Hyenas: maternal masculinization makes birth no laughing matter (part 1)

Birth of their baby is usually fast & uncomplicated in higher primates such as gorillas, chimpanzees, & orangutans. However in man a natural birth is usually prolonged & painful. The evolution of these processes has been discussed in my previous blogs on  birth in humans & apes.

Human females often have a very hard time bearing a baby, but female hyenas apparently go through hell when delivering their first baby. Obstructed labor is very common problem & often results in the death of the pup due to asphyxia & the mother due to the obstruction & rupture of the genital tract. Obstruction can potentially occur along the whole genital tract in any animal. In man the problem is often the narrow bony outlet but in the hyena the problem is in the soft tissues i.e. the extremely long & kinked clitoris through which the baby must pass.

birth passage man & hyena

In man a birth can be prolonged by the passage through a relatively narrow bony pelvis. In hyenas the obstruction is produced by the soft tissues i.e. the masculinized clitoris (click to enlarge)

Figure 1 schematically compares the factors influencing the “flow”  of the human baby & hyena pup through the birth passages. We have previously simplified the explanation of  birth process by “stretching” the fluid flow analogy i.e. pressure,  & resistance due to diameter of a tube & viscosity of a liquid will influence flow, to explain factors influence the passage of a baby at birth. The pressure the mother can create to overcome the resistance of the birth canal & “viscosity” of the baby will determine the ease of passage. The female hyena has ambiguous genitalia i.e. excessive masculinization of the external genitals results in a urogenital outlet that resembles the males. The clitoris & labia fuse to form a “pseudo penis & pseudo scrotum”. Thus in humans the baby passes from the uterus thru the cervix into the vagina & into the world.  In the hyena after passing thru the uterus, cervix, & vagina the pup encounters a further obstacle. It now enters a “masculine world”, passing into the very long & kinked clitoris with its narrow urethral opening i.e. equivalent to the penis in males. This narrow passage is the cause of the obstetrical problems in the hyena. They have attempted to adapt to this obstruction by development of very strong uterine muscles &  special mucous glands to lubricate the passage. But obstruction is very common. It is interesting to note that the umbilical cord is relatively short for the pups long & twisted journey & tearing of the umbilical cord & consequent hemorrhage is an extra hazard during its difficult journey

Spotted Hyena’s female genitalia

The masculinized external genitalia are drawn over a photograph of a proud hyena mother and her beautiful newborn pup in Kgalagadi Transfrontier Park, South Africa. Idea for drawing triggered by the illustration in the book by Richard C Francis, “Why men won’t ask for directions” (Click to enlarge)

Figure 2 shows a simple schematic drawing of the hyena urogenital tract with the long & kinked clitoris superimposed on the photograph of a mother who did deliver a super pup. It is easy to understand why the birth can be obstructed. Rupture of the clitoris during the first birth is common and makes subsequent deliveries easier.

In next blogs the hormonal mechanisms underlying this masculinized urogenital tract will be discussed. The disadvantages are evident but what are the advantages for the hyenas of a masculinized and very dominant female in hyena society?

Useful references on the above are the clear discussion by Richard C Francis in his book “Why men won’t ask for directions” (Princeton University Press 2004) and the detailed research by  Prof SE Glickman’s group & coworkers in California over many years (e.g. Mammalian sexual differentiation: lessons from the spotted hyena. Trends Endocrinol Metab. 2006;17:349 )

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