular formation of what looked like erectile tissue, complete with red blood cells and nerves, while dissecting the genitalia of a female death adder (Acanthophis antarcticus).
Closer examination under a microscope and 3D imaging showed it wasn't part of the scent glands that sit nearby, nor was it an underdeveloped hemipenes, as might be seen in an intersex individual.
The structure, Ms Folwell and her colleagues postulated, had to be a clitoris.
And when the team looked for similar organs in eight other snake species, they found every single one had a clitoris of some size and shape.
So … what's it for?
The presence of a functional clitoris indicates that the female snake genitalia is "more than an empty cavity that's to receive sperm", Professor Lee said.
Dr Crowe-Riddell and her crew are now keen to get a better handle on how the snake clitoris differs between species.
They've already seen glimpses of this variation, with a couple of species sporting small "pockets" at the top of their clitoris.
But knowing snakes have a clitoris is one thing. Finding out why it's there, and what it does, is another.
"We can only speculate, and looking across other animals like dolphins, it's used for pleasure in sex ," Dr Crowe-Riddell said.
"But we don't know if that's what snakes are using it for."
To find out, Dr Crowe-Riddell is keen to more closely examine the nerves running to and from the snake clitoris.
Mapping where they go, how densely they're packed and the kinds of signals they send could tell biologists if the clitoris is stimulated during sex.
And who knows? Maybe snake sexy time isn't a completely coercive act.
"Most hemipenes have spines at the base like a backward-facing hook, so they go into the vaginal cavity fine, but to come out, they presumably cause a lot of damage, and that's quite common across the animal kingdom," Dr Crowe-Riddell said.
"So a lot of our understanding of snake mating has been dominated by the thought that it's all coercion.
"But I think this study wedges the door open for seduction.
"And if you look at other animals, there's plenty of examples of the female choosing to mate, and even prolonging mating, which can lead to more successful reproductive outcomes for the male and female."
