Inside the bone-crushing mating rituals of dinosaurs
A handful of unique fossils are spilling the secrets of dinosaur courtship and reproduction.
Whatever happened to the duck-billed dinosaur, it was a real pain in the butt. Many of the spines sitting atop the animal’s tail vertebrae had been fractured. The dinosaur, known as Olorotitan to paleontologists, wasn’t simply clumsy; too many bones were broken. The breaks were more consistent with some heavy downward pressure right near the base of the tail. At 26 feet long and three tons, another Olorotitan would have been large enough to do such damage.
Institute of Natural Sciences in Brussels paleontologist Filippo Bertozzo had ventured to Blagoveshchensk, Russia’s Paleontologic Museum in 2019 to study the dinosaur’s bones when he noticed the strange injuries. “When I realized what I had in front of me, I had a cry of joy,” says Bertozzo. He had seen busted bones like these before in his research and in the scientific literature—fractured vertebrate near the hadrosaur’s hips. In 1989, paleontologist Darren Tanke suggested that similar breaks were the result of mating as one dinosaur mounted the other. At the time, there were too few hadrosaur fossils with such injuries to tell whether the broken bones were outliers or really were caused by mating.
Bertozzo’s Olorotitan discovery revived the notion. The broken bones could be the long-sought evidence of how dinosaurs mated. Paleontologists have never doubted that dinosaurs copulated just as modern reptiles and birds do. Given how fleeting such moments between dinosaurs must have been, however, no dinosaurs have ever been found preserved in a mating position.
“Reconstructing the courtship and mating behaviors of dinosaurs is inherently challenging because these activities were brief, seasonal, and left few durable traces in the fossil record,” says Universidad de Cantabria paleontologist Ignacio Díaz-Martínez.
(The secrets of dinosaur sexes may be locked in bone.)
Bone fractures point to dinosaur mating rituals
How dinosaurs mated has been a difficult puzzle for paleontologists, one that cannot simply be resolved by smashing dinosaur models together. Until now, all experts had to go on were skeletons and comparisons to modern animals, hindered by the fact that there are no land-dwelling animals alive today that have the big, thick tails that characterized dinosaurs and would have made mating an acrobatic challenge. How armored dinosaurs like Stegosaurus managed to connect is anyone’s guess.
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Even the duck-billed hadrosaurs, lacking armor, confounded experts trying to figure out how the reptiles copulated. “There aren’t other living animals with a hadrosaur-like tail, a large, long, muscular tail kept high and horizontal to the ground, Bertozzo says. The clues would have to come from the bones themselves.
Hadrosaur bone injuries are more common in the fossil record than other dinosaur species, partly due to the fact that experts have excavated so many of them. Looked at alone, the breaks in individual fossil specimen might be linked any number of different traumas, from predator bites to stumbling falls. But together, they tell a different story.
After studying hundreds of injured hadrosaur fossils from North America, Europe, and Russia, Bertozzo, Tanke, and colleagues proposed in an iScience study that mating was the most likely cause of the pathologies. “To discover broken tail bones in hadrosaurs from totally different regions in the world meant that the pattern was present in virtually all hadrosaurs,” Bertozzo says.
The tail injuries lacked bite marks or embedded teeth, which makes predation unlikely. The fact the tail vertebrae principally were located near the hips across species, continents, and timespans also suggested a common cause rather than more variable injuries that would come from falling down or other accidents like being caught under a toppling tree. Big hadrosaurs were so heavy that the bottom dinosaur’s tail was bent down thirty degrees or more, indicating close body contact where the weight of one dinosaur heavily rested on the other. Dinosaur hugs seemed out of the question, making mating the most likely explanation.
(Dinosaur vomit has solved these prehistoric mysteries.)
Why was hadrosaur mating so painful?
The reason anatomical reason the hadrosaurs had to lean on each other so forcefully was only recently uncovered in the fossil record itself. Every dinosaur, much like their living bird relatives, had a cloaca.
A cloaca is a specific feature of baby-mating anatomy seen not only in birds, but crocodilians and other reptiles. It’s only visible on the outside as a slit or opening under the base of the tail. It’s a single covering for the outlets to the excretory, urinary, and reproductive tracts, and also encloses the genitals. It was logical that non-avian dinosaurs would have the same sort of setup, given that both birds and crocodiles are dinosaur relatives that have cloacae, but no one had ever found the fossil proof.
