Female Complex

The spotted hyena of Africa has a very peculiar attribute pertaining to gender. Long thought to be true hermaphrodites the females of the species (which are by far the dominant gender) actually posses masculinised genitalia. The females have no vagina, and have an elongated clitoris that resembles the male penis in nearly every way.

From Wikipedia:

The female Spotted Hyena's urogenital system is unique among mammals: there is no vagina, and the clitoris is as large and as erectile as the male's penis - only the shape of the glans makes it possible to visually tell the sexes apart. The female urinates, mates and gives birth through this pseudo-penis (it dilates for mating, the opening widening to admit the male's penis). It was thought that the development of this structure depended on a masculinisation process triggered by the action of androgens on the female fetus, but experiments with anti-androgens show that it still forms in the effective absence of the hormone, so it is now ascribed to normal morphogenesis and sexual mimicry. Since it is impossible to penetrate without the female's cooperation, female hyena have full control over who they choose to mate with.

Birth is very difficult: the internal birth canal extends almost to the subcaudal location of the vulva (which in Crocuta is fused to form a scrotum containing fatty pseudo-testes) before turning abruptly towards the clitoris, and the clitoris itself is narrow (although it ruptures with the first parturition, making subsequent births easier). In captivity, many cubs of primiparous mothers are stillborn because of the long labour times involved; in the wild, survival rates of females seem to fall sharply around the age of first giving birth, suggesting that the process is hazardous for the mother also. This suggests that at some point there must have been powerful selective pressures driving the evolution of masculinisation.

Researchers originally thought that one of the things that causes this characteristic of the genitals is androgens that are expressed to the fetus very early on in its development. However, it was discovered that when the androgens are held back from the fetus, the development of the female genitalia was not altered. Other hyena species lack this adaptation, making it a fairly recent one in the hyena line. Masculinised female genitalia also appears in some lemurs, spider monkeys, and the binturong but the fused vulva is unique to the hyena.

There are a few other mammals with similarly masculinised genitals, but the pseudo-scrotum is unique among the spotted hyena. It should also be noted that no other genus of Hyena possesses this characteristic.