Probably one of the most fascinating cases of genitalia among mammals is that of the false penis of the female spotted hyena ( Crocuta crocuta ).
It is the only mammal whose female does not have an external vaginal opening. Although their internal structure is different, the external similarity with the real male penis is so high that, except to the expert eye, or unless it is a lactating female, they are indistinguishable to the naked eye .
So rare is this occurrence, that hyenas were long thought to be hermaphrodites . The famous corsair Sir Walter Raleigh excluded hyenas from among the animals that boarded Noah’s ark because they were not pure breeding animals. He assured that it was a kind of unnatural chimera between dogs and cats. Today we know that, mythology aside, the closest evolutionary group is that of the herpestidae, commonly known as mongooses.
The origin of this peculiar anatomical structure , exclusive to this species —the other two species of hyena, the striped ( Hyaena hyaena ) and the brown ( Parahyaena brunea ) do not have it— was an enigma for a long time . It was thought that it must be some kind of evolutionary adaptation, but it could not be identified. It was easier to discover how it was formed than how it evolved.
The formation of the false penis
Actually, this pseudo-penis is an extreme modification of the clitoris. The lips of the vulva are welded behind, and generate the appearance of a scrotal sac like that of males, and the clitoris closes forming a tube, which can exceed 17 centimeters, through which both the urethra and the genital duct pass. .
This fascinating structure is formed during embryonic development . All mammals share a common embryonic pattern of sexual organs, in which, initially, the embryo is sexually ambiguous and contains all the precursors and structures necessary to develop either of the two systems, male or female—and even, exceptionally, both, one on each side, or even remain ambiguous.
The internal genitalia—ovaries or testicles—develops, in principle, according to what the genes designate. In the external genitalia , if nothing interferes, the natural course is for the female genitalia to form —vulva and clitoris—. The male route -scrotum and penis- is the one that involves a hormonal alteration, normally caused by the secretion of testosterone by the incipient testicles.
Another organ that produces testosterone is the placenta . In female mammals, an enzyme is present, called aromatase , which degrades this testosterone, allowing the embryo to develop as it should. But a mutation prevents spotted hyenas from degrading it , transporting this hormone through the umbilical cord, and causing their offspring, both male and female, to develop the apparent external anatomy of a male .
Both copulation and childbirth occur through this conduit . And both involve difficulties. Managing to penetrate an erect clitoris with such a penis is a complicated feat, not for novices. In this way, the females not only avoid being forced, but can allow themselves to be selective with the males . If a female regrets having received a visit from a male, she has the ability to eliminate the sperm, or most of it, simply by flushing urine.
As for childbirth, it is extraordinarily complex , frequently the pups are not born alive due to the long journey they must travel —especially among first-time mothers—. And occasionally, tears occur during childbirth that later leave very characteristic longitudinal scars.
Social functions of having a false penis
Spotted hyenas maintain a matriarchal society , the females are more aggressive than those of other species, and more so than the males, and they are usually the leaders of the pack and the first to feed . Most females dominate almost all males.
Hyenas have a ritualized greeting similar to that of dogs. They stand parallel and in opposite directions, and sniff or lick each other’s anal and genital regions.
They often raise their hind leg to make it easier for their partner. During that greeting, the subordinate individual declares submission to the dominant . And one of the gestures that indicates that submission is the erection of the penis or the pseudopenis —depending on the sex of the person greeting—.
When evolution does not seek adaptations
For a long time it was thought about what could have been the adaptation that had as a consequence this pseudopenis of hyenas . However, the event does not respond to an adaptation, but to the maintenance of a non-selective trait.
It is not the female with the false penis who has the genetic trait that expresses the character, but her mother. If an adaptation existed, it would not be having that false penis, but rather having the mutation that causes their offspring to develop that false penis. Therefore, the selective pressure does not act so much on the carrier of the mutation, but on its offspring.
As it is an effect that occurs during pregnancy, it is the genetics of the mother that is modeling her daughter . A trait caused by a mutation, which due to genetic drift , and completely stochastically , ended up spreading to the entire species, but without generating, as far as is known, an adaptation .
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