Most male birds do not have a penis inside their cloaca and instead pass semen to females by pressing their orifice to the female's in a “cloacal kiss.” But some male birds are well endowed, and, intriguingly, their lineages are all near the base of the bird family tree. According to Patricia Brennan of the University of Massachusetts Amherst and her colleagues, this pattern means that ancient birds had penises and that other lineages of birds lost this trait later in their evolution. Like waterfowl and other archaic bird lineages, male crocodilians have penises, too, which they use to inseminate females in the same fashion. Thus, male dinosaurs almost certainly had them. And if the genitals of crocs and endowed birds are any indication, the dinosaur phallus took the form of a single, unpaired organ with at least one long runnel down which semen flowed during sex. That being said, with more than 1,850 genera of dinosaurs estimated to have lived between 245 million and 66 million years ago, there were probably numerous variations on this theme.
Boy or Girl?
Reconstructing the mating habits of dinosaurs requires more than just an understanding of their reproductive organs, however. Scientists need to be able to tell males apart from females, which in the absence of genitalia is easier said than done. Investigators have long sought skeletal characteristics that might distinguish one sex from the other in lieu of soft tissue. But most of the traits proposed to fit this bill—such as a large crest atop the head marking Lambeosaurus individuals as male—have turned out to be unreliable indicators of gender.
Because skeletal differences between male and female dinosaurs are so elusive, if there are any at all, the only way that we can identify dinosaur sexes is through more direct evidence. Finding developing eggs inside a dinosaur's body cavity—as with a rare oviraptorosaur specimen from China—is one way to pinpoint a female dinosaur. But there is another option. In 2000 a special specimen of Tyrannosaurus rex finally yielded a way to identify hidden female dinosaurs. When some species of female birds are growing eggs, they develop a thin layer of tissue called medullary bone inside the shafts of the long bones in their hind limbs. This tissue is calcium-rich and acts as a store of raw material for creating eggshells. When Mary H. Schweitzer of North Carolina State University examined the broken thighbone of the T. rex, she spotted medullary bone. The specimen must have been a female who was pregnant when she died. Not only did the discovery mean that this physiological response to pregnancy evolved in the dinosaur ancestors of birds, but it also revealed a means of identifying female dinosaurs—at least pregnant ones.
Building on Schweitzer's discovery, Andrew Lee, now at Midwestern University, and Sarah Werning of the University of California, Berkeley, investigated when in their development dinosaurs became sexually active. Previous work had shown that dinosaurs have rings in their bones that can be used to estimate their age at death. These so-called lines of arrested growth (or LAGs), most likely represent a yearly slowdown in growth during tough times, such as a dry season when water and food are scarce. The LAGs, along with reconstructions of dinosaur growth curves, indicate that many dinosaurs grew rapidly during their early lives and slowed as they approached skeletal maturity.
Looking at the LAGs in the pregnant T. rex, as well as two other dinosaurs containing traces of medullary bone—a beaked herbivore called Tenontosaurus and the carnivore Allosaurus—Lee and Werning concluded that all three dinosaurs were young moms when they died. Tenontosaurus perished at around eight years of age, Allosaurus at 10 and Tyrannosaurus at 18. All these dinosaurs were still growing—their skeletons had not yet developed to full maturity. And the medullary bone only indicated the latest date at which each female started having sex.