The Truth About the Sex Lives of Dinosaurs

Dinosaurs are often thought of as aggressors—giant beasts that dominated our planet for millions of years. But these prehistoric animals almost certainly had a softer side. In the last decade, researchers have gained tantalizing insights into the sex lives and mating habits of these ancient reptiles.

In this episode, Smithsonian contributing writer Riley Black describes new evidence that reveals how and when dinosaurs mated—including ancient behavior recorded in rock, a new theory around dinosaur horns and spikes, and a prehistoric cloaca.

A transcript is below. To subscribe to “There’s More to That,” and to listen to past episodes on the sticking power of stories about animal behavior, what happens when the Colorado River goes dry and how asteroid dust might tell us about the origins of life, find us on Apple Podcasts, Spotify, iHeartRadio or wherever you get your podcasts.

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Ari Daniel: So, I’m curious, why are you so interested in dinosaur sex?

Riley Black: I think, as a subject, dinosaur sex is just, it’s fun and kind of funny to think about.

Daniel: Riley Black is a science journalist who often writes about dinosaurs for Smithsonian magazine.

Black: Like, how did animals that were multiple tons and up to more than 110 feet long, how did they make the bed rock? You know that these dinosaurs, like Patagotitan, the biggest dinosaur that we know about, more than 70 tons, more than 110 feet long, they had sex. How do you do that when you’re that massive and you have a big, heavy tail in the way? It’s almost like an engineering problem, trying to figure this out. And yet, we know that they did. And in just entertaining that question, we start to find neat stuff.

Daniel: Part of what’s fascinating about dinosaurs is their biological armor—the sometimes massive plates, horns and crowns that cover their heads and bodies. For a long time, scientists assumed these features were intended for fighting and self-defense, but recent discoveries have started to challenge this idea, including a new dinosaur discovered this past summer.

Black: That was named Lokiceratops, that had these really peculiar horns. They’re kind of blade-like.

Daniel: Lokiceratops, so named after the Norse god Loki, right?

Black: I think it has a little bit more to do with the Marvel show at this point, but yes, that’s the specific scientific reason.

Daniel: Just like the Norse god Loki or the Marvel character, Lokiceratops sports some pretty impressive horns on its head.

Black: That Marvel manifestation is always wearing the helmet with the curved horns. And Lokiceratops has brow horns that are kind of like that. But really, that’s just the beginning of its ornamentation. A lot of these horned dinosaurs, they have horns that are going all around their frills that shield a bone off the back of their head, and on their cheeks, and on their noses. They have hooklets and spikes and nasal bosses, and things like that, and they all have their own kind of dinosaurian fashion.

Daniel: I’m looking at a sketch of what Lokiceratops may have once looked like, and it is really dramatic. Can you tell me about what we think these horns may have been used for?

Black: So 100 years or so ago, the answer might’ve seemed obvious, that these would’ve seemed to be defensive weapons, and that might be partially true. But now, we think they have far more to do with social signaling, particularly what dinosaurs found sexy among their species.

Daniel: What dinosaurs found sexy … Well, that’s something I’d never really thought about before.

Black: The reason that we think this is because when you look at a dinosaur like Lokiceratops, at the same time, in different parts of ancient North America, where these animals are mostly found, no two look exactly alike. In each basin, you kind of have a different horned dinosaur with a different horn fashion. And what that’s telling us is that these were social signals. Yeah, sure, a pointed horn could discourage a predator or make it look like you’re harder to eat. But what’s really driving all these shapes is that these dinosaurs are interacting with each other. They’re showing off. We don’t know exactly how, but the anatomical signal is there—that basically pressures from within the species, what these dinosaurs found compelling, interesting …

Daniel: Attractive.

Black: Yes, would make a good mate. That is what’s driving the evolution of all these different forms. The fact that we often admire, are interested in these dinosaurs for their bizarre structures, right, it’s like, well, dinosaurs probably were just as impressed by those things, and now we’re just beginning to get the fossil evidence for how this worked and how the mating lives of dinosaurs—this point where they kept making more dinosaurs to be around for 235 million years—now, we’re just starting to get that insight.

Daniel: From Smithsonian magazine and PRX Productions, this is “There’s More to That,” the show bold enough to discuss the prehistoric birds and bees. I’m Ari Daniel. In this episode, what paleontologists know about dinosaur sex and what they still want to know.

