The Spotted Hyena exhibits some interesting characteristics when it comes to the evolution of appendages. Namely, both the male and the females of the species possess elongated sexual organs that resemble a “male-type external genitalia”. The uro-genital development of this “male-type external genitalia” occurs prior to Gonadal differentiation. Despite the morphological similarities, there are differences between the genitalia of the female and male spotted Hyena. For instance, the male of the species has an enclosed urethra that extends the entire length of the penis and is surrounded by erectile tissue incapable of expansion. On the other hand, the female spotted hyena has “a large pleated urogenital sinus is surrounded by unconstrained connective tissue” (Glickman 405). In addition to this, the penis of the male spotted hyena is thinner than the female clitoris. Researchers have used these unique characteristics as an opportunity to study the effect of androgens (specifically Androstenedione) on sexual differentiation when administered to pregnant spotted hyenas in an attempt to affect the embryo. The results are somewhat surprising. According to the study, the injection of androgens in pregnant spotted hyenas did little to stop or hinder the growth of “male-type external genitalia” leading researchers to believe the growth of the appendage is entirely genetic and not determined by hormones. Although the injection of androgens did not stop the growth of an external clitoris in the female, it did increase the elasticity of the appendage allowing for a significant decrease in failed first births. This prompted the researches to search for a reason why the female spotted hyena did not evolve with higher levels of Androstenedione. They concluded that although first births are more likely to fail with a decrease in Androstendione, there is more benefit in the increased social aggressiveness exhibited by the female spotted hyena with lower level of the Androgen. This goes to show that reproduction is not all that matters in the sexual differentiation of a species.
|*An image which depicts the “male-type genitalia” of both the male and female spotted hyena. photo taken from (Glickman 2005)|