The majority of hyena species show little sexual dimorphism, with males being only slightly larger than the females. The Spotted Hyena is an exception to this, with females larger than males. One unusual feature of the Spotted Hyena is that females have an enlarged clitoris, called a pseudo-penis, demi-penis, or sometimes mistakenly referred to as a nanophallus. Female hyenas give birth, copulate, and urinate through their protruding genitalia, which stretches to allow the male penis to enter for couplation; it also stretches during birth. The anatomical position of the genitalia gives females complete control over which males are allowed to mate with them.
Researchers originally thought that one cause of this characteristic of the genitals was androgens that were introduced to the fetus very early on in its development. However, it was discovered that when the androgens were held back from the female fetus, the development of her genitalia was not altered. Spotted Hyenas have a matriarchal social structure that some biologists speculate evolved because it is in the best interests of the female hyena to dominate the male hyena as it provides no assistance in rearing the cubs.