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The evolutionary tango of animal genitalia.
Can you guess what you're looking at?
If you answered "duck vagina," you'd be right.
Although the bird's outward appearance may not strike you as especially odd,
it uses this strange, intricate,
cork-screw shaped contraption to reproduce.
We see similarly unbelievable genitalia in insects,
mammals,
reptiles,
fish,
spiders,
and even snails.
Apparently, no organs evolve faster and into more variable shapes
than those involved in procreation.
Superficially, it makes sense because evolution works via reproduction.
When an animal leaves more offspring, its genes will spread.
And since genitalia are an animal's tools for reproduction,
any improvement there will have immediate effect.
And yet, what's the point of having such decorative nether regions?
After all, the function of genitalia seems simple.
A penis deposits a bit of sperm
and a vagina receives it and delivers it to the egg.
A pipette-like thingy on the male
and a funnel-like gizmo on the female should do just fine for any animal.
And yet, that's not what we see.
The penis of a chicken flea, for example, looks nothing like a pipette,
more like an exploded grandfather clock.
And the vagina of a featherwing beetle
resembles something you'd find in a Dr. Seuss book.
Throughout the animal kingdom,
genitalia are very complex things,
much more complicated than seems necessary for what they're meant to do.
That's because genitalia do more than just deposit and receive sperm.
Many male animals also use the penis as courtship device, like crane flies.
In some South American species,
males have a tiny washboard and scraper on their penis,
which produces a song that reverberates throughout the female's body
when they mate.
It's thought that if female crane flies enjoy this unusual serenade,
they'll allow the male to father their offspring.
This way, the genes of the most musical penises spread,
leading to rapid evolution of insects' phalluses.
Similarly, some beetles have two little drumsticks on either side of the penis.
During mating, they'll rub, slap, or tap the female with these.
And some hoofed mammals, like rams and bulls,
use a whip-like extension on the penis's left side
to create a sensation during mating.
But how can females really choose between males
if she can only assess them after mating?
This is where the power of female adaptation comes into play.
In fact, insemination is different to conception,
and the female genitalia exploit this distinction.
For instance, in some dung flies,
the vagina contains pockets for separating sperm from different males
depending on how appealing they were.
Males using their penises for courtship
and females controlling their own sperm management
are two reasons why genitalia evolve into such complex shapes.
But there are others
because genitalia are also where a sexual conflict is played out.
A female's interests are best served if she fertilizes her eggs
with the sperm of the best fathers
and creates genetic variability amongst her offspring.
For a male, on the other hand, this is bad news.
For him, it would be best if a female used his sperm
to fertilize all of her eggs.
So we see cycles of adaptation in an evolutionary arms race
to retain control.
Black widow spiders have a disposable penis tip
that breaks off inside the vagina blocking the attempts of his rivals,
and bed bug males bypass a female's genitalia altogether
using a syringe-like penis to inject sperm cells directly into her belly.
Not to be outdone,
females have evolved their own countermeasures.
In some bed bug species,
the females have evolved an entirely new set of genitalia
on their right hand flanks where the males usually pierce them.
That allows them to maintain the power to filter out unwanted sperm
with their genitalia.
And duck vaginas are shaped like a clockwise spiral
so that when the male inflates his long, counterclockwise coiled penis into her,
and she disapproves,
all she needs to do is flex her vaginal muscles
and the penis just flubs out.
So, genitalia differs so much, not just to fascinate us,
but because in every species,
they're the result of a furious evolutionary tango of sex
that has been going on for millions of years
and will continue for millions of years to come.
コツ：単語をクリックしてすぐ意味を調べられます！

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B2 中上級 英

【TED-Ed】動物の生殖器の演化 (The evolution of animal genitalia - Menno Schilthuizen)

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April Lu 2017 年 8 月 7 日 に公開

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