by Brian Switek
The unique specimen provided a glimpse of the internal reproductive workings of female dinosaurs. Unfortunately, though, she did not illustrate the Tab A, Slot B anatomy actually used during sex. To figure that out, we must turn to their closest relatives, birds.
Even though we usually think of birds as an entirely different category of creatures, they are truly a specialized lineage of dinosaur that evolved around 150 million years ago and continues to thrive today. Together with crocodylians—the alligators, gharials, and crocodiles that are the closest living relatives to the Dinosauria as a whole—birds create a set of evolutionary guideposts called an extant phylogenetic bracket. The logic is very simple. It basically means that a trait present in both birds and crocodylians is likely to have been present in non-avian dinosaurs as well. We can use what we know about living species to investigate prehistoric creatures.
Take the cloaca, for example. This charming-sounding orifice (from the Latin for “sewer”) is the single endpoint for the reproductive, urinary, and intestinal tracts in both sexes of birds and crocodylians. Following the logic of the extant phylogenetic bracket, we can deduce that dinosaurs probably had these organs, too. You wouldn’t be able to see anything hanging low, or wobbling to and fro, as a male Apatosaurus plodded by. The dinosaur’s genitals would be kept inside the cloaca, which would have only looked like a slit beneath the dinosaur’s tail. And since scientists have been unable to find any definitive skeletal differences between male and female dinosaurs, this means that you wouldn’t be able to tell a female Allosaurus from a male one unless you looked inside the dinosaur’s genital opening.
So what did male dinosaurs hide inside their cloaca? Whenever I bring up the subject of dinosaur sex—which may be too often—nervous laughter and horrifying speculation about the members of Spinosaurus, Brachiosaurus, and kin soon follow. Stegosaurus and other well-armored dinosaurs always generate the most startling ideas. Covered in immobile plates and spikes, these dinosaurs had to avoid being impaled during their dangerous liaisons and certainly could have used a reproductive work-around. More than once, my friends have indulged my curiosity, jokingly suggesting that male dinosaurs had a prehensile phallus that could safely inseminate females from a distance—an idea that shocks my mind into waking anime nightmares. Unfortunately, there’s not a scrap of fossil evidence to say whether such terrifying organs actually existed.
We have to go back to birds and crocodiles for clues. Birds are a highly varied group of animals with different reproductive arrangements and strategies. While most male birds don’t have a penis at all—they pass semen to females by way of a technique given the cringe-inducing name “cloacal kiss”—some birds are impressively endowed. The Argentine lake duck Oxyura vittata has the longest penis relative to its body length of any known vertebrate. According to the ornithologist Kevin McCracken, who actually went to the trouble of investigating the bird’s infamous appendage, the length of the duck’s spine-covered penis can rival that of the bird’s body. Other ducks, male and female, are similarly famous for having prominent, complex sexual organs that fit together like a key in a lock or a limber corkscrew in a labyrinth—flashy outcomes of a sexual evolutionary arms race between males and females. And while not all male birds have penises, the lineages with well-endowed males are all near the base of the bird family tree. As determined by the University of Massachusetts (Amherst) ornithologist Patricia Brennan and her colleagues, this means that a penis is the ancestral state for birds.
Like waterfowl, male alligators, crocodiles, and gharials have penises, too. In their survey of crocodylian genitals, zoologists Thomas Ziegler and Sven Olbort wrote: “For correct sex identification, the male external genital organ of an immobilized crocodile must be felt out and protruded from the cloaca, and subsequently compared to the similar female clitoris.” Not an enviable task. But together the “intromittent organs” of crocs and birds indicate that male dinosaurs almost certainly had penises. And even though we don’t know exactly what these organs looked like, both crocodylians and endowed birds have a single, unpaired organ with at least one long runnel down which semen flows during sex.