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Black: Up until really just a couple of years ago, we had no idea what the genital anatomy of dinosaurs looked like at all. We could look at living crocodiles and we can look at birds, so the two closest relatives of things like T. rex and Triceratops, and say, “OK, in both crocs and birds, they have a cloaca.”

Daniel: The cloaca is the egg chute?

Black: It’s an egg chute and many other things. So it’s the Latin term for “sewer,” and it was originally named that because this is the outflow point for the excretory tract and also the reproductive tract. So you basically have these plumbing lines all coming, emptying out into the same chamber. We expect non-avian dinosaurs to have that as well, but there was no fossilized test of this. So basically, if you looked at a dinosaur and you wanted to figure out, “OK, what anatomy does this dinosaur have? What sex is this dinosaur?,” you wouldn’t see anything externally to tell you that, inside the cloaca would be your answer. And if dinosaurs were anything like modern crocodilians, which we think they were, it would be really hard to sex a dinosaur. Modern crocodiles, males typically have a phallus, females typically have a clitoris, and they look so anatomically similar that you need to be an expert to tell them apart.

Daniel: No one had ever seen a dinosaur cloaca.

Black: Until a little-horned dinosaur called Psittacosaurus. It had a beaky snout, it had these big flaring cheeks, it walked on two legs, had these bristles growing out of its tail, and there was a particular specimen that was so well preserved that the skin from under its hips was preserved, and we could tell that this dinosaur had a cloaca, just like crocodiles and birds do today. It was finally a confirmation of what we were basically making educated guesses about, based upon dinosaurs’ nearest relatives. And so far as we can tell, dinosaurs probably had the same kind of setup.

Daniel: So what does finding that cloaca in this dinosaur mean for our understanding of the reproductive lives of these animals?

Black: It’s likely that dinosaurs had what we often euphemistically call in scientific circles an “intermittent organ.” In order for male dinosaurs to fertilize female dinosaurs, they probably had some kind of phallus or something to bridge the distance between their bodies, which would make a lot of sense, given those big, heavy tails that would be hard to move out of the way. If we look at alligators and crocs and we look at many kinds of birds, things like emus and cassowaries, what we think of as more archaic birds, in the sense ones that are further down on the evolutionary tree, they all have phalluses and clitorises that work together when they mate.

Some birds today, they reproduce by way of what’s called a cloacal kiss, where they basically just touch the orifices together and sperm is passed during this brief moment of connection. But that seems to be what we call a derived trait, something that changed through songbird evolution over the past 50 million years or so. For all our favorite dinosaurs, they probably had a setup much more like what we see in crocs and alligators. And that brings up all kinds of questions of like, “OK, what shape were these organs? How did they vary from species to species?”

Especially when we think of armored dinosaurs, things like the Ankylosaurus or Stegosaurus, did they have a longer intermittent organ, because it’d be more difficult to get around all those plates, and spikes, and things like that? So even though the cloaca itself doesn’t give us these answers, it at least points us toward some of these additional questions. And it’s possible these fossils are out there. As strange as it might seem to have this on a wish list, yeah, it’s entirely possible that a dinosaur was preserved in such a way that if their genitals were everted out of the cloaca, it could have left an impression or been preserved in some way. And that would tell us a great deal about how the mating behavior of these animals played out, because as far as sort of dinosaur sex positions or how they actually did it, that is entirely a matter of art and kind of mashing sculptures together until we have something more substantial to go on.

Daniel: So many people have been captivated by dinosaurs from a young age because they see their skeletons reconstructed in museums. And we feel like we have some understanding of what they look like. But to be honest, Riley, I never really considered their genitalia. Nothing I ever saw a museum talk about. So I’m wondering, was this content just a bit too spicy for natural history museums to consider discussing, or was it just this kind of big, blank space for a while that we knew nothing about, so there was really nothing to discuss?

Black: I mean, this is the question, right? Why was nobody talking about dinosaur sex?

Daniel: Why was nobody talking about dinosaur sex? That’s the question.

