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Over 95% of bird species don't have penises. And to us, that might sound bizarre, but it works:.

They bring their nether regions together, much like mammals do, but there's no penetration involved. And to be up front: we don't have one, definitive answer as to why. But we have some pretty good ideas.

And by looking at those ideas, we can learn a lot about birds and evolution in general. Now, to be clear, we're talking about penis loss because scientists are pretty confident that, around 310 million years ago, the land-dwelling ancestor of all reptiles, mammals, and birds had a penis. The ancestor also laid eggs, which were filled with fluid and had water-proof shells.

That was a key development that allowed animals with backbones to move onto land, since they needed eggs that could survive outside of water. But, it meant they couldn't fertilize those eggs the way many fish and amphibians do: by dousing them with sperm as the eggs are laid, or releasing egg and sperm into the water and assuming they'll meet up. A tough eggshell means no sperm's getting in after the egg leaves the female.

So fertilization has to happen internally, before that shell forms. And penises are very good at delivering sperm right where they're needed to do that. Still, chickens and the vast majority of their avian kin lack them.

Tuatara, the last living remnant of an old reptile order that's only distantly related to birds, are also penis-less. Meanwhile, all other lineages in this group, collectively known as the amniotes, have them, though, they vary kind of a lot. Like, snakes have twin hemipenes.

So, there's that. Given such wildly different-looking anatomies, scientists used to think that the penis evolved separately in different amniote groups. But when scientists started to study chicken and tuatara embryos more closely, they discovered something that basically sealed the deal on a single, early penis evolution: both species start out making penises.

See, during embryonic development, an embryo starts with two swollen areas of tissue near the hind limbs. These later merge to form a single genital tubercle, which, in males, becomes the penis. Or, in lizards and snakes, they remain unfused to become those two hemipenes we talked about.

Reptiles, man. Anyway, in 2013, researchers discovered that in chickens, this early tubercle switches from actually growing to dying part way through development, leaving a little nub, if anything at all. And just a few years later, a team from the same lab confirmed something similar happens with tuatara.

That pretty strongly suggests that all amniotes had penises before the lineage split into birds, reptiles, and mammals. And it was that universal penis that was modified by different groups, sometimes dramatically… which, in retrospect, makes sense. Since reproductive anatomy can have a large effect on whether an individual breeds or how many offspring it has, penises and other involved parts can evolve and change surprisingly fast.

These findings also made it clear that penis reduction and loss occurred separately at least four times in amniotes! That's in spite of the fact that internal fertilization is pretty crucial to being able to live full-time on land. And early amniotes would have faced strong selective pressures to develop reliable methods of getting sperm and eggs together.

We don't have a time machine, so it's challenging for scientists to figure out why so many groups would do away with something so practical. Luckily, researchers can make clever predictions based on hypotheses, and then test those predictions against data they can collect to see if their ideas hold up. And there have been at least seven suggested explanations for why so few bird species have penises.

But few withstand scrutiny. Like, it's been suggested that waterfowl maintained them to keep sperm safe and dry because they mate in aquatic environments. But that doesn't explain why ostriches have them, or why tuatara lost them.

Or, maybe, the external appendage just had to go to reduce weight and drag for efficient flight. But duck penises can be half as long as their bodies, and yet, they're a mere 0.3% of their body weight and don't seem to impact their flying ability. There are still a few strong contenders, though.

The first of these has to do with sexually transmitted diseases, or STDs. See, birds and reptiles, and even egg-laying mammals, use a single orifice for sex, urination, and excretion. This one-stop shop is called a cloaca.

And it has the potential to be a nasty breeding ground for bacteria, especially in birds, since they keep their body temperature around 40 degrees Celsius, a temperature germs can thrive at. So, it could be that penis reduction was naturally selected for because it reduced the amount of contact males had with this potentially-deadly orifice. We don't actually know a ton about avian STDs, but the ones we do know about can be super bad.

Goose venereal disease, for instance, carries a 10% mortality rate. And if the bird survives, the infection can cause reduced weight and fertility in both sexes. Studies have also found that, for some species that do have penises, males have a nine-fold increase in specialized immune system cells in the penis during the breeding season.

That suggests mating via the cloaca is a disease threat for them. In fact, even in birds without penises, bacteria spread relatively easily during mating, so much so that females seem to have strategies for eliminating unwanted bacteria that hitch a ride on the male's seminal fluid. So, this hypothesis makes sense.

But, we don't have enough information about avian STDs to feel totally confident or to rule out alternative explanations. It also doesn't explain why monotremes, mammals that also have cloacas, and are similarly warm-temperatured and egg-laying, kept their penises. But that could be a whole separate video.