That changed in 2022 when Phil Bell, a paleontologist at the University of New England in Australia, and his colleagues described an exquisitely-preserved small horned dinosaur called Psittacosaurus. The fossil preserved enough skin impressions that researchers could detect several different scale patterns over the dinosaur’s body. Among the preserved stretches of soft tissue was what appeared to be a cloaca, a vertical slit located just behind the hips just as in modern crocodiles.
In order to mate, then, two dinosaurs would have to bring their cloacae into close contact with each other. The positions must have been awkward, no doubt resulting in the broken bones detected by Bertozzo and colleagues. From here, however, the fossil trail has run out for now.
The soft tissue fossils of the Psittacosaurus only preserved the external part of the cloaca. Paleontologists expect that different sexes of non-avian dinosaurs had a phallus or clitoris, just like crocodiles and many birds do, but those soft tissues have not yet been found as fossils. Even then, it will be tricky to work out dinosaur sexes. In crocodiles and many birds like cassowaries, the phallus and clitoris of different sexes so closely resemble each other that only experts can tell the difference, and such organs vary widely by species. It’s probably all dinosaurs had a cloaca, but the anatomy of what was behind the cloaca was likely as variable between dinosaur species as zoologists have documented among modern reptiles.
How did dinosaurs woo mates?
Dinosaur reproduction is not merely about the mechanics of mating, of course. Many of the ornate dinosaur features that paleontologists used to think of in terms of defense, such as the horns of ceratopsids like Styracosaurus and the pointed armor of dinosaurs like Stegosaurus, have had at least a dual role as visual display structures. The high degree of variation in arrangements of horns, crests, spikes, and other features suggest that the conspicuous features were not honed for a specific form of combat but were essentially dinosaurian fashion, meant to signal and communicate to other members of the same species as much as discourage hungry tyrannosaurs.
(This stunning dinosaur likely used armor to flirt as well as fight.)
Features that paleontologists have long found sexy about dinosaurs were probably sexy to the dinosaurs themselves. Even feather patterns on some dinosaurs, such as the striped tail of the fuzzy dinosaur Sinosauropteryx, hint at visual signals being very important to dinosaurs in terms of socializing and impressing potential mates. The discovery of vast dinosaur display arenas underscores how dinosaurs went to lengths to show off.
The scratches in the ground were originally thought to be failed attempts at making nests. Upon closer examination, however, paleontologist Martin Lockley and colleagues realized that there were multiple such scratch marks in close proximity to each other at multiple fossil sites across Colorado. The marks were made by large theropod dinosaurs, perhaps akin to the giant carnivore Acrocanthosaurus, that gathered in one place to display to each other. Some modern bird species still engage in the same behavior today, avians like puffins coming together in an area called a lek to scratch at the ground. The dinosaurs danced to impress.
“Scrape displays often being as soon as a pair occupies a territory,” Díaz-Martínez says, usually with male birds demonstrating they could ably dig out a nest. The large dinosaurs, around the size of the 30-foot-long Allosaurus, may have scratched at the ground for the same reason, as a sample of the nest they could build. Since the traces were first reported in 2016, paleontologists continue to document additional display arenas where dinosaurs courted each other prior to mating and nesting.
Even with all paleontologists have learned, however, the known case studies apply to specific dinosaur species or groups. In the case of the scrapes, paleontologists have not yet identified the exact species of the dinosaurs who left the ruts behind. But each new find raises new questions about the intimate moments of dinosaur love lives.
More evidence is almost certainly out there, in the rock record. While undoubtedly rare, it might be possible to identify dinosaur mating habits from tracks. “If mating involve positions that left distinctive impressions such as overlapping trackways, claw marks from grasping, or localized deformation to the sediment, these could be preserved under exceptional circumstances,” Díaz-Martínez says. The dancing dinosaur scrapes reveal courtship, but other forms of footsteps, scratches, and other traces would uncover the actual moment of dinosaur mating.
Bertozzo is optimistic that more mating-related pathologies, as well, will be recognized in previously excavated specimen off the back of this new research. “My biggest hope,” he says, “is that this study will inspire more researchers to re-check their collections.”
From scratches to busted bones, though, the evidence of dinosaur coupling continues to mount. The encounters likely lasted only seconds, maybe minutes, but rock and bone have held the intimate moment safe for millions upon millions of years.
