How Dinosaurs Coupled Up

Animals today do all kinds of different things to try to catch the eye of a mate.

Some male mammals like deer and bighorn sheep literally butt heads to compete for females.

Other male mammals do things like post pictures of themselves holding fish on social media.

Lizards flash brightly colored neck flaps and bellies.

And birds sing, dance, and even build and decorate elaborate structures made of twigs and rocks to win hearts.

But what did the non-avian dinosaurs do?

Y’know, the extinct ones.

Dinosaur mating behavior has been the subject of a lot of speculation, including BY ME.

But what we can actually say about it from the fossil record has been pretty limited.

It’d take the description of a new kind of fossil in 2016 to give us a peek into that part of the lives of these animals.

The fossils were the 98 million year old traces left by big theropod dinosaurs, cousins of the famous T. rex and Velociraptor, in Colorado.

And they didn’t really match the fierce reputations of most theropods.

So prepare to learn what little we know about dinosaur sex.

I’d rather you hear it from me than, like, on the playground or whatever.

Dinosaur mating has been on the minds of paleontologists for more than a 100 years.

It’s been on MY mind since I first learned what a cloaca was.

I’ll get back to that in a minute.

But, scientifically speaking, we know dinosaurs did it, but we don’t have a lot of evidence for how it all happened.

Like, on the tree of life, dinosaurs are close relatives of crocodilians and birds, so they probably shared some of their reproductive anatomy and behaviors with these two groups.

When we make inferences based on evolutionary relationships like these, it’s called phylogenetic bracketing.

And it’s useful because it gives us a starting point for coming up with hypotheses about what dinosaurs might’ve done.

But then we need actual, physical evidence to test them.

And that’s where those fossils published in 2016 come in.

A team of paleontologists was working in Colorado, looking for dinosaur trackways in a geological formation that preserves a lot of trace fossils.

These are things like footprints and other marks left behind by extinct animals.

Now trace fossils might not seem as exciting as a dinosaur skeleton, but they give us a window into the past, because they record an organism’s behavior.

Now, we have found hints about mating behavior in the bones of some dinosaurs, like tyrannosaurs with bite-marks on their faces, because those could be signs of fighting over mates.

But, they easily could’ve come from something else too.

But that site in Colorado was crisscrossed by dinosaur trackways, and there the team noticed some odd scrape-marks on the ground.

First one set, and then another, and then another.

All told, they would find around 60 scrapes there.

And it turns out it wasn’t the only site like this.

They’d also discover two more close by, in western Colorado, and one in the eastern part of the state.

And these scrapes were a whole new kind of trace fossil, one that hadn’t been recognized before.

They had a distinctive shape - with a deep groove down the center, as if the maker had sharp claws on their toes.

And associated with some of those scrapes were footprints with three toes.

So this convinced the paleontologists that the tracks were made by theropods.

But then they had to figure out why big, probably-meat-eating dinosaurs would’ve been digging in the dirt.

And they proposed four hypotheses.

First, maybe the scrapes were nests or parts of nest colonies.

Now, we have found dinosaur nest colonies before, and we know what bird nest colonies look like.

The nests tend to be a standard shape, with regular spacing between them.

Nests also tend to have eggshells in or around them, and sometimes the remains of unlucky hatchlings, too.

But the scrape sites don’t have any of these things, so the researchers think they weren’t nests.

So OK, hypothesis number two: Maybe the dinosaurs were digging burrows, or looking for food or water.

But digging doesn’t seem to have been a common behavior for most dinosaurs, and the little evidence we have suggests that dino burrows would’ve been small -- certainly smaller than these big dinos could’ve used.

And since we think the scrape-makers were theropods, they probably weren’t digging for plant food, like roots.

There aren’t any burrows belonging to things like small mammals there, either, so it doesn’t look like they were digging for prey.

And the environment was wet enough that they wouldn’t have needed to dig for water.

Hypothesis the third: Could the scrapes have been made to mark territory?

Well, this isn’t a behavior we see in birds or reptiles today, either -- it’s more common in mammals that mark those scrapes with the scent of their urine.

So, we’re left with the fourth hypothesis: The scrapes were marks made by mating behavior.

Now, we know living birds are dinosaurs, and they’re even technically theropods.

Which means we can use bird mating dances as a model for what might’ve been going on.

A lot of ground-nesting birds, from shorebirds to the nocturnal parrot from New Zealand known as the kakapo, do what are called “nest scrape displays.” As part of their courtship behavior, males scrape the ground with their feet, and move around a certain area, over and over.

Sometimes they do this to show off to females, in hopes of being chosen as a mate, and sometimes they do it to warn rival males away.

And sometimes the females get in on the action too, as part of the courtship dance.

This leaves behind traces a lot like those at the Colorado site, just on a much smaller scale -- the dinosaur scrape sites were about two meters long on average, much larger than any living bird could make.

And the paleontologists argue that the fact that so many different birds make these scrapes today is pretty compelling evidence that this behavior has a deep evolutionary history.

In which case, these scrapes could be some of the best evidence we have of the lead-up to dinosaur mating -- literally, the courtship dances of big theropods, preserved in stone.

And since that study was published, we’ve even found more scrapes like these, in different sizes and in other countries!

Paleontologists have described scrapes from sites in Canada and South Korea.

And the South Korean scrapes are a lot smaller than the ones from Colorado - though they also show a three-toed foot - suggesting that this behavior might’ve been something common to theropods.

So this fills in a missing piece of the puzzle of dinosaur reproduction.

Before, all we really knew was the outcome -- that dinosaurs built nests, laid eggs, and at least some of them cared for their young.

Now, we have a better sense of what happened to get to that point -- a kind of before and after snapshot of dinosaur mating.

But we still don’t know what happened during the main event itself ... although that too might be coming into slightly clearer focus.

In early 2021, researchers published a description of a dinosaur cloaca, the all-purpose opening at the ends of the digestive and reproductive tracts shared by birds and reptiles.

Their description was based on a specimen whose skin had been preserved as a film around its body.

This exceptional preservation allowed them to reconstruct details of the dino’s soft tissue, including the anatomy of its cloaca.

They compared it to the same structure in egg-laying mammals, amphibians, reptiles, and birds.

And while it had some similarities in shape to the cloacae of crocodilians, it also might’ve shared a certain lobe in common with birds.

So this might be physical evidence that dinosaurs probably mated the way crocodilians and birds do - by putting their cloacae together, with the male transferring his sperm into the female that way.

Unfortunately, the details of how that happens has a lot to do with soft tissue structures inside the cloaca, which weren’t preserved.

But they did find two intriguing things that might’ve played key roles in dino mating.

First, the dino cloaca seemed to have a set of bumps on either side that could be musk glands, which crocodilians have.

Crocs of both sexes release stinky secretions from these during social displays.

And second, the scales on the exterior of the cloaca were pigmented, leaving behind a visible residue on the skin film itself.

This suggests it might’ve been a visual signal, like it sometimes is in living birds.

So where does that leave us on the evidence for dinosaur mating?

Well, it looks like we’re closer to understanding dinosaurs as animals that did all the normal stuff that living animals do.

And that, as is so often the case, what they were doing was something between what birds and crocs do today.