The clitoris is an organ of the female reproductive system primarily defined by its composition of highly innervated erectile tissue. This structure is homologous to the male penis, developing from the same embryonic tissue, the genital tubercle. Its function in many species involves sensory stimulation due to an abundance of nerve endings. A homologous structure is found across a wide range of the animal kingdom, particularly in amniotes.
The Clitoris Across Mammalian Species
The presence of a clitoris is nearly universal across placental and marsupial mammals, although its size, location, and visibility vary. The general structure consists of a glans (tip) and a body made of the corpus cavernosum, which engorges with blood during arousal. The external form is subject to significant evolutionary pressures, resulting in remarkable anatomical diversity that often exceeds that of the male penis.
The most extreme example is the female spotted hyena (Crocuta crocuta), which possesses a massive, elongated clitoris. This organ, averaging about 17 centimeters, is fully erectile and is the sole external opening for urination, copulation, and giving birth. The glans features small spines, similar to those on the male’s penis.
Other mammalian species show variations in prominence and mechanical support. Many non-human placental mammals, including rodents and bats, have a small bone embedded in the clitoris, the os clitoridis. This bone provides structural rigidity, functioning as a female analogue to the male baculum. The clitoris of the female bonobo is notably large and externalized, proportionally much bigger than the human clitoris. In squirrel monkeys, the clitoris becomes erect during dominance displays, suggesting a function beyond copulatory stimulation.
Beyond Mammals: Comparative Anatomy in Vertebrates
A homologous structure is present in many other vertebrate groups. The most compelling examples are found in reptiles, particularly the Squamates (lizards and snakes). Female squamates possess a paired structure called the “hemiclitores,” which are developmentally equivalent to the male hemipenes.
In many lizards, the hemiclitores are eversible, meaning they can turn inside out, structurally resembling the male organ. This structure also exists in snakes, where they are non-eversible and separated medially by connective tissue. Histological analysis of snake hemiclitores, such as in the Australian death adder, reveals erectile tissue and nerve bundles, suggesting a role in tactile sensitivity during mating.
A distinct clitoris is also found in other reptiles, such as crocodilians and turtles, situated on the ventral side of the cloaca. This structure contains well-developed erectile tissue and innervation, likely allowing for functional stimulation. In most birds, a distinct external clitoris is largely absent in adults, though it is present during embryonic development. A rudimentary form persists only in certain groups, such as waterfowl and ratites, where the male also retains a phallic structure.
Evolutionary Origins and Functional Roles
The widespread presence of the clitoris across Amniota (reptiles, birds, and mammals) is explained by shared developmental biology. The structure arises from the genital tubercle, which is present in the early embryo of all these groups. Depending on the hormonal environment, this single developmental precursor differentiates into either a penis or a clitoris.
The functional role of the clitoris in the animal kingdom may represent a vestige of an older reproductive system. One theory suggests the clitoris initially played a direct role in reproduction, potentially helping to induce ovulation in ancestral mammals. As species evolved to have spontaneous ovulation, this direct reproductive function became redundant, and the organ was often repositioned further away from the copulatory canal.
The structure has been either co-opted for new purposes or retained as a non-essential feature. For instance, the large erectile clitoris of the squirrel monkey is used in social contexts, such as signaling dominance. The retention of sensory tissue in the hemiclitores of snakes and lizards suggests a role in female receptivity or stimulation that coordinates successful mating. This homologous structure is maintained through deep evolutionary history, even as its size, exposure, and primary purpose adapt to diverse ecological and reproductive needs.