The female spotted hyena, Crocuta crocuta, possesses one of the most unusual reproductive systems in the animal kingdom. She gives birth through a pseudo-penis, a process unlike that of any other placental mammal. This unique anatomy means the external female genitalia closely mimic those of the male. The female must urinate, copulate, and give birth through this single, highly specialized structure.
Anatomy of Female Spotted Hyenas
The female spotted hyena’s anatomy is characterized by a high degree of masculinization of the external genitalia. She lacks an external vaginal opening, which is typical for most mammals. Instead, her labia are fused, creating a structure that resembles the male’s scrotum, often called a pseudo-scrotum.
The clitoris is greatly enlarged, forming a long, pendulous organ nearly indistinguishable from the male’s penis in size and shape, hence the name pseudo-penis. This organ contains erectile tissue and is capable of erection. Running the entire length of this pseudo-penis is a narrow channel known as the urogenital canal. This single canal serves as the only external opening for all reproductive and excretory functions. The male hyena must insert his own penis into the female’s retracted pseudo-penis during mating.
The High-Risk Birthing Process
Giving birth through the female’s narrow urogenital canal introduces significant mechanical difficulty and high risk for both mother and cub. Spotted hyena cubs are relatively large at birth compared to the mother’s body weight, further complicating their passage through the long, constricted canal. The urogenital canal is only about one inch in diameter in a nulliparous, or first-time, mother.
For a first-time mother, the rigid, cartilaginous pseudo-penis is too narrow for the cub’s head to pass, requiring the tissue to tear. This tear is necessary for the first cub to exit and often results in a permanent wound. This birth trauma contributes to a mortality rate for first-time mothers, with estimates ranging from 9 to 20 percent. Subsequent births may be easier because the canal has been stretched or scarred open.
The risk to the offspring is even greater, as the narrow, extended birth canal increases the chance of suffocation. The umbilical cord, which is relatively short, often detaches from the placenta while the cub is still in the canal, cutting off its oxygen supply. Approximately 60 percent of cubs, particularly those from a mother’s first litter, die from asphyxiation during birth.
Hormonal Drivers of Female Virilization
The development of the female hyena’s masculinized anatomy is directly linked to high levels of androgens, or male hormones, during gestation. Female hyenas naturally have significantly higher concentrations of circulating androgens, such as testosterone and androstenedione, compared to other female mammals. This hormonal environment causes the fusion of the labia into the pseudo-scrotum and the enlargement of the clitoris into the pseudo-penis during fetal development.
The hyena placenta plays a unique role by actively synthesizing androgens. The placenta converts the precursor hormone androstenedione into testosterone. The activity of the enzyme aromatase, which typically converts androgens into estrogens, is substantially lower in the hyena placenta. This limited aromatase activity allows high concentrations of androgens to remain active, leading to the virilization of the female fetus’s external genitalia.