The Squamate Clitoris: A Review and Directions for Future Research

Abstract

The clitoris is a part of the genitalia of female amniotes that typically functions to stimulate sensory arousal. It usually consists of a small organ that is dimorphic and homologous to the penis. The developing amniote embryo forms a genital tubule, then sex hormones initiate a developmental cascade to form either a penis or clitoris. In squamates (lizards and snakes), the genital tubule develops into a paired hemiphallus structure called the "hemiclitores"in the female and the "hemipenes"in the male. The complex evolution of squamate hemipenes has been extensively researched since early discoveries in the 1800s and this has uncovered a huge diversity in hemipenis size, shape, and ornamentation (e.g., protrusions of spines, hooks, chalices, and cups). In contrast, the squamate hemiclitoris has been conspicuously under-investigated, and the studies that describe this anatomy are fraught with inconsistences. This paper aims to clarify the current state of knowledge of the squamate hemiclitoris, providing a foundation for further research on its morphology and functional role. We show that while several studies have described the gross anatomy of hemiclitores in lizards, comparative information is entirely lacking for snakes. Several papers cite earlier authors as having reported discoveries of the snake hemiclitores in vipers and colubrid snakes. However, our examination of this reveals only erroneous reports of hemiclitores in snakes and shows that these stem from misinterpretations of the true anatomy or species involved. An especially problematic source of confusion is the presence of intersex individuals in some snake populations; these form reproductively functional ovaries and a single hemipenis, with the latter sometimes mistaken for a hemiclitoris. (The intersex hemipenis is usually smaller and less spinous than the male hemipenis.) Further research is recommended to identify the defining anatomical features of the squamate hemiclitores. Such studies will form a vital basis of future comparative analyzes of variation in female genitalia in squamates and other amniotes.