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