Some scientists think a different hypothesis is more likely: that penis loss is really about female choice. In sexual selection, the preferences of members of one sex of a species can drive the evolution of traits in members of the other sex. Think of a peahen's preference for the beautiful feathered tails of peacocks.

Who don't have penises, by the way. Well, some researchers think female birds have good reason to pick the smallest penises they can find. See, female birds have to invest a lot of energy to make egg yolks and shells.

So, they may want a say in who fathers those chicks, to ensure they're getting the best quality genes for their offspring. And that means they might prefer their eggs were fertilized by males that they chose, rather than, say, ones that used a longer appendage to force copulation. In a nutshell: a preference for ever-smaller penises could have given females greater control over the parentage of their chicks.

This only works because birds fertilize and lay eggs one at a time, and up to a day or more apart. So a female can know which male provided the sperm for each egg, and therefore, could decide to abandon or destroy an egg laid after mating with an undesirable partner. All this means that even though longer penises have reproductive advantages, like, they get sperm in prime position for fertilization, the females' distaste for them could lead to their reduction or loss.

The catch here is that, for birds with relatively large eggs, the cost of abandoning is quite a bit higher! High enough, perhaps, that it would never be worth it. So this hypothesis predicts that large-egged birds should have retained their penises.

One analysis tested exactly this by comparing relative egg size and penis presence across bird species. And lo and behold: it turns out the birds with the largest eggs relative to their bodies, like kiwis, do have penises! There's other support, too, like, that in ducks and other smaller birds with penises, females have evolved tactics to avoid forced copulation, and males have continued to evolve traits to achieve penetration anyway.

Still, that's a bit short of a slam dunk explanation. After all, it's possible that sexual selection is at play, but from the other side. Male birds also want to be sure that they're the parent of their partner's eggs and chicks.

And most of them have that assurance, because over 90% of bird species exhibit biparental care. When both parents stay together, incubate the eggs, and feed the chicks, a male can be pretty certain the offspring are his, and he has less need for an elaborate sperm delivery mechanism. But in birds without coparenting, or when extra-pair copulations are common, there may be evolutionary pressure for a longer penis, one that allows a male to get his sperm closer to the eggs than the sperm of any other males a female mates with.

This sperm competition hypothesis could explain the existence of penises in some birds while also explaining their loss in ones that have mating and parenting styles which better guarantee paternity. But there are enough exceptions across bird families to make this hypothesis a little shaky. And taxonomists are still debating what method of parental care, if any, was common in birds' ancestors.

Our final hypothesis has nothing to do with sex. Which might seem weird since we're talking about penises, but hear me out. It's possible that the missing penises are just side-effects of some other selected-for trait, a phenomenon called selectional pleiotropy.

Pleiotropy can happen when the genes that control one trait that is under evolutionary pressure also happens to control something else kind of unrelated. And we actually know this is the case for genes involved in penis development. Research in mice, lizards, and snakes has revealed that key genes in penis formation are also involved in limb development.

Like, in chickens, a gene called Bmp4 dictates the death of the genital tubercle. In fact, if you activate it in duck embryos, they fail to grow a penis! And Bmp4 also helps dictate limb growth because it's involved in building bones.

Oh, and it also governs the loss of teeth in birds. So it's possible that selection for changes in limb development or teeth just so happened to impact avian penis size. And so long as that didn't cause much of a reduction in reproductive success, then this kind of incidental loss could occur.

While this hypothesis is plausible, some think it's a little unlikely to have occurred so many times, and especially unlikely to happen with something so central to reproduction. But further study of Bmp4 expression patterns in birds with and without penises could bolster support for this hypothesis. It's also good to remember that not every trait we see is adaptive.

We tend to automatically think that every aspect of an organism is tuned to perfection. But that's just not how evolution works. Non-harmful mutations or weird side-effects of selection could explain a lot of things that seem evolutionarily counterintuitive.

And we already know that evolution isn't always about something being adaptive. Flukes of who lives and dies in a population can have large effects on the prevalence of traits or gene variants, a phenomenon called genetic drift. The idea of penis loss being a fluke might be hard for our species to grasp, especially since they've been a pretty big part of mammalian reproduction.

But, clearly, thousands of bird species have been surviving and breeding just fine without one, demonstrating that internal fertilization doesn't require a penis. So, they could have lost them on accident, or essentially in exchange for something way more useful to them. Or, maybe, it was natural or sexual selection, because of how they mate, who they want to mate with, or who takes care of the offspring.

We're still trying to pin down exactly why so many birds ditched penises. But one thing is for sure: the more we learn, the more intriguing the story gets. Thanks for watching this episode of SciShow, and learning more than you ever thought you wanted to know about bird genitals.

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