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Why Do Boys Have Wieners?

The questions kids ask about science aren’t always easy to answer. Sometimes, their little brains can lead to big places adults forget to explore. With that in mind, we’ve started a series called Science Question From a Toddler, which uses kids’ curiosity as a jumping-off point to investigate the scientific wonders that adults don’t even think to ask about. I want the toddlers in your life to be a part of it! Send me their science questions and they may serve as the inspiration for a column. And now, our toddler …

Q: Why do boys have wieners? — Katelyn S., age 3-and-a-half

Wieners exist because they make it easier to make babies. How are babies made? Why don’t you take that question to your parents? Go on. I’ll wait.

Back so soon? Yeah. You’re right. Making babies is pretty weird. And penises (that’s the technical, scientific term for the wiener) are a great example of just how weird it is. For instance, wieners exist because they make it easier to make babies … but you don’t need a wiener to make babies. Humans have figured out lots of technological end-runs around the wiener. Plenty of animals get by just fine without one. Other animals have things that are wiener-esque, shall we say, but aren’t officially wieners.

Even in humans, wiener ownership isn’t as simple as “boys have this; girls have that.” That’s because humans have both gender and sex, and those two things don’t always match. Somebody can be born with a wiener, but not be a boy. Somebody else can be a boy, but not have a wiener. Some people are born with reproductive anatomy that is neither wiener nor hoo-ha (technical name: vagina). There is a lot of diversity out there.

So let’s concentrate on a different question: Why are there wieners? For all their exposure, wieners are still mysterious things. Here’s what we do know: Wieners exist because they are a pretty efficient way to get sperm cells from a male body into a female body and close to an egg cell. Scientists call that internal fertilization, and it first evolved millions of years ago.

The first living things that had sex probably did it in a very impersonal way, similar to how sea animals such as coral and sponges do today — everybody just squirting their eggs and sperm into the water and then sitting back and hoping for the best. “But your sperm and eggs might never meet. Then that’s just wasted effort,” said Diane Kelly, a research assistant professor at the University of Massachusetts Amherst.

Some animals solved this problem by depositing their eggs and sperm into little nests. Female catfish, for instance, lay eggs in tree roots, under rocks or in holes in the riverbank. Males deposit sperm in the same place. That way, the eggs and sperm are closer together from the start, and the parents can reasonably expect their reproductive cells to have a chance.

Internal fertilization is kind of an extension of this. Instead of a crevice in a rock, you’ve got a crevice in the female’s body. Once you’re mating in this way, it helps to have something to get the sperm up in there effectively. Wieners do that. But here’s the thing: If it were simply about reproductive efficiency, then all wieners everywhere would look the same, Patricia Brennan, visiting lecturer of biological sciences at Mt. Holyoke College, told me. “You only need a little tube.” But that isn’t what we see in nature. “They’re all so totally different,” she said.

There’s a huge amount of diversity in the organs that animals use to facilitate internal fertilization. Octopuses use a modified version of a tentacle. Sharks use modified pelvic fins called claspers. Spiders spin a special web, spatter it with sperm, and use modified mouthparts to suck it up and deposit it into a female. “It’s very complicated oral sex,” Kelly told me. There are even a couple of species — a fish called a sculpin and a type of lice that lives in caves — where it’s females who stick their organs into the males, siphoning sperm back into their own bodies.

But those things don’t technically count as wieners. Wieners belong to amniotes — a class of vertebrates that includes us, as well as other mammals, turtles, snakes and birds. And those wieners all came from the same place. Somewhere, back in our history, a shared ancestor of birds, reptiles and mammals developed a proto-wiener. That single organ blossomed and bloomed into the magical garden of wieners we have today. There are big wieners and small wieners, pointy wieners and stubby wieners. There are wieners with distressing-looking spiky things all over them. There are wieners that come in pairs. (Some of these are wieners you may not want popping up on your work computer. So consider that before you click the previous links.)

This revelation about the origin of wieners is new information, Brennan and Kelly told me. Scientists used to think that all those different wieners had evolved independently of one another (which is true of the nonwiener organs like those possessed by octopuses, spiders and sculpins). We don’t know what shared ancestor we can thank for this, but we know it existed. Scientists have found that all amniotes use the same genes to grow their wieners from the same types of tissue. “The genes that control the formation of the penis are pretty much the same as genes that control formation of the limbs. It’s not like they’re arm and leg genes. It’s more like they’re sticky-out-y part genes,” Kelly said. That’s true even for snakes, whose ancestors evolved smaller and smaller limbs until they had none at all. But even without legs, snakes still use the same genes and same tissues to make wieners.

It wasn’t until late last year that scientists were able to show that all the lineages that make up the amniote family tree have the genetic capability of making a wiener, even if the animal doesn’t develop one. There are a lot of birds, for instance, that have no wiener. That’s also true of a reptile called the tuatara, which is the last surviving member of its evolutionary subgroup. In 2015, scientists showed that tuatara embryos start to develop wieners just like humans, cats, snakes and other amniotes. But the wiener gets reabsorbed before birth.

This is all important because it suggests that wieners matter for reasons other than just making internal fertilization a little more convenient. As Brennan said, if that were all there was to it, then there would be no reason for a single proto-wiener to have evolved such diversity, for some species to lose it, for other species to find it again. There must be other evolutionary drivers selecting different physical features for different wieners in different species. And nobody knows what those are.

There are guesses, of course. Brennan, who studies the evolution of duck wieners, thinks it has to do with sexual conflict — a coevolutionary arms race where females of a species and males of a species are both trying to take control of reproduction and end up evolving genitalia that help them have babies with who they want to have babies with. But we don’t know if that’s true. That’s partly because it’s difficult to study how male and female genitalia interact during sex. For example, Kelly told me that, in order to study fruit flies, scientists drop mating pairs into liquid nitrogen to freeze them mid-flagrante. That is not a technique you could use with many other species.

But our lack of knowledge also has to do with cultural bias, Brennan said. For how little we know about the wiener, we know even less about female reproductive anatomy. Female amniotes, for instance, have a thing called a clitoris. It’s a wiener. Same genetics. Same tissue. Just (usually) smaller. In humans, the clitoris (which sadly lacks its own comical, kid-friendly nickname) is a big part of how some bodies experience pleasure during sex. But is that why birds have a clitoris? What about snakes? They have a clitoris. Do they have orgasms? Nobody knows. And there’s a lot of debate about whether making sex fun is, itself, enough of a reason for females to develop a clitoris, or if that’s a side effect of some other driver.

Even the hoo-ha is very little understood. Before Brennan came along, scientists knew that duck wieners were these crazy, coiling things, often longer than the duck itself. But they hadn’t realized duck hoo-has were equally labyrinthine. Because nobody had ever looked. Hoo-has might play a big role in determining what wieners look like. But we don’t know, because science ignored hoo-has for a really long time. “Our understanding of the role of females is lagging way behind because there is a cultural bias,” Brennan said. “I think we’re moving forward with that. People realize it’s a problem of not having had enough female researchers asking the questions.” Maybe, Katelyn, when you grow up, you can find out the answers.

CORRECTION (April 22, 1:30 p.m.): An earlier version of this story misquoted Patricia Brennan. She did not say that a cultural bias assumes males are more important.

Maggie Koerth-Baker is a senior science writer for FiveThirtyEight.