Dinosaur sex: T. Rex Autopsy on National Geographic Channel.

How Did Tyrannosaurus Rex Have Sex?

How Did Tyrannosaurus Rex Have Sex?

Wild Things
Slate’s animal blog.
June 5 2015 1:09 PM

How Did T. Rex Have Sex?

The business end of a T. rex statue at the World's Biggest Dinosaurs park in Cabazon, California.

Courtesy of Flickr user Sam Howzit

Dear reader, you and I have had some good times here. Together we’ve learned about oral sex in bats, golden showers with porcupines, and even the real meaning of “doggy style.”

Now I have three words for you: Tyrannosaurus rex sex.


I recently had the opportunity to watch a team of paleontologists and large animal vets cut into a life-size, hyperrealistic model of the king of the Tyrannosaurs as they filmed a special event called T. Rex Autopsy for the National Geographic Channel.

At the end of the first day’s filming, the beast had been gutted. Fake blood and silicone viscera lay everywhere. And the monster’s eye, a combination of Jurassic Park and Sharptooth, stared dully at the ceiling.

But all I could think about was the dinosaur’s cloaca.

Teeth, skull, feet, tail: Fossils can tell us what these looked like on a T. rex. But the cloaca—the all-inclusive organ out of which dinosaurs would have urinated, defecated, had coitus, and laid eggs—this beautiful organ is the gateway to many mysteries.


Let’s get the biggest question out of the way first: Did T. rex have a penis?

“I think we can give a firm ‘yes’ to the penis question,” says Brian Switek, science writer and author of the excellent My Beloved Brontosaurus. “And I apologize for the pun that just happened.”

Unlike many mammals, dinosaurs did not have bones in their penises. And because soft tissue doesn’t fossilize as readily as the hard stuff, we’re left without a specimen that could settle the penis debate. But we don’t actually need one.

By using what scientists call extant phylogenetic bracketing, or looking at T. rex’s closest living relatives, we can infer that the predator had a penis tucked up inside that cloaca.


“Crocodilians all have what’s politely called an ‘intromittent organ,’ ” says Switek. “And if you look at birds, the most basal or primitive lineages—the ratites, the waterfowl—they also have penises.”

It’s very unlikely penises would have evolved separately in crocs and cassowaries. Instead, having a penis is more likely the default setting among all of them, T. rex included, and their common ancestor.

As to what that monstrous member would have looked like, well, that’s still up for debate. Penis size is extremely variable across the animal kingdom. Gorillas, though they can grow up to 400 pounds, have penises that are just 1.25 inches long. (Yes, that would be erect.) Ducks, on the other hand, have relatively large sex organs for their small body size, not to mention explosive erections—but that’s another story.

Of course, the question of T. rex penis size is of more than just prurient interest. This detail would inform what sorts of positions were anatomically possible for the animals. For instance, if it turned out T. rex had some sort of long, prehensile penis, like whales do, then it’s possible they could just sidle up to one another and inseminate from relatively afar. (This is an especially appealing scenario for the armored and spiked dinosaurs like stegosaurus, if a little unlikely.) Otherwise, the cloacae would have to be in close contact.


And for that, T. rex would have to pretty flexible.

John Hutchinson is a professor of evolutionary biomechanics at the Royal Veterinary College in London and a consultant on T. Rex Autopsy. (He also has one of the most fascinating dissection blogs around, because that’s a thing.)

Much of Hutchinson’s work has focused on reconstructing the arrangement of muscles, ligaments, and bones that made up T. rex’s impressive rump—not to answer sex questions, mind you, but to understand how fast those suckers could run. Still, he had some insight.

“I’d think there’d be some twisting of tails,” says Hutchinson. “The tail was flexible, especially at its base. The female could tilt her cloaca toward the male, the male could tilt his toward her, and any sort of phallus could be everted from there.”  


Position-wise, this would jibe with the way crocodilians mate. But then crocodilian cloacae aren’t 13 feet off the ground and attached to a frame that weighed as much as an African elephant. And if you look at the primitive birds, like ostriches, the male essentially climbs on top of the female. (Side note: People set animal sex videos to the weirdest music.)

“Did they do it standing up? Did they squat? These are good questions, but all we’ve got is speculation,” says John Long, paleontologist and author of The Dawn of the Deed: The Prehistoric Origins of Sex. “The biomechanics of a big dinosaur like that indicate that they were quite capable of squatting.”

So you have female T. rex squatting down on her haunches, leaning forward, tail in the air and twisted to the side. Essentially, downward-facing dino. But what happens when the male saunters over and leans all that meat-eating mass onto the female’s haunches?

“We’re really on the frontier of Tyrannosaur biology here,” says Hutchinson. (Ask scientists about something as speculative as the mating habits of animals that have been dead for 67 million years and you get a lot of caveats.)


“You’re looking at seven tons or so on two legs. There’s nothing like that around today,” he says. “But I have no doubt that if an animal had the strength to walk, it could probably have sex standing up.”

Unfortunately, although T. Rex Autopsy will teach you tons about the anatomy of a T. rex, it does not provide answers to any of these prehistoric sex riddles. As consolation, please enjoy the embedded clip below where paleobiologist Tori Herridge goes armpit-deep inside a Tyrannosaur cloaca. It may be the best thing I’ve ever seen happen on television. (I only wish they could have referenced Ian Malcolm’s horrible how-you-sex-a-dinosaur joke.)

As is often the case with science, there is no final word on T. rex sex. Switek says he’s optimistic that someday, someone will happen upon a fossil that provides more insight into the “big bang theory” (which is also one of the chapter headings in his book).

In the meantime, Switek says he’s working with paleontologist Heinrich Mallison at the Natural History Museum in Berlin to build digital models of different dinosaurs. With these, the two hope to test out every position in the dinosaur Kama Sutra and determine what’s even possible.

“It’s technically feasible to figure out what dinosaur sex positions could work,” says Switek. “We use biomechanics to learn about biting, or clawing at each other, or running, but no one’s thought about using it for probably the most important element in dinosaurian life.”

Until then, I’m afraid the more intricate details of T. rex’s sex life must remain a mystery. Male-to-male competition, courting dances and displays, female selection, coital duration, pillow talk—all of it lies within the fog of prehistory.

Using the sex lives of living animals as a guide, I think there’s at least one thing we can say with certainty: T. rex sex was likely appalling. And awesome.  

T. Rex Autopsy airs this Sunday night on the National Geographic Channel.