Female spotted hyenas do not possess testicles. Instead, they exhibit highly masculinized external genitalia that can be easily mistaken for male organs. This unique anatomical feature is a defining characteristic of the species, challenging typical mammalian sexual dimorphism.
The Unique Female Anatomy
Female spotted hyenas have a unique urogenital system, including an enlarged clitoris that resembles a male penis. This structure is often referred to as a “pseudo-penis” or “peniform clitoris.” The pseudo-penis is functional and can become erect. Their labia are fused, forming a structure that resembles a scrotum. These scrotal-like sacs, however, do not contain testicles; they are filled with fat and connective tissue.
This anatomical similarity makes it challenging to differentiate between male and female spotted hyenas externally. The female’s pseudo-penis is typically shorter and thicker than a male’s, with a more rounded glans. Despite the superficial resemblance to male genitalia, these are distinctly female reproductive organs.
Functional Adaptations of the Pseudo-Penis
The pseudo-penis in female spotted hyenas serves multiple biological functions. Females use this organ for urination, just as males do. Mating is complex, requiring the female’s full cooperation. The female must retract her pseudo-penis inward to create an opening for the male’s penis, making forced copulation impossible.
Giving birth through this narrow, elongated canal presents significant challenges. The birth canal is only slightly larger than the cubs, and the process is often protracted and difficult. This can lead to high mortality rates for both first-time mothers and their offspring. Approximately 15% of first-time mothers may die during childbirth, and up to 60% of first-born cubs may not survive, often due to asphyxiation or birth trauma. The pseudo-penis often tears during the first delivery, but this tearing can make subsequent births easier.
Hormonal Basis and Evolutionary Advantages
The unique anatomy of female spotted hyenas is largely influenced by high levels of androgens, or male hormones, during prenatal development. These hormonal exposures contribute to the masculinization of their external genitalia and are linked to their aggressive behavior and social dominance. Late-gestational increases in androgens impact the differentiation and masculinization of the clitoris. Studies show that interfering with prenatal androgen action can lead to more “feminized” genitalia in females, facilitating easier births.
This anatomy is intertwined with the spotted hyena’s social structure, where females are dominant over males. Females are generally larger and more muscular than males; even the lowest-ranking female outranks the highest-ranking male. The pseudo-penis plays a role in social interactions, particularly in greeting rituals where hyenas sniff or lick the erected pseudo-penis, signaling submission or reinforcing social status. Some theories suggest the masculinized genitalia evolved as a byproduct of strong selection for increased female aggression and dominance, with dominance benefits outweighing reproductive costs. The pseudo-penis also grants females significant control over mate selection.