Black: A few people have over the years, but I feel like it was often due to something we didn’t have very much direct evidence about, and so scientists didn’t want to go too much out on a limb about it. And also, that is sex. This is something particularly in the United States that, look, our sex education for ourselves is bad. For dinosaurs, I don’t think anybody was out there advocating like, “We need to know these things.” And thankfully, I think it’s beginning to change.

Daniel: A conversation about the sexual characteristics of dinosaurs doesn’t end with their genitalia. Going back to the horns and armaments we were discussing earlier, Riley says that the Triceratops has yielded some interesting insights there, too.

Black: So Triceratops, that one that we all know and the first dinosaurs we’re ever introduced to, paleontologists were curious, “OK, what were they actually using their horns for? What’s the behavior?” And a paleontologist by the name of Andy Farke took Triceratops models, these exact models of the skull, and he put two of them together, said, “OK, if these dinosaurs were locking horns, what would it look like? What were the arrangements that would actually work if they were engaging in combat?”

And based upon the positions that seemed to work on the models, he actually went and looked at the skulls. And those skulls have basically scratches, lesions, bone damage exactly where you would predict if these animals were locking horns. So it seems that Triceratops weren’t so much fighting tyrannosaurs as they were fighting each other. And why do animals tend to fight within the same species? It usually has something to do with territory and mating.

Daniel: From what I understand with modern animals, and I don’t know if this is the case with dinosaurs, maybe fighting would’ve been a last resort, that often these plates and horns and accessories were a signal both to potential rivals that they could take someone on, and also to potential mates that they were a good prospect, but that if push came to shove, they would push and shove.

Black: Absolutely. When we look at modern animals, where you have competition for mates or for spaces to live and they get into conflicts, it’s usually not when there’s a really big one and a small one, or they seem unevenly matched. It’s when they’re relatively evenly matched in size, ornamentation, what have you, that there’s more likely to be an actual fight.

Other than that, there’s all sorts of social signaling, bellowing, throwing their heads around. We don’t know exactly what they were doing, but given that Triceratops seems to be one of the few that actually could engage in combat in this way, the rest of them were probably posturing. They’re trying to seem big, shaking their heads or doing something interesting, to use all those flashing ornaments to say something and avoid potential injury.

Daniel: And is it the case that perhaps male dinosaurs would look different than female dinosaurs in this regard if it was actually due to some sort of attraction?

Black: This is a big debate that spilled out in the literature about ten years or so ago. When Darwin came up with the idea of what we call sexual selection, one of the hallmarks of sexual selection, the idea that sort of mate choice was causing evolutionary change was something that he thought of as sexual dimorphism, that basically you’d wind up with two different forms. We see that among some animals today, like living dinosaurs, many birds—particularly the males will be more brightly colored, or they’ll have these long trains of feathers.

When we look at the bones of non-avian dinosaurs, we don’t find these differences that we would expect. It’s actually really unusual. We would expect to see some form of sexual dimorphism, or different anatomies or shapes based upon different sexes, and we don’t see that. It seems that skeletally, at least, males and females seemed pretty similar to each other. So whatever differences there were between dinosaurian sexes most likely were in coloration or aspects of behavior, something that didn’t leave its hallmark on the skeleton. Then again, our sample size is very, very small.

Even the best known dinosaurs, like T. rex, for example, we only have about 50 partial skeletons from a two-million-year span, from a geographic range that goes from Saskatchewan to just about northern Mexico. If you talk to a modern biologist who studies variation in the species or tries to figure out sex differences or things like that, that sample would be pitiable. It’s great for a dinosaur, but we usually don’t have these population-level sort of metrics on dinosaur variation. So it’s possible we’ll be surprised in the future. But as for right now, no one has ever convincingly found any evidence of sexual dimorphism among dinosaurs. They were doing some things that are relatively familiar, but it was getting expressed in a way that we’re still trying to understand.

Daniel: Does that cast any doubt on the idea that these structures may have been used for mate attraction?

Black: So there are some paleontologists who have argued yes, because they had a stricter view that sexual selection requires sexual dimorphism in order to identify. There are other paleontologists who disagree with that, and I tend to lean more in their direction—that social signaling can result in these arrays of form without, “Males look like this and females look like that.” I mean, just even looking at humans, everybody has nipples, and people will usually think, “Well, nipples are for breasts and breasts are for breastfeeding,” and that’s a very reductive way of thinking about this.

