The number one question on the mind of every organism on earth,
if that organism happens to have a mind,
is: how do I make more of myself?
It's bigger than all the other questions combined,
including how am I going to feed myself?
And what's the meaning of life?
Because, from a biological perspective,
we know what the meaning of life is.
Biology has answered that question. It's reproduction.
Different organisms go about reproducing in different ways:
You can make more of yourself by yourself,
a strategy called asexual reproduction,
OR you can team up with somebody else and make a baby that's
genetically different than both of you through sexual reproduction.
From liver flukes to pine trees, 99% of the eukaryotic organisms
on earth use sex to reproduce, at least some of the time,
because by creating offspring with a slightly different genome,
helps the new generation stay one step ahead
of pathogens or competitors.
Or if you're the pathogen, it helps you stay ahead
of that pesky host that's always trying to kick you out.
But still, sex is inconvenient and it's a lot of work:
First you have to find somebody to mate with, which means
you have to get out of bed and brush your teeth and stuff.
Then, if you're an animal, you have to find somebody who's willing
to mate with you, and then figure out whether he or she
is going to provide higher or lower quality genes than yours.
Thankfully, and unsurprisingly, animals' reproductive systems have
evolved to streamline all of those inconveniences to address one,
and only one, aim: to get your sex cells where they need to be.
So, sex. How does it work?
I thought you'd never ask.
Reproductive systems, like all the other systems we've discussed,
take on an incredible diversity within the kingdom Animalia.
For instance, some female spiders mate with a bunch of different
males and stash their sperm in different storage units.
When she's ready to fertilize her eggs, the female spider
will choose which male spider she liked the best
and let his sperm out the storage unit to fertilize her eggs!
Hyenas, meanwhile, have female-dominated social system,
and it's the alpha female who chooses who she mates with.
And she has sex using an enlarged sensitive sex organ, a clitoris,
that looks exactly like a penis, called a pseudopenis.
And a duck's penis can be a quarter of the length of its body,
and shaped like a corkscrew.
Want to know why? Look it up! Actually don't!
Google that with care! Just don't press play on the video.
The point here is that while the delivery systems may be somewhat
different from animal to animal, the fundamentals are the same.
In order to do the sex, an organism needs to find another of its
species that has a different type of gamete,
or sex cell, than their own.
Gametes, you'll recall, are haploid cells, meaning they have only one
set of chromosomes, and they're formed by the process of meiosis.
And there are only two kinds of gametes: One is the ovum, or egg.
In plants it's called an ovule.
The egg is always a large cell that takes a lot of time and energy
investment to make, and it's usually not very mobile.
The other type of gamete, sperm, are smaller, a lot more plentiful,
easy to make, and always more mobile than eggs.
Most animals have either one or the other type of gamete,
though hermaphroditic species, like garden snails
and some flowering plants, can produce both.
In the magical moment that one of these sperm finds
one of those eggs, the two fuse together to create
a single diploid cell that has all of the instructions
to make a new seahorse or secretary bird or whatever it is.
But let me get your mind right about what we really mean
when we talk about sex.
Because we humans have external sex organs, called genitals, we tend
to think of them as key indicators of who's male and who's female.
But the fact is, genitals are only one byproduct
of a much, much more important and fundamental distinction:
From a biological perspective, the only thing that makes sexes
different is that females produce big, not very mobile gametes,
and the males make smaller, much more mobile gametes.
Across the spectrum of all things that reproduce sexually, that's
pretty much the only consistent difference between boys and girls.
Therefore, all reproductive systems and reproductive behavior
are designed entirely around the production, storage,
and delivery of these gametes.
For instance, because sperm are really mobile, males within a species
are generally the more mobile ones who go out to find a mate.
This is even true for plants: female gametes of a flowering plant
generally stay in one place while the pollen, which ends up producing
the sperm, gets picked up by a pollinator, or sometimes just sprays
out every which way in the wind,
hoping to bump into the right kind of ovule.
In animals, we see all kinds of crazy behaviors
where mating is concerned.
And of course not every animal goes about courtship in the same way,
but one thing is pretty consistent: Females tend to be pickier
about the quality of their mates, because while a male animal could
conceivably fertilize thousands of eggs every year,
a female has only a limited number of eggs,
and she's spent a lot of energy developing them,
so she wants them to be fertilized with high quality genes.
Plus, in cases where both parents stay together after fertilization,
she also wants those genes to be attached to a high-quality provider.
This often results in males having to do a lot of showing off
in order to get a lady's attention.
Males of a species are generally louder, larger, brighter,
more combative than the females.
Basically, they're putting on a big show
so the females can size up how awesome that guy's genes are.
But for all those differences, during the development of the embryo,
there are actually very few physical differences
between males and females, at least at first.
You and I, we didn't start out being a male or a female.
While you were hanging out in our mom's uteri,
you didn't have a sex at all until about two months.
Before that, we had all the pieces to become either male or female,
but our genes hadn't gotten together to determine whether or not
our gonads, the glands that make the gametes,
were going to become ovaries or testes.
In mammals, that decision is made by the sex-determining chromosome:
If an offspring has two of the same kind of sex-determining
chromosome, called XX, it will be female.
And if it has two different chromosomes, XY, it will be male.
The same is true for some other animals like fruit flies,
and even some plants like Gingko trees.
However, the opposite is true for birds:
boy birds have XX and girl birds have XY.
Go figure.
In mammals, the default setting for sex is always female.
Absent a signal from the Y chromosome,
ovaries form and begin working on developing female structures.
