Female Genital Variation Far Exceeds That of Male Genitalia: A Review of Comparative Anatomy of Clitoris and the Female Lower Reproductive Tract in Theria

Abstract

A review of the literature on the anatomy of the lower female genital tract in therian mammals reveals, contrary to the general perception, a large amount of inter-specific variation. Variation in female external genitalia is anatomically more radical than that in the male genitalia. It includes the absence of whole anatomical units, like the cervix in many Xenarthra, or the absence of the urogenital sinus (UGS), as well as the complete spatial separation of the external clitoral parts from the genital canal (either vagina or UGS). A preliminary phylogenetic analysis shows two patterns. Some morphs are unique to early branching clades, like the absence of the cervix, while others arose multiple times independently, like the flattening out or loss of the UGS, or the extreme elongation of the clitoris. Based on available information, the ancestral eutherian configuration of the external female genitalia included a cervix, a single vaginal segment, a tubular UGS, and an unperforated clitoris close to the entrance of the genital canal. The evidence for either bilobed or unitary glandes clitorides is ambivalent. Despite the wealth of information available, many gaps in knowledge remain and will require a community-wide effort to come to a more robust model of female genital evolutionary patterns.

The hierarchical basis of serial homology and evolutionary novelty

“…Paramorph hierarchies can be found at multiple biological levels of organization, from paralogous genes to cell type families, tissue paramorphs, and organ paramorphs. At still higher levels of organization, sexual homologies between male and female genitalia are paramorphic (Pavlicev et al, 2022), as are caste types in insect colonies (Abouheif, 2021). Given that paramorph hierarchies comprise characters that are developmentally individualized from one another, their existence implies a corresponding hierarchy for the associated ChIMs.…”

Gross Anatomy of the Female Reproductive System of Sugar Gliders (Petaurus breviceps)

“…He also described a clitoris in Petaurus breviceps, located on the ventral wall of the urogenital sinus, near its posterior extremity. According to Pavlicev et al (2022) [36], there seems to be a relationship between the anatomy of the penis and that of the clitoris in marsupials: when the glans penis is bifurcated, the clitoris is also forked. Thus, a bifurcated clitoris was found in female short-tailed opossums [14], the Tasmanian devil (Sarcophilus satanicus) [29], the Tasmanian tiger (Thylacinus cynecephalus) [27] and the sugar glider (Petarurus breviceps).…”

“…The existence of a reduced cloaca, such as the sugar glider one, seems to be a common feature of marsupials [39], although in some South American species, such as Didelphis albiventris [38], Chinorectes minimus [40] and Monodelphis [36], the urogenital sinus and the anus open separately on the surface of the body.…”

The female orgasm and the homology concept in evolutionary biology

“…From a phylogenetic perspective, the ovulatory‐homolog hypothesis is based on phylogenetic data suggesting that male‐induced ovulation is the ancestral condition in mammals, while spontaneous ovulation is a derived mode of the ovarian cycle originating later in several eutherian clades. More recently, Pavličev and collaborators have strengthened the phylogenetic support of the ovulatory‐homolog hypothesis with a comparative anatomy of the development of male and female external genitalia in different mammalian species (Pavličev et al, 2022 ). While after early joint development, male genital and urinary tracts always integrate into the phallus, in females there is a lot of structural and positional interspecific variation.…”