The evolution of animal genitalia - Menno Schilthuizen

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Genitals are the fastest-evolving organs in the animal kingdom. But why is this so? And what’s the point of having decorative private parts? Menno Schilthuizen explains how the evolutionary biology of nature's nether regions uncovers a hidden world of seduction, conflict, and rivalry.

Lesson by Menno Schilthuizen, animation by Mette Ilene Holmriis.

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.