the most successful mammalian predator in Africa -- 8/02/17
Today's selection -- from Cannibalism: A Perfectly Natural History by Bill Schutt. The hyena is the 'most successful mammalian predator in all of Africa.' A genetic mutation that modifies the development of the hyena's sex hormones and organs is the likely reason that the male hyena will not attempt to kill and consume unrelated cubs -- which contributes to the success of this species:
"Interestingly, females of a similarly social predator, also found on the African savannah, possess a potent defense against infanticide and cannibalism. This adaptation has also enabled females of this species to become the dominant clan members. How this phenomenon works is fascinating, although it requires a brief review of development genetics. An additional bonus for this approximately 60-second commitment will be an answer to one life's great mysteries, namely, 'Why do men have nipples (or penises, for that matter)?'
Male and female reproductive systems of the spotted hyena
"During early embryological development, mammal embryos are genderless. At a certain point, tiny buds of tissue grow into precursors of the penis and mammary glands. Sex determination is based on the embryo having one of two combinations of the X and Y sex chromosomes. These combinations, XY or XX, act like two versions of a blueprint. The XY blueprint results in the production of the male hormone testosterone, a chemical messenger that stimulates the growth of the penis bud into a penis. Since testosterone prevents the further development of the mammary glands, this explains why males still have the nipples they grew as genderless embryos -- but don't produce milk. Alternately, having two X chromosomes results in the production of estrogens, the primary female sex hormones, and these stimulate the production of mammary glands. The female hormones also put a halt to the growth of the penis bud, leaving behind the clitoris, a tiny, erectile structure, which unlike the penis, is not involved in urination.
|Adult Spotten hyena|
"In the spotted hyena (Crocuta crocuta), the developmental scenario described above has been flipped on its ear. Scientists believe that at some point in their evolutionary past, a genetic mutation initiated the production of higher levels of the male sex hormone testosterone in female spotted hyenas. As a result, bulked up, hyperaggressive females dominate every interaction with their male counterparts, with males even coming up short in the external genitalia department. Female hyenas develop a remarkably elongated clitoris, which resembles a longer version of the penis. Additionally, the normally liplike vulva is fused closed, thus enabling females to urinate through their pseudopenises (or pseudopenes), the tips of which are also penetrated by the Real McCoy during copulation. Completing the he-man look, the sealed-up vulva forms a matching pseudoscrotum, within which deposits of fat stand in for a functioning pair of testicles.
"The female hyena's uniquely shaped external sex organs actually gave rise to a myth that these mammals are hermaphrodites. Although this is definitely not the case, the birth process is an extremely painful and dangerous experience for first-timers, and by now you may have guessed the reason. Large, full-term hyena fetuses must pass through the clitoris, which, if things proceed smoothly, causes it to tear open. Reportedly, stillbirths and instances of maternal mortality during delivery are high, but after the successful birth of the first litter, the clitoris never fully closes again, making subsequent births somewhat easier. So, while some aspects of this adaptation sound counterproductive, the fact remains that Crocuta crocuta is the most successful mammalian predator in all of Africa. One reason may be related to the fact that, unlike in lions, there is little danger that males will attempt to kill and consume unrelated cubs. Females, on the other hand, have been known to do just that."