The spotted hyena (Crocuta crocuta) is a bit of a strange creature. It has a distinctive call that sounds like hysterical human laughter, will readily fight off lions from a kill, and looks like a dog, but isn’t one. Perhaps the most unusual characteristic, though, is that females have genitals that looks outwardly male- capable of erection and all. They don’t have a true penis, but a modified clitoris called a psuedopenis, through which the female urinates, has intercourse, and gives birth. Modified labial tissue (that looks rather like a scrotum) completely seals the vaginal opening. What could have lead to the evolution of such an unusual case? The following questions (in bold) were proposed for discussion, and here are some of my thoughts.
Explain the extra androgen hypothesis and evidence that supports or refutes it.
The extra androgen hypothesis proposes that the reason the female spotted hyenas develop a psuedopenis is because there are raised androgen levels present during embryonic development that act upon the clitoral/genital tissue (which, in the homologous male tissue, forms a penis). Extra androgens would in fact cause these changes, however, evidence has been found that shows that this isn’t the case. Female hyenas had less testosterone in their blood than males, and when pregnant females were given an androgen receptor antagonist, there was no change in the daughters’ psuedopenis development.
Give some alternative explanations for how the pseudopenis may not be adaptive.
Spotted hyenas are highly aggressive animals, and in hyena populations, the females are dominant. Male hormones are attributed with aggression. Perhaps an altered male hormone metabolism developed in females to promote this. Females who had higher levels of aggression likely defended their territories more successfully, got more access to food, etc., and therefore raised more young. The extra androgens in the body would act on the genital tissue and transform “normal” female genitalia into highly masculinized forms. The positive selection on the higher androgen levels would, in this case, outweigh the reproductive dangers.
How could sensory exploitation have played a role in the adaptive origin of the pseudopenis?
Hyenas greet one another by sniffing the genital area. Presenting the genitals is a sign of non-threatening intentions in several species. Perhaps the males had a behavior in which they displayed their penis that benefited them, so females with larger clitorises were positively selected upon throughout history.
What is one possible adaptive value of this structure that does not involve sensory exploitation?
The psuedopenis makes rape physically impossible, so the males can mate with a female only if she permits it. This is advantageous to the female because she can select the only most fit males as mates.