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 from both of you through sexual reproduction. From liver flukes to pine trees ninety-nine percent of eukaryotic organisms on Earth use sex to reproduce at least some of the time. That's because 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 gotta 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.

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Reproductive systems, like all the other systems we've discussed, take on an incredible diversity within kingdom Animalia. For instance, some female spiders mate with a bunch of different males, and then stash their sperm into 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 of the storage unit to fertilize her eggs.

Hyenas, meanwhile, have a female dominated social system, and it's the alpha female who chooses who she mates with. She has sex using an enlarged sensitive sex organ, a clitoris, that looks exactly like a penis called a pseudo-penis.

And a duck's penis can be a quarter of the length of its body and shaped like a cork screw. Want to know why? Look it up. Actually, don't. Google that with care. Just don't press play on the video.

Gametes: Ova and Sperm
(2:16) The point here is that while the delivery systems may be some what 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 that they only have one set of chromosomes, and they're formed by the process of meiosis. And, there are only two types of gametes. One, is the ovum, or egg, in plants it's called the 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 and 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 con 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 by-product of a much, much more important and fundamental distinction. From a biological perspective the only thing that makes sexes different is that the females produces 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 into 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 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, and more combative than the females. Basically, they're putting on a big show so that the females can size up how awesome that guy's genes are.

Sex Determination
(4:59) 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 we were hanging out in our mom's uteruses, we didn't have a sex at all until about two months. Before that, we had all the pieces to either become 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 chromosomes called XX, it will be female, and if it has two different chromosomes,  XY, it will be male. The same is true for other animals like fruit flies, and even some plants like ginkgo 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 tissues in it actually becomes part of a penis. But it is 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 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 an females that will later determine how both estrogen and testosterone are used by 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.

Secondary Sexual Characteristics
(6:48) 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 examples being breasts.

In other animals, secondary sex characteristics include things like manes on male lions, a big ol' 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.

Female Reproductive Structures
(7:25) 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 all this works with human people, and of course, ladies first.

Uterus & Oviducts
(7:40) 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.

Endometrium (Menstruation)
(7:51) Maybe the biggest different between women's and men's reproductive setup is that women have 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 find it and fertilizes it there is 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 seventy percent 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 is 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.

Cervix & Vagina
(8:32) This material leaves the female reproductive system through the narrow lower end of the uterus, the cervix, and then out and 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.

Male Reproductive Structures
(8:45) While a women's body is busy all month developing the next egg, getting it ready for fertilization, shedding her uterine lining if its not fertilized, males are undergoing a completely different process that calls on a lot of other highly specialized reproductive structures.

Scrotum, Seminiferous Tubules & Epididymis
(8:59) We start, of course, with the testes which are largely made up 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 two degrees cooler ceases than inside 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 three weeks coiled in tubes in the scrotum called the epididymis, and that's where they mature and grow flagella, the little whip like tails that sperm are so famous for, which make them able to move around and swim.

Penis (Coitus)
(9:37) 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 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.

Vas Deferens To Urethra (Emission)
(10:13) 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 mucous and coagulating enzymes, ascorbic acid and sugars that the sperm are going to need for their trip. Now, the semen is complete, and it travels down 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, the archaea, and the 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.