- Scientists historically skirted the subject of how dinosaurs had sex, out of modesty and an absence of evidence.
- But studies of the closest living relatives of dinosaurs are providing insights into their probable reproductive anatomy.
- And computer models can test the plausibility of putative mating positions.
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Adapted from My Beloved Brontosaurus: On the Road with Old Bones, New Science, and Our Favorite Dinosaurs, by Brian Switek, by arrangement with Scientific American/Farrar, Straus and Giroux, LLC. Copyright © 2013 by Brian Switek
I was shuffling through Chicago's O'Hare international airport when I saw it: a magnificent, towering skeleton of a dinosaur. At first I thought it was a mirage created by my travel-addled brain. But the scene did not evaporate as I approached. Pillarlike forelimbs and brawny shoulders supported a long swerve of neck bones leading up to the dinosaur's small, boxy skull, which peered over the top of a banner touting the airport's Wi-Fi, as if looking to the tarmac beyond to check the latest departures and arrivals. I stopped and stared at the behemoth—a replica of Brachiosaurus inherited from the Field Museum in Chicago—mentally filling in the internal organs, muscles and skin of a creature that at 85 feet long is one of the largest dinosaurs ever found. And then a strange thought bubbled up in my mind: How did such a gargantuan animal have sex?
Giddy and tired, I envisioned a pair of amorous Brachiosaurus standing in a clearing in a conifer forest some 150 million years ago during the Jurassic period, each one waiting for the other to make the first move. But try as I might, I couldn't quite figure out the mechanics of what should come next. Could the male rear up to mount the female? Could the female support his weight? Wouldn't her massive tail get in the way? Alas, my flight started boarding, so I had to part ways with the skeleton, but I continued to ponder the mating mystery on the plane. It has captivated me ever since.
Dinosaurs must have had sex to reproduce. As in nearly all modern-day reptiles, males would have deposited sperm inside females, which would later lay fertilized eggs containing developing dinosaur embryos. Yet although scientists have managed to deduce quite a bit about dinosaur biology, the nuts and bolts of dinosaur sex remained largely unknown—in part because studying the sexual behavior of animals was taboo historically and the topic seemed so beyond the reach of science that very little could be said about dinosaur mating with confidence. Not all hope is lost, however. Dinosaur fossils have furnished clues to such intimate details as when during development these reptiles reached sexual maturity and how they attracted mates. Meanwhile studies of birds and crocodilians—the closest living relatives of dinosaurs—hint at what the external reproductive anatomy of dinosaurs looked like. And computer modeling offers the possibility of testing theories about how these giants managed to do the deed itself. Much remains to be discovered, but scientists are slowly drawing back the curtain on dinosaur amour.
Lock and Key
Signs of sex are hard to find in the fossil record of any creature. Among the rare examples are 47-million-year-old turtles that died while copulating and a pair of 320-million-year-old sharks that might have been courting when they were rapidly buried. Sadly no dinosaur skeletons have been found locked in romantic embrace. And not even the most beautifully preserved of these beasts retain remains of their reproductive organs.
For insights into the private parts of these extinct animals, scientists have had to turn to their closest extant relatives: birds and crocodilians. Birds are living dinosaurs, a specialized lineage that evolved around 150 million years ago and continues to thrive today. Crocodilians—a group that includes the alligators, gharials and crocodiles—are the closest living relatives of the group formed by extinct dinosaurs and modern birds. A trait present in both birds and crocodilians is likely to have been present in nonavian dinosaurs as well. One such trait is a cloaca—the single end point for the reproductive, urinary and intestinal tracts in both sexes of birds and crocodilians and probably, by extension, dinosaurs. Thus, an Apatosaurus's genitals would not be visible as it plodded by. Instead they would have been concealed in the cloaca, which would have appeared only as a slit underneath the dinosaur's tail.