How Did the Biggest Dinosaurs Get it On?

In 1991, the American Museum of Natural History unveiled one of the most fantastic fossil displays ever created. Placed at the center of the renovated Theodore Roosevelt Rotunda, an adult Barosaurus rears back to protect its offspring from an oncoming Allosaurus. The defending sauropod’s head is 50 feet up in the air, although whether or not such an immense, long-necked dinosaur could have pulled off such a feat has been a continuing point of contention. Even in a typical posture, Barosaurus must have had a powerful heart to pump blood along its 25-foot neck, and who knows how hard the dinosaur’s heart would have to work to continue bloodflow to the animal’s head if it reared up? Some paleontologists consider such a feat physically impossible, but as paleontologist William Gallagher pointed out while teaching my Paleontology 101 class at Rutgers University, male Barosaurus had a good reason to rear up. How else would the huge dinosaurs have positioned themselves to mate?

Exactly how dinosaurs got it on has inspired no small amount of speculation. The largest dinosaurs of all, the sauropods, have been especially perplexing. We often say that these dinosaurs “shook the earth” with their footsteps, but did they also make the bed rock with their lovemaking? (I apologize for that joke, and will keep the geology puns to a minimum. Promise.) Paleontologist Beverly Halstead famously wondered about dinosaur sex in public lectures and articles, and he suggested that standard “dinosaur style” was for a male to come alongside the female and throw its leg over the female’s back as she lifted her rump into the air to move her tail out of the way. In the case of sauropods such as Diplodocus, Halstead even imagined that the amorous dinosaurs might intertwine their tails. While other paleontologists have considered the tail-twisting aspect unlikely—sauropod tails were balancing organs and were too stiff to intimately coil around each other—the basic dinosaur position Halstead promoted has remained a prominent possibility for the dinosaur kama sutra.

But not everyone agreed that giants such as Apatosaurus and Brachiosaurus were capable of such nupital acrobatics. These animals were so immense—Apatosaurus, not even the largest sauropod, is estimated to have weighed more than 23 tons—that some researchers thought the kind of positions Halstead was promoting would give the dinosaurs fractured legs and broken spines. At a symposium of vertebrate morphologists held at the University of Chicago in 1994, biologist Stuart Landry, Jr. gave a short presentation entitled “Love’s Labors Lost: Mating in Large Dinosaurs.” He did not see how sauropods could have mated on land. A large, rearing sauropod, he told his audience, “would have to support 10 to 20 tons in a precarious position two or three meters off the ground.” A male Apatosaurus would be liable to tip over and possible take the female with him. Instead, Landry suggested that the largest dinosaurs looked for muholes or bodies of water to buoy themselves up. When a conference attendee asked if he was proposing that all dinosaurs mated in water, Landry responded, “I would say the very large ones must have.” Of course, this hypothesis required a large number of Jurassic and Cretaceous hot tubs of just the right depth for sauropods to reproduce, and scientific models of sauropods have suggested that these dinosaurs were actually quite buoyant and unstable in water. Sauropods were diverse, disparate and widespread animals that roamed in terrestrial habitats all over the world—there’s no reason to presume that the largest dinosaurs had to seek out the nearest deep lake when they got the itch.

Biomechanics expert R. McNeill Alexander also considered the weight problem in his 1989 book Dynamics of Dinosaurs & Other Extinct Giants, but came to a different conclusion. Even though a male sauropod would have rested a great deal of weight on the back of the female during mating, Alexander pointed out that the stresses and strains would not have been any more severe than those caused while the female dinosaur was walking. (After all, walking requires shifts in weight as the dinosaur balances and goes through each step cycle, and so a dinosaur’s skeleton had to be strong enough to cope with these shifts.) “If dinosaurs were strong enough to walk they were strong enough to copulate,” Alexander wrote. “They were presumably strong enough to do both.”

Without living specimens to observe, we will never know all the intimate details of sauropod sex. Still, there are only a limited number of positions that could have worked for the dinosaurs. For reasons that I’ll write about later this week, the consensus among paleontologists is that male dinosaurs probably had relatively small penises. (Shhh! Don’t tell Tyrannosaurus. He’s already upset about all those “useless forelimbs” jokes.)  An amorous male would have to position his cloaca—the orifice used for both expelling waste and mating in crocodylians, birds and probably dinosaurs—right up to the cloaca of a female, and the female’s tail would have undoubtedly presented an obstacle. Rather than simply leaning straight against the top of a female like an elephant or rhinoceros does, a male sauropod would probably have to rear up at a relatively oblique angle, and the female would have to assist by moving her tail (which is also a way in which female dinosaurs could have exerted mate choice and confounded any hot-under-the-collar males they would rather not mate with). Perhaps some museum will look into the problem and try to mount a pair of coupling sauropod skeletons—much like the mating Tyrannosaurus at Spain’s Jurassic Museum of Asturias—but even then we are limited by what we can imagine. Whether we want to imagine a pair of Brachiosaurus in flagrante delicto is another matter altogether.

This post is the first in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.

References:

Alexander, R. M. 1989. Dynamics of Dinosaurs & Other Extinct Giants. New York: Columbia University Press. pp. 57-58

Anderson, J. The Perplexing Puzzle of Maladroit Mating. Chicago Tribune. August 30, 1994.

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.