Everything You Want to Know About the Sex Lives of Dinosaurs, From Scratching the Ground to Battling With Horns

For more than a century, paleontologists have wondered about how dinosaurs mated. When the iconic Tyrannosaurus rex was named in 1906, for example, the dinosaur’s description mused that perhaps the carnivore’s tiny arms served as a “grasping organ in copulation.” The notion never caught on, but it still recognized that even the grandest and most fearsome of dinosaurs must have bred and perhaps even had unique sexual behaviors. But studying the sex lives of animals that have been extinct for more than 66 million years is a challenge.

To date, no one has yet discovered an exquisitely preserved dinosaur with its reproductive organs intact. The closest we’ve been able to get is a small horned dinosaur with a preserved cloaca, a single orifice that housed the urinary, excretory and reproductive tracts behind it. Comparisons to living birds and crocodylians hint that individual dinosaurs usually had a phallus or a clitoris, although hard evidence of such structures has not yet been uncovered. Until such evidence is found, not just for one species but for many, accurately envisioning the mechanics of how dinosaurs did it will be difficult.

Of course, dinosaur sex lives were not merely about anatomy. Much like birds and crocodylians, dinosaurs must have courted each other and expressed a variety of different mating systems throughout the many millions of years they lived on our planet. Many of these Mesozoic moments are lost to time, but in 2016 paleontologists announced that they had found evidence of broad mating grounds where large, carnivorous dinosaurs similar to Acrocanthosaurus gathered to scratch at the ground just as some ground-nesting bird species do today. The dinosaurs picked a spot and scuffed up the dirt in prehistoric Colorado, either to attract potential mates or to intimidate neighboring dinosaurs during mating season. Since then, similar scrapes have been found in Canada, and small scrapes have turned up in South Korea, suggesting that the method was a favorite among theropods.

The vastly different ornamentation of horned dinosaurs such as Triceratops and Styracosaurus suggest that dinosaurs took care to choose their mates, as well. Early in paleontology, horns and spikes were often perceived as defensive weapons against large carnivores. If this were the case, however, paleontologists would find horned dinosaurs with relatively similar horn arrangements that would seem optimized to defend against tyrannosaurs. Instead, experts recognize the many ceratopsid species by their widely varying arrangements of horns, many of which seem like they would be of limited use as defensive weapons. Sexual selection, in which mate choice influences anatomy, is a much more likely driver of the fantastic dinosaur traits that so often grab our imagination.

In fact, evidence that some horned dinosaurs used their horns for combat is a hint that interactions between dinosaurs of the same species might have been more significant in driving unusual dinosaur anatomy than pressures like predation. Studies of Triceratops skulls indicate that the horned reptiles must have frequently locked horns in conflict with each other, perhaps over territory or in mating contests. Other dinosaur species have left their mark on each other, too. Multiple tyrannosaur fossils have been uncovered with bite marks on their snouts or wounds from tussles with other tyrannosaurs, and at least one damaged ankylosaur skeleton indicates that the armored dinosaurs bashed each other with their club-like tails. While some of these injuries might be signs of other rough encounters, perhaps over territory or a food source, it’s likely that many outstanding dinosaur ornaments and features were molded by dinosaur courtship and mating conflicts within the same species.

Even dinosaur plumage and color were likely influenced by the mating lives of the creatures. Dinosaur feathers contained tiny structures called melanosomes, some of which have been preserved in microscopic detail in fossils. These structures are also seen in the plumage of living birds, and they are responsible for colors ranging from black to gray to brown to red. As long as a dinosaur specimen has well-preserved feathers, we can compare its arrangements of melanosomes with those of living birds to determine the feather’s palette, and a study published in 2010 did this for the small, feathered dinosaur Anchiornis. It looked like a modern-day woodpecker, the analysis showed: mostly black with fringes of white along the wings and a splash of red on the head.

Anchiornis, a strange feathered creature from the Jurassic period

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So far only one specimen of Anchiornis has been restored in full color, but so many additional specimens have been found that paleontologists will eventually be able to determine the variation in color within the species, specifically looking for whether there was a difference between males and females or whether the flashy red color might be mating plumage. Through the discovery of dinosaur color, we may be able to understand what was sexy to an Anchiornis.

A discovery regarding an avian dinosaur that lived about 125 million years ago underscores the possibility that dinosaurs were flashy animals with ornate structures and colors meant to impress potential mates. Paleontologists have found multiple fossils of the early bird Confuciusornis sanctus with feathers. Some of the specimens have long, trailing feathers coming off the tail, and others do not. A 2013 study found that a specimen lacking the long tail feathers had medullary bone, which means it was likely a female, indicating that the long tail feathers were probably a feature of male Confuciusornis—in the same way that males of many modern bird species often have elaborate feathers or colors compared with other sexes. Such stark differences between different sexes are often an indicator of sexual selection, when courtship or mating behaviors lead to the evolution of striking displays.

The Mass Extinction That Wiped Out the Dinosaurs

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Paleontologists will surely keep making new discoveries of dinosaur sex lives for years to come. Researchers may even uncover two dinosaurs that perished while mating. Experts have discovered prehistoric turtles that died while copulating, and if sleeping dinosaurs have been preserved then it’s possible mating dinosaurs might one day be found. Tracks and trace fossils might be helpful, as well. Courting and copulating dinosaurs could have left their distinctive footprints on an ancient surface, showing how they moved and positioned in relation to each other. For as many mysteries remain about dinosaur sex, the fossil record still likely holds a wealth of surprises about how dinosaurs made the earth shake.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.