Brachiosaurus is estimated to have weighed close to thirty metric tons. Put two of these behemoths together, and you're dealing with upwards of 125,000 pounds of dinosaur.
Now, try to imagine all 125,000 of those pounds coordinating for the sake of copulation, and you'll begin to appreciate how baffling the mating mechanics of sauropods (the most colossal creatures to ever live on land) truly are.
Over on Dinosaur Tracking, Brian Switek has written two posts (in an ongoing, four-part series) examining how, exactly, these dinosaurs might have done the deed — and it is an outstanding read. Included here is an excerpt from the first installment, but you'll definitely want to check out both posts in their entirety.
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?
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.
You can check out the rest of the sauropod sex series — which will be continuing through Valentine's day — over on Dinosaur Tracking.