It's more that everybody has the genes for this aspect of development, which will get expressed differently based upon any number of other environmental factors. So the fact that these bizarre dinosaur structures are present in different dinosaur sexes really makes sense in the fact that yeah, there’s something about where their genes met the environment. And it worked for them, and it worked for hundreds of millions of years.

Daniel: I am just curious, what does knowing about dinosaur mating behavior, dinosaur sex, what does that tell us about this moment in evolutionary time and about these remarkable organisms?

Black: So one of my favorite recent discoveries in this involves these nesting grounds. They found some in Colorado. There are similar marks that have been made in Korea and in Canada. They’re scratch marks in the ground. They were originally thought to be failed nesting attempts, that dinosaurs are trying to build a nest in the ground, they gave it up. But there are too many of them, and they don’t really make sense in terms of their distribution if it’s just nests. They look just like what ground-dwelling birds do today, things like puffins, some things like turkeys. They gather together in these broad, open spaces, and they start scratching at the ground, and that’s what they did to show off.

And these are animals that were like Allosaurus or Tyrannosaurus. We don’t know exactly what species they were because the bones were not preserved with the tracks. But if you imagine an animal like that—something with a big, toothy head, eats lots of meat, tiny arms, big chicken-like feet, multiple tons, standing out in the middle of the ancient landscape, just scratching away, trying to do their best job to impress some other dinosaur … And I think it’s fantastic that we have these moments of prehistoric time—that that’s fossilized behavior that we otherwise totally would’ve missed.

There are surprises in the fossil record, that there are things in there that we never would’ve expected to find, but if we start asking the right questions, maybe we can pick up on. I love that aspect of dinosaur biology, that these are moments that you would think would not be preserved, just these fleeting bits of Mesozoic time. And yet, there they are, right in the rock.

Daniel: Talking about these scratch marks, they would scrape the ground as they kind of strutted around, trying to impress mates?

Black: So if you’ve ever seen like a sage-grouse …

Documentary voice: The sage-grouse simultaneously scrape their wings back and forth against their flanks, expel air from twin fleshy chest sacks the size of tennis balls, and call softly. The resulting sound combines swishing, popping and cooing.

Black: These are birds that gather together in these broad, open areas, and they kind of dance around, and shake their feathers, and things like that a bit. It was something similar to that where every dinosaur kind of had their place at the audition and were scratching at the ground, much the way that if you’ve ever seen a chicken out in the yard do so. It’s kind of funny to think of an animal that we regard as so big, and impressive, and even intimidating as these carnivorous dinosaurs doing something so simple, but it seems to have worked for them. And birds are carrying on this tradition.

Daniel: Sage-grouses also produce vocalizations in those gatherings.

Black: Yes.

Daniel: So I can imagine that the dinosaurs too may have bellowed or hooted or something in order to encourage mating as well?

Black: Yeah, I wish we could hear it. Dinosaur sound is so complicated to get at. But the same way that we figured out dinosaurs should probably have a cloaca, if we look at their modern relatives—do something what we call extant phylogenetic bracketing, which just means that you look at their nearest living relatives, and if the same trait is present in both, it was most likely present in the common ancestor, at least we can hypothesize that it was.

So, many birds today, especially when they’re mating or courting, they sing. They have all kinds of wonderful, lovely vocalizations. And even birds that we normally don’t think of as making sounds certainly do. I knew someone once who kept emus, and to hear them drumming at night in those low bass notes—it’s like, those birds can certainly make sound. Modern alligators and crocodiles also bellow. If you go to the Florida Everglades during alligator mating season, you will hear this. They even make the water kind of dance on their back from the vibrations, from these low bellows. So the fact that crocodilians do this, living dinosaurs do this, non-avian dinosaurs probably did the same.

Daniel: I’m wondering, what do we know about when dinosaurs started having sex in their lives?

Black: This makes dinosaurs very relatable to me in that they started having sex before they hit complete adult maturity. Dinosaurs were not waiting until the equivalent of their 20s and 30s to start being sexually active. They basically were becoming sexually active in their teenage years. We know this because there’s a specific kind of bone tissue called medullary bone—it’s related to egg laying. We see this in modern birds, where when a bird is getting ready to lay their eggs, they develop this calcium store in their bone, this bone tissue inside the long bones, and it’s temporary. They build up this resource, and then a lot of the calcium from the eggs is mobilized from this medullary bone tissue.

