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their sex—male one year, female the next. Others proposed that hyenas
were true hermaphrodites, capable of taking either role in copulation.
Even as recently as the twentieth century, Ernest Hemingway and others claimed spotted hyenas were bisexual.
Though Pliny and the others may have erred about the sexual nature
and capacities of spotted hyenas, they were right about one thing: spotted hyenas are most unusual mammals. They are not hermaphrodites
or shape shifters, nor do all hyenas have penises and scrotums. Spotted hyenas do push the sexual envelope, though, and they do all have
the appearance of males. Between the rear legs of every spotted hyena
there is a penis-like shaft about seven inches long, and a fleshy sack that
looks very much like a scrotum. And in about half the hyenas in every
pack, that’s just what they are—penises and scrotums. But in the other
half, the apparent phalluses are fully erectile, large clitorises (or pseudopenises), and what from a distance appeared to be scrotums are in
fact labial-scrotal fusions holding two pads of fatty tissue that look like
testes. In effect, half of all spotted hyenas are intersex—40,XX females Where Our Sexes Come From 101 (forty being the normal number of chromosomes among hyenas) whose
genitals look like those of males. So, much like women with congenital adrenal hyperplasia, female spotted hyenas sport a normal female
complement of chromosomes but look physically like males.14
How does that happen to spotted hyenas?
Unlike most mammals, among spotted hyenas females are dominant. All power passes matrilineally, with females assuming their mother’s rank after the mother’s death.15 After a kill, males don’t eat until the
females have finished. Laurence Frank at the University of California
at Berkeley described a situation in which one juvenile female held
five fully grown adult males at bay while she had her fill from a buffalo carcass.16 In fact, except during copulation, female spotted hyenas
completely dominate males.
To ensure their dominance, as female hyenas are developing they
get an overdose of male hormones—in particular the male androgen
androstenedione. In other mammals, including humans, during female
development much of this androstenedione gets converted into estrogen. But in spotted hyenas most of the androstenedione becomes testosterone. And the higher the female hyena’s status, the more testosterone
she passes to her offspring, both male and female, but especially to
the females. All of that testosterone, along with some other, less well
understood contributors including genetic factors, masculinize developing female fetuses in several ways. Females do develop vaginas and
uteruses, but their clitorises enlarge to equal the reach of male penises,
and the labia become a little more like scrotums and never fully separate as they do in many other female mammals.17 Body size increases,
as does aggression.
An interesting experiment in mice, pigs, rabbits, and other litterbearing species supports the idea that as female spotted hyena fetuses
bathe in testosterone they become more aggressive. During the development of a litter it is possible for a female fetus to develop in the
uterus situated between two male fetuses (2M), between a female and a
male fetus, (1M), or between two female fetuses (2F). It turns out that
if you are female, who you sit next to in utero makes a difference. A 2M
female produces fewer living offspring, has higher blood levels of tes- 102 Between XX and XY tosterone and larger scent glands, and is much more aggressive than 2F
females—all apparently because of testosterone spillover from her littermates. There may be some evolutionary benefit to this. Among these
same litter-bearing species, during times of high population density, the
most aggressive females are also the most likely to reproduce in spite of
their reproductive problems. That makes it appear that when food gets
scarce the most aggressive females are the most successful. That would
be an enormous advantage to any female living in a place where food is
never abundant. And all of it arises simply from sharing a placental bed
with a brother or two.
Among female spotted hyenas there is a hierarchy. Alpha females
are the most aggressive of all the hyenas, and while they are pregnant
they give their fetuses the largest doses of testosterone. That testosterone ensures that her offspring will be equally aggressive and dominating. But there is a price; female spotted hyenas’ ovaries are partially
stunted, increasing the difficulty of conception. In addition, the females’
genitalia look a lot more like those of males, including the enlarged
clitorises, the labioscrotal fusions, and the failure of the ureter to separate from the vaginal opening, creating a long penislike vagina beneath
the hypertrophied clitoris. This also increases the difficulty of successful mating. Among the four species in the hyena family, only spotted
hyenas are built like this.
So how do spotted hyenas reproduce? Wit...
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This document was uploaded on 02/04/2014.
- Spring '14