If there is a Y, the ovaries instead form into testes,
and parts that would be female turn into male structures,
for instance, the clitoris I mentioned, which is sensitive
and has spongy tissue in it, actually becomes part of a penis.
But it's worth pointing out that by this time,
some features are already in place before the sex is determined.
Nipples, for instance, form before this point,
so that's why men have them, even though they don't do anything.
Now, once the sex is determined, the ovaries and testes
pump out estrogen and testosterone.
Meanwhile, the brain is growing and creating receptors,
organized differently in males and females, that will later
determine how both estrogen and testosterone are used in the body.
Soon after a baby girl is born, she'll have half-formed versions
of all the eggs she's ever going to have for her whole life,
then at puberty, once a month,
one of those eggs will finish forming and be released.
But for baby boys, the sperm-making does not begin
until around puberty.
Most of the time when a young animal starts getting close
to sexual maturity, secondary sex characteristics crop up:
In humans more body hair appears, boys all of a sudden develop
facial hair, while both sexes get more pubic hair.
Also muscle and fat get redistributed around the body,
the most obvious example being breasts.
In other animals, secondary sex characteristics include things
like manes on male lions, a big old funky rack of feathers
on male peacocks, antlers on male deer.
Males really have the market cornered
on fancy, showy secondary sex characteristics.
So, by the time an animal has reached sexual maturity,
the males and females of a species often look pretty dissimilar.
Not just of each other, but of their previous
non-sexually-developed forms.
Basically showing the world that their different reproductive
structures that they were born with are now in full gear, and they've
got some really different jobs to do, based on what sex they are.
So let's go over how this all works with human people.
And of course, ladies first.
As you know, the gonads of a female embryo turn into two ovaries,
one on either side of the uterus, with its oviducts,
or fallopian tubes, reaching out toward them.
The ovaries are where those precious eggs are kept.
Maybe the biggest difference between women's and men's
reproductive set-up is that women have a menstrual cycle,
typically a four-week process in which one egg matures in an ovary
and is released to be drawn into the fallopian tubes,
a process called ovulation.
If, while the egg makes its way down the fallopian tube
to the uterus, a sperm finds it and fertilizes it, there's a chance
that the fertilized egg will implant on the endometrium,
a tissue layer inside the wall of the uterus and a baby will grow.
However, it's estimated that up to 70% of fertilized eggs
don't take hold in the endometrium.
This could be because women's bodies have sort of a built-in
genetic testing: If something's suspected to be wrong with the
growing embryo, the lining of her uterus that she's built up over
the past month will shed, and the woman will menstruate as usual.
This material leaves the female reproductive system through the
narrow lower end of the uterus, the cervix,
and then out into the muscle-lined tract of the vagina,
and those are of course the same structures through which
a newborn baby passes and through which the sperm enter.
While a woman's body is busy all month developing the next egg,
getting it ready for fertilization, and shedding her uterine lining
if it's not fertilized, males are undergoing a completely different
process that calls on a lot of
other highly specialized reproductive structures.
We start of course, with the testes, which are made up largely
of a bunch of coiled tubes called seminiferous tubules,
which are where the sperm form.
Unlike a woman's ovaries, the testes are outside of the body,
because in order to make sperm, they have to be kept
at a specific temperature, usually about 2 degrees celsius
cooler than inside of the body cavity.
For that reason, the testes are kept in a pouch called a scrotum
that's in charge of keeping the testes at the perfect
sperm-making temperature.
After being produced in the testes, human sperm spend about 3 weeks
coiled in tubes in the scrotum called the epididymis,
and that's where they mature and grow flagella,
the little whip-like tails sperm are so famous for
and which make them able to move around and swim.
Now the sperm stay here until they're ready to leave the body,
so before we, or they, can go any further,
we have to set the stage for that.
As you know, in humans and some other animals,
the penis usually sits around, not doing much except
for letting urine out of the bladder from time to time.
But every so often a male realizes that he's totally going
to get the chance to mate!
At this point, spongy tissue in the penis
fills with blood, and BAM, erection!
Some animals like raccoons, whales and walruses actually
have a literal bone in their penis to help the erection along.
But either way, the point is to allow the penis to enter the vagina,
which scientists call coitus,
and deposit the sperm he's put so much into making.
These sperm travel in a special fluid, semen,
whose ingredients aren't combined until they're ready to be released
by a series of muscular contractions that cause emission,
more commonly known as ejaculation.
At this point, the contractions carry the mature sperm
from the epididymis, through two muscular ducts called
the vas deferens, which carry them up from the testes,
up and over the bladder, and down past the seminal vesicles.
Here, with contributions from the nearby prostate gland,
they pick up a bunch of fluid that contains mucus,
coagulating enzyme, ascorbic acid and sugars that the sperm
are going to need for their trip.
Now the semen is complete, and it travels down
the short ejaculatory ducts to the urethra to be released
at the end of the penis, where if the timing is right,
one among the hundreds of millions of sperm in that emission
can find and fertilize an egg.
That, my friends, is how we all get our start.
To find out, or to remind yourself, what happens after fertilization,
you can always check out this video on embryonic development.
But, fittingly enough, this wonderful beginning marks the end
of our treatment of the Animal Kingdom.
Please join us next week when we go deeper
into the other kingdoms that we share this planet with
the bacteria, archaea and protists.
Thank you as always for watching Crash Course Biology.
If there's any sex stuff that you want to go over again,
there's a table of contents.
Thanks to everyone who helped make this video,
especially to Amber for illustrating all of those gonads.
If you have any questions, ideas, or comments for us,
Facebook, Twitter, or the comments below.
We'll see you next time.