It's been found in T. rex, it’s been found in Allosaurus, it’s been found in a beaked herbivorous dinosaur called Tenontosaurus, and I think it’s been described in a few other dinosaurs and prehistoric reptiles as well. And it only exists in a dinosaur when that dinosaur basically has fertilized eggs that it’s developing. So you can find this bone tissue and tell, “OK, this dinosaur has had sex.” When you look at how old these dinosaurs were in terms of their individual ages, we do this similar to counting tree rings, where dinosaurs laid down what we call these lines of arrested growth where they’d grow during the wet season or where there’s plenty of food and then they’d stop growing during the dry season where there was less food.

And by counting basically these rings and looking at the nature of the bone tissue—is it growing fast? Is it growing slow? Fast-growing bone often looks messy; slower-growing bone often looks a little bit more precise and put together. It was obvious that each of these dinosaurs was reproducing well before they hit skeletal maturity, before they stopped growing. This is different from what we see in many birds. Many birds, they will grow incredibly quickly to become adults within one year, and then they start reproducing. Dinosaurs were reproducing well before they hit their full adult size and skeletal maturity. And this was also part of why they were so successful in the wake of several mass extinctions. There were still our predecessors, the earliest mammals and proto-mammals were around.

They reproduced much more slowly and had a smaller number of offspring, whereas dinosaurs were reproducing earlier in their lives and having more offspring. So basically, they’re able to flood the environment, and that’s what created what we think of as the Age of Dinosaurs. They basically bred so quickly that they filled up the world and fundamentally changed its ecology before our relatives and ancestors could really even get their feet off the ground.

About 20 years or so ago, there was a pair of hips from an oviraptorosaur. If you kind of think of a parrot that looked like Velociraptor, and it had two eggs preserved in the hips. And No. 1, that’s really, really cool, because that shows that this dinosaur was about to lay eggs, hadn’t yet, perished and became preserved before making that part of the nest. And it showed that non-avian dinosaurs probably had two basically ovarian follicles making one egg in each at a time, and then laying them down in the nest. And it matched the nest pattern, where for these dinosaurs, the oviraptorosaurs, usually see pairs of eggs arranged in a circle around the nest. So it was really clear that they were making two at a time, two at a time, two at a time, and kind of arranging them so they could sit in the middle of it.

And this is between what birds and crocodiles do in that birds have one functioning oviduct, they make one egg at a time. Alligators and crocodiles have two functioning oviducts, but they make many, many eggs at a time. They make clutches of eggs all at once. So once again, dinosaurs both resemble living things and were doing their own unique kind of behaviors. And we would have no clue about it whatsoever if this one, sadly tragic dinosaur had not died just before it was about to lay eggs. And it’s those sorts of finds that can kick off all sorts of other questions or areas of inquiry, whereas before it was just kind of a black box where we weren’t really sure what was going on.

Daniel: It’s striking to think about an animal so big sitting on an egg.

Black: Yeah. Dinosaurs, so far as we know, had all different kinds of ways of nesting. Oviraptorosaurs were among the best dinosaur parents that we know of. They would lay these two eggs at a time, arrange them in a circle, and they’d sit right in the middle of it. And some of these fossils have actually been found on the nest with their arms spread out around them. And we know these dinosaurs were feathered, so that’s another feather function. They’re keeping their nests warm. They kind of made the spot in the middle and kind of laid so that their arms and their tail were covering the nests and keeping them warm.

Daniel: What a beautiful image.

Black: I know, it’s really fantastic. It’s sad when you find these fossils in a way, because there’d be like dune collapses. These dinosaurs would be sitting on their nest, and a whole sand dune would collapse on them. And that’s why we know about these moments, so it’s always a little bittersweet. Then you have the long-necked dinosaurs, things like Apatosaurus or Brachiosaurus. So far as we can tell, no parental care whatsoever—that they scratched nests out of the ground using their front feet. They laid a whole bunch of eggs, sometimes in the hydrothermal plains, basically places like if you think of Yellowstone, with all their geysers, and fumaroles, and things like that, where the ground was naturally kept warm, they congregated here, they built their nests, left the eggs. And it’s kind of like what you see in sea turtles today, where like all at once, you have this flood of tiny hatchling, which the carnivores knew about, and would eat up prehistoric popcorn. And that’s why we don’t have a whole lot of baby dinosaurs. So it really ran the gamut for these animals from very dedicated parental care to just what we call, “Lay them and leave them.”

