Dinosaurs lived fast and died young. Rapid growth and early reproduction, Lee and Werning suggested, might attest to difficult, dangerous lives in which mating early was required for a dinosaur to pass along its genes to the next generation. Early breeding would have been particularly important for the biggest dinosaurs. If an 80-foot dinosaur such as Apatosaurus took decades to grow to sexual maturity, there would be very few of them left to mate by the time they matured. Instead, Lee and Werning estimated, these dinosaurs probably started copulating long before they reached maximum size, probably by 19 years of age. Teenagers will be teenagers, after all.
Big Bang Theories
Before getting down and dirty, however, dinosaurs had to attract mates. Paleontologists have wondered whether many of the extravagant adornments of dinosaurs—including crests, spikes, plates, horns and feathers—could have served to seduce. The long necks of sauropods might have functioned similarly. Although the extravagant necks most likely evolved to allow these dinosaurs to reach a wide range of food, they could have been co-opted during the mating season, possibly developing striking color patterns to advertise their good health to potential partners. (Many immense sauropods were too big to be threatened by predators and so could afford to ditch the camouflage in favor of flashy hues.) Other dinosaurs probably showed off, too. Maybe the spiky dinosaur Kentrosaurus found the plates and spikes of the opposite sex arousing, and perhaps females of the sauropod Amargasaurus looked for males with the longest neck spines. Yet for all their appeal, these extravagant traits surely complicated the act itself. Which brings us back to the question that I found myself pondering when I came across Brachiosaurus at O'Hare. After all the posturing and showing off, how did dinosaurs actually mate? Hypotheses for exactly how this occurred depend on what feats of strength scientists think dinosaurs were physically capable of.
Biomechanics expert R. McNeill Alexander of the University of Leeds in England imagined that dinosaurs mated just like today's elephants and rhinoceroses—females had to bear the extra weight of the mounting male. The main difference would be that dinosaurs had those big, relatively stiff tails. Working from the idea that male dinosaurs threw one of their legs over the back of the female, Alexander pointed out that the weight of the male would have rested on the female's hindquarters. This would have been a massive load, but, as Alexander noted, the resulting stresses involved would not have been any worse than those that arise from walking because, during the step cycle, the dinosaur's weight would be supported by just one hind leg as the other swung in the air during a step. “If dinosaurs were strong enough to walk, they were strong enough to copulate,” Alexander wrote in 1991. “They were presumably strong enough to do both.”
British paleontologist Beverly Halstead also argued that male dinosaurs had to mount the females to inseminate them. But rather than likening them to elephants and rhinos, he believed that dinosaurs did as lizards and alligators do today. Males threw one hind leg over the back of their partners, he surmised, and this move would push their hips underneath the tails of the females to bring their cloacae together. Longer-tailed species may have even intertwined their tails for more tactile stimulation, just as some snakes twist their bodies around one another.
Personally, I have never been satisfied by this standard explanation of dinosaur sex. Scientists do not really know if the legs and tails of sauropods could have bent and flexed enough to achieve the traditional position. Bipedal carnivores such as Allosaurus also looked like they would have required a good deal of balance and cooperation to make this kind of mating work. It is easy enough to draw a two-dimensional image of flexible dinosaurs, but no one had tested these ideas against the bones or evaluated the plausibility of other positions. Did female dinosaurs lie down on their sides during the act? Or might the lovers have backed up into each other? Researchers had no shortage of ideas but exhibited seemingly little interest in going beyond line drawings.