The spotted hyena, \(Crocuta\) \(crocuta\), is a highly social carnivore known for its unique, female-dominated social structure. The term “hyena style sex” refers to the mandatory use of a specialized reproductive organ, often called a pseudo-penis, that necessitates a mating process unlike that of nearly any other mammal. This unusual biological feature has profound implications for the animal’s behavior, social dynamics, and the significant risks involved in reproduction.
The Unique Female Anatomy
The female spotted hyena possesses a reproductive tract that is virtually indistinguishable from the male’s genitalia from an external view. This is due to the presence of an elongated, enlarged clitoris that forms a pendulous organ, correctly termed a pseudo-penis. The clitoris contains erectile tissue and is capable of erection, further mimicking the male structure.
The labia are fused together, forming a bulbous structure that resembles a scrotum, often called a pseudo-scrotum. There is a complete absence of a separate, external vaginal opening in the perineal region. Instead, all urogenital functions—urination, copulation, and parturition—must occur through a single, narrow urogenital canal that runs the entire length of the pseudo-penis. This anatomical configuration is the foundation for the species’ unusual reproductive challenges.
The Mechanics of Copulation
Successful copulation requires the full, active cooperation of the female. Due to the presence of her pseudo-penis, the male cannot simply mount and penetrate the female. The female must first retract or invert her pseudo-penis inward, much like a shirtsleeve, to expose the opening of the urogenital canal.
The male must then perform a difficult maneuver, often squatting beneath the female and angling his erect penis upward and backward to align with the retracted canal opening. The female typically maintains a head-down posture and must remain exceptionally still, demonstrating a high degree of patience and trust in her partner. The physical difficulty makes forced copulation impossible, giving the female complete control over who sires her offspring.
Evolutionary Hypotheses
The Byproduct Hypothesis
One major theory suggests the masculinized genitalia are an incidental result of high androgen levels in the female. These elevated levels of male hormones, such as testosterone, are thought to be adaptive because they confer increased aggression and dominance. These are beneficial traits in the female-led, highly competitive hyena society.
The Social Signaling Hypothesis
A second leading theory proposes the pseudo-penis evolved to serve a function in the hyena’s complex social rituals. Female hyenas frequently use the organ in greeting ceremonies, where one animal will lift its leg to allow the other to inspect its erect pseudo-penis. This display often functions as a signal of submission, which is useful for mitigating aggression and maintaining social hierarchy within the clan. The structure also may have conferred an advantage by making female cubs appear male, potentially reducing aggression or infanticide from dominant females.
Consequences for Parturition
The most significant biological cost of the female’s anatomy is incurred during parturition. The cub must pass through the narrow, rigid clitoral canal, which is only about 2.5 centimeters in diameter. This process is extremely difficult, particularly for first-time mothers.
The difficulty leads to high rates of maternal injury, as the clitoris often tears during delivery, which can take weeks to heal. Infant mortality is also notably high, with estimates suggesting that up to 60% of firstborn cubs may die. This mortality often results from suffocation because the umbilical cord detaches before the cub can pass through the long canal.