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How would Rex have sex?

By Kristy Koehler, February 5 2020—

Ever look at Rex, the University of Calgary’s beloved dinosaur mascot and wonder how he’d get it on? Well, I did. I started by Googling “Dinosaur sex” and came across some uh, “artistic” renderings of how dinos might have mated but then, recognizing that U of C is full of dino-mite researchers, I asked an expert. For science of course… 

Last year, I brought you Rex’s PG rated-origin story. This year, I’m getting to the tricera-top — and bottom — of how he and his fellow dinosaurs came to be. 

François Therrien, Curator of Dinosaur Palaeoecology at the Royal Tyrell Museum, and adjunct professor at the University of Calgary, spoke to the Gauntlet about the mating habits and sex lives of dinosaurs. It was the highlight of his year, I’m sure.

Beginning at the beginning — the land before time, if you will — Therrien said that dinosaurs were hatched from eggs.

“Dinosaurs are reptiles — they’re closely related to crocodiles and birds,” he said, and based on the way their closest relatives came into the world it would have been possible to guess that dinosaurs also laid eggs. Guessing, however, isn’t necessary.

“We do find lots of dinosaur eggs especially here in Southern Alberta,” said Therrien. 

South of Lethbridge, at Devil’s Coulee — the largest dinosaur nesting site in Canada — plenty of evidence has been found to show that dinosaurs did in fact build nests, lay eggs and hatch their young.

“It’s a very little-known site that’s very significant but is not as popular as Drumheller or Dinosaur Provincial Park. You can find lots of nests of duck-billed dinosaurs as well as the small, meat-eating dinosaurs called Troodon.”

As for dinosaur gender roles, it’s hard for researchers to tell who took on what role.

“We don’t really know if the male or the female protected the nest,” said Therrien. “For the majority of dinosaurs we don’t even know if they were good parents and watched over their young.”

Even if dinosaur bones were found near the nest, Therrien said, it would be difficult to discern whether the animal was a male or female. Why? Dinosaurs lack external genitalia.

“If we look again at their closest relatives — crocodiles and birds — all of these animals have internal genitalia. From the outside, they all look the same. The male has a penis that comes out only for copulation, for mating. Otherwise, everything is internal and it’s almost impossible to tell whether you’re dealing with a male or a female,” said Therrien. “In looking at birds, individuals of different sexes can be of dramatically different colours and that’s the only way to tell them apart.”

So far, there aren’t any dinosaur um, bones, if you will.

“We know that some mammals have a real bone inside the penis — dogs and walruses are some of them and some primates also do. That’s something that’s unique to only some species of mammals — all other animals don’t have bones associated with their genitalia,” explained Therrien. “It’s possible if you’re studying fossil mammals some of them will have the bone called a baculum. If you find one, you know for sure you’re dealing with a male. For every other animal there’s no hard parts associated with the genitalia.”

There’s a couple of animals that come to mind when imagining how dinosaurs got down and dirty, and one is the T-Rex. Since dinosaurs were the originators of doggy style, I’ll call it dino style. I have a hard time wondering where T-Rex puts his little hands in the middle of mating? He surely can’t reach down and grasp her hips.

“The forelimbs — the arms of a T-Rex — we call them vestigial because they are getting so short it’s hard to imagine they could have been functional,” said Therrien. “If you look at just the scale of things — a fully grown T-rex that’s 10–12 meters long, its arm is the same length as the arm of a human so it’s tiny in comparison to the size of the animal. It’s hard to imagine that the arm could have been used for restraining the female — maybe to guide or position, but to restrain a female that weighs 6,000 kilograms, I doubt the arms would have been able to truly restrain them.”

The other dinosaur that stuck out as seemingly impossible to picture romping around was the stegosaurus.

“That’s a question that’s puzzled paleontologists for a long time,” said Therrien. “These animals have all those spikes and plates on their back — it’s the same for Ankylosaurus, the dinosaurs with all that armour on their backs. It’s hard to imagine how a male could mount a female.”

However, Therrien added that in again looking at the dinosaurs’ living relatives might give a clue. Crocodiles and birds have a body part called the cloaca — a posterior, do-it-all orifice that serves the digestive, reproductive and urinary tract.

“There’s other lizards that align their rear ends, but they kind of lie side by side and don’t actually climb on top of the other partner,” said Therrien. They may align these openings against one another and the penis would have been inserted into the cloaca of the female. 

Another thought I had was how on earth gigantic sauropod dinosaurs like the brontosaurus didn’t break a hip when getting mounted from behind. Turns out, they might have. Therrien spoke to some findings in the hadrosaurs — the duck-billed dinosaurs.

“We find at the base of the tail close to the rear end of the animal, lots of vertebrae that have been broken and have healed,” he said. “The spine along the back has been bent, broken then refused or healed. It has been suggested that maybe those are injuries related to mating. Some of those animals are massive — we’re talking about several tonnes — so when one climbs on top of the other, there’s a risk of injury. So, at least for some hadrosaurs, there’s evidence that maybe some could have been injured in the process. Obviously, it wasn’t fatal but some of those bigger animals could have been injured while mating.”

How did a dino find a mate to make his “heart saur?” Therrien says dinosaurs had display structures such as frills and crests to attract members of the opposite sex. 

“Initially we thought that all those structures were used to differentiate members of different species,” said Therrien. “There’s lots of duck-billed dinosaurs that have crests of different shapes and those dinosaurs needed to recognize one another on the landscape. Now, what we’ve discovered is that many of those differences are between animals that did not live together at the same time so it’s probably not for species recognition. We now think that many of those features are actually related to display — to try to attract a mate. Those structures were probably very flashy and very colorful when the animal was alive. The fact that the structures develop only at the time when the animal reaches sexual maturity is one reason why we think it’s related to display.

“Now we have lots of dinosaurs that we know actually had feathers. We thought they were used to keep the animals warm, especially meat-eating dinosaurs, as they seem to be fully covered with those primitive feathers that look like down feathers or like hair,” continued Therrien. “We thought that those feathers evolved for insulation.”

Therrien and another U of C researcher, Darla Zelenitsky, collaborated on a project and found that ostrich-mimic dinosaurs called ornithomimids, had wings on their forelimbs in adulthood that weren’t present in juveniles.

“That told us that wings only developed later and during the lifespan of the animal and that’s something that you typically observe in a display-structure,” said Therrien. “When you’re young and you’re not involved in reproduction, those structures are useless so you don’t have them — when you reach sexual maturity it’s important to have wings or a crest to be more attractive to a potential mate.”

Therrien noted that some dinosaurs may also have had a display ritual.

“There’s a very curious site in the US where there are lots of dinosaur footprints preserved in the ground. Some look like scratch marks, like the animal kept scratching repeatedly in the ground. That’s very similar to some behaviour that we observe in some birds. During the mating ritual, when they face off with other males or face females to try to impress their potential mate, they jump around on the ground and scratch. Scientists have interpreted the really weird footprints with scratch marks as being a ritual dance by those meat-eating dinosaurs. We have evidence that they had a mating ritual that did not involve big showy structures but actually just walking around and scratching the ground to try and impress a partner.”

Though, adds Therrien, more research in that area is needed.

I saved the most important question for last. Since Therrien published a paper in 2007 entitled “My theropod is bigger than yours… or not: estimating body size from skull length in theropods,” I figured he’d be the one to ask about estimating the size of something else on a dinosaur. No luck, but he is probably re-evaluating his life choices after getting a PhD only to have me ask how big T-Rex’s penis would have been.

“I don’t think it would have been proportional to the size of the animal — but I actually have no idea,” he said.

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