Daniel: The other thing that this conversation is making me reflect on is how often I feel like the narrative surrounding dinosaurs is one of aggression and one of fighting and conflict. But what you’re describing is a whole series of moments that accompany that other set of narratives—moments that are tender, moments that are evocative, racy even. It’s adding this whole other kind of flavor and texture to this time period of when the dinosaurs lived.

Black: The most common phrases that we use around dinosaurs are ones of dominance: “When dinosaurs ruled the earth.” The famous images are often of aggression and predation. And any time a new carnivorous dinosaur is described, there’s more than half a chance it’s going to be depicted with its jaws open, and drool, and blood, and everything else. And those things certainly did happen, they were part of dinosaur life. But dinosaurs would not have existed for as long as they have without these more tender moments, without courtship and mating, without taking care of their offspring, without evolving different ways of reproducing.

We have dinosaur bones. We have dinosaur footprints and fossils. Those are the remnants of the real animals that we try and understand. But our image of what dinosaurs are and how they lived fundamentally changed from the time that we started to find and describe them. We have always played a role in shaping the image of what dinosaurs are. So, for example, during World War I, dinosaurs were generally considered to be big reptiles who were not very smart, dragging their tails, living in swamps, violent creatures that were just tearing at each other. And there was a pacifist group called the Anti-Preparedness [Committee] who adopted a Stegosaurus that they named Jingo as their mascot because the whole idea is, “All armor plate and no brains”—that these were not very smart animals, they’re incredibly violent, “Don’t be like the dinosaurs.”

Further through the 20th century, a lot of that imagery remained, but I feel like it’s been in the past 20, 30 years or so that we’re starting to expand beyond this. All those messages about dinosaurs being dominant and kind of tearing into the world, they’re still there in pop culture because it is evocative imagery. But I want to do my part in this effort to shift the image of a dinosaur or at least bring the complexity to it, that yes, there was conflict and predation and they were incredibly successful animals. But it’s not as if the first dinosaur hatched out of the egg and started eating everybody, and that’s how they came to rule the planet or something like that.

It was the fact that there was parental care, that they had these sort of complex courtship and behavioral systems. And I think if we look at dinosaurs less as these animals that took on the world, nature red in tooth and claw, but were successful because so many of them were behaviorally complex, because they were caring for their offspring in so many cases, it paints a fundamentally different picture.

Daniel: Riley, thank you so much. This has just been fascinating, and I love how it’s changed the way that I think about these thundering beasts that sauntered and swayed and sashayed across the landscape millions of years ago.

Black: Always a pleasure. Thank you so much for letting me rhapsodize about dinosaur romance a little bit.

Daniel: You can read more about dinosaurs and their sex lives from Riley Black online at SmithsonianMag.com. We’ll put a link in our show notes, as well as a link to subscribe to future issues of Smithsonian magazine.

“There’s More to That” is a production of Smithsonian magazine and PRX Productions. Find us on Apple Podcasts, Spotify, the iHeartRadio app and wherever you get your podcasts.

From the magazine, our team is me, Debra Rosenberg and Brian Wolly. From PRX, our team is Jessica Miller, Genevieve Sponsler, Adriana Rozas Rivera, Sandra Lopez-Monsalve and Edwin Ochoa. The executive producer of PRX Productions is Jocelyn Gonzales.

Our episode artwork is by Emily Lankiewicz. Fact-checking by Stephanie Abramson. Our music is from APM Music. Sage-grouse and alligator recordings courtesy of American Archive. I’m Ari Daniel. Thanks for listening.

You know, I’m thinking maybe they should have called it Jurassic Spark.

Ari Daniel | READ MORE

Ari Daniel, the host of Smithsonian magazine's "There's More to That" podcast, is an independent science journalist who has reported across six continents and contributes regularly to National Public Radio among other outlets. In a previous life, he trained grey seal pups and studied wild Norwegian killer whales.