Uncovering the primate clitoris

All female mammals have a clitoris. The clitoris plays a vital role in arousal, sexual pleasure, and orgasm. Yet we know very little about this structure and how it has evolved. The 'pea-sized' external part of the human clitoris is just the tip of the iceberg: 90% of the structure lies hidden under the skin. Such details of the human clitoris were only revealed in the last two decades, following the use of medical imaging technology, such as MRI and X-ray CT. Even less is known about the clitoris of our closest kin, the primates. Whilst distinctive skin swellings around the genitals have been documented in females of many species, we know almost nothing of the internal anatomy of the clitoris. How many nerve endings are present? Where is the clitoris most 'sensitive'? How might tissues change shape during arousal? Without this information from other primate species, it is impossible to understand the evolutionary forces that have shaped the form and function of the human clitoris, nor human sexual behaviour.

The lack of understanding does not extend to male reproductive organs. The equivalent structure in males, the penis, is comparatively well-studied. This may reflect a societal bias, where female genitalia were historically thought to be less variable than male counterparts and therefore overlooked. This one-sidedness may also relate to technical challenges associated with studying the clitoris. Compared to male genitals, female tissues are often small, delicate, and hidden from view. Recent advances in biological imaging, however, ensure these hurdles can now be overcome.

In this project, we will develop new methods for studying the primate clitoris. They will allow us, for the first time, to understand the structure of the clitoris, from gross anatomy to microscopic features, across the family tree of primates. Cadavers will come from the National Museum of Scotland, which is the main repository for deceased UK zoo animals. Sampled females will include lemurs, loris, marmosets, macaques, gibbons, and apes. Meticulous dissection of the perineal region will be required, to both describe the clitoral anatomy and to extract the tissues without damage for further analysis. An anatomical guide exists only for human cadavers, and dissections will therefore be extremely novel and exploratory.

Once dissected out, the clitoral tissue will be sliced very thinly, stained with dyes and viewed under a microscope, a process referred to as histology. These 'thin sections' will be used to document the number of nerve endings and their distribution across several areas of the clitoris. To better understand the 3D shape of the primate clitoris, the pelvic regions from additional cadavers will be x-ray microCT scanned. This will allow for a non-destructive 'virtual dissection' of the whole clitoris (including the hidden tissue), allowing us to better understand its shape and function by ensuring clitoris remain undisturbed within the animal's pelvic region. The resulting models will be the first ever glimpse into 3D structure and shape of the primate clitoris, beyond our currently limited understanding of human females.

Having established the feasibly of extracting such valuable information from highly prized museum specimens, the proposed project will open several avenues for further research. In mammals, reproduction ultimately requires the direct interaction of male and female genitals. The idea that female and male reproductive structures should evolve together is generally accepted and yet, remarkably, has never been tested in our closest relatives, the primates. Using the techniques developed in this project, future research will explore the size and shape of primate genitals, as well as interactions between male and female anatomy during sex. These advances will revolutionise the study of sexual selection in this group and transform our understanding of the natural history of human genitalia and human sexuality.