How dinosaurs varied from this basic structure is unknown. I doubt there was a one-size-fits-all appendage. In 2006, Steve Wang and Peter Dodson estimated that more than 1,850 genera of non-avian dinosaurs lived during a span of over 150 million years. With so much diversity, there must have been startling variations of penis structure and female variations to match. (Sadly, finding a dinosaur clitoris seems as unlikely as finding a fossil penis. Some problems are eternal.)
Reconstructing the mating habits of dinosaurs requires a bit more than a basic understanding of the anatomical tackle they used, however. We need to know which dinosaurs were males and which were females, and, more important, if there were any broader physical differences between the sexes. Among living birds and crocs, the cloaca is the key to sex identification. Paleontologists obviously don’t have that option. So, in lieu of soft tissues, and given the rarity of obviously female skeletons with eggs preserved inside, paleontologists searched for skeletal characteristics that might distinguish one sex from another. Until recently, the key was thought to be sexual dimorphism—differences between the sexes other than the anatomy of the genitals—including size, ornamentation, and other features that might distinguish males from females.
There’s only one problem: identifying sexual dimorphism requires a large sample size from a population of dinosaurs that lived at a particular place and time. Paleontologists rarely get such detailed samples. Even our knowledge of Tyrannosaurus, arguably the best-studied dinosaur, comes from about fifty specimens spanning 2 million years. Unless a large number of animals died together in the same place at the same time, we’re out of luck.
These frustrating circumstances haven’t stopped paleontologists from trying. A study done back in 1975 by the paleontologist Peter Dodson highlighted the trouble with distinguishing male and female dinosaurs. During the previous hundred years, paleontologists had named three genera and twelve different species of crested hadrosaur from the roughly 74- to 76-million-year-old strata of Alberta, Canada’s Oldman Formation. When Dodson compared measurements of the dinosaurs’ skulls, however, he found only three species of hadrosaur present in the same place at roughly the same time—Corythosaurus casuarius, Lambeosaurus lambei, and Lambeosaurus magnicristatus. Each of these dinosaurs had a similar body shape, but they differed in their crests—Corythosaurus had a low, rounded crest, while the Lambeosaurus species had hatchet-shaped and large, curved crests, respectively. What paleontologists had believed to be species differences were just variations from one individual to the next, or represented age stages of the same species. But Dodson also believed that he had uncovered sexual dimorphism in each of the three valid species he proposed. In Lambeosaurus magnicristatus, for example, one sex had a bulging pompadour of a crest while the other seemed to have a more abbreviated version of the same structure. Dodson suspected that the more prominently crested form was a male and the poorly endowed form was a female.
Other paleontologists even went so far as to suggest that the two species of Lambeosaurus should really be one, with the larger-crested forms being males and the smaller-crested types, females. After all, crests are very prominent visual signals, and male Lambeosaurus might have flashed their decorations as signs of dominance, vigor, and power. The larger the crest, the more desirable the male.
But these paleontologists had misread the evidence. In their analysis of some of the fossils Dodson studied, the Canadian paleontologists David Evans and Robert Reisz found that Lambeosaurus magnicristatus truly was a separate species, not just a male of another. Not only did this species live at a different time than the other Lambeosaurus species, but the skull Dodson designated “female” had been broken so that it only appeared smaller. There was no evidence for sexual dimorphism in these dinosaurs.
Even Tyrannosaurus rex suffered from some sex-identity confusion. During the 1
990s, some paleontologists suggested that female T. rex had a wider space between the base of their tail and their hips to allow eggs to pass through. If true, this would be a way to identify female dinosaurs, but the argument crumbled. As paleontologists later found by studying fossils and the anatomical differences between male and female alligators, the proposed gap didn’t actually exist.
Other supposed indicators of dinosaur sex have not fared well, either. Sexual skeletal differences have been proposed for a variety of dinosaurs, from the sleek little early theropod Megapnosaurus to Mongolia’s famous ceratopsian, Protoceratops, and the super-spiky stegosaur Kentrosaurus, but none of the traits in these dinosaurs has turned out to be a reliable sex indicator. While dinosaur sample sizes are small, and cases of sexual dimorphism may be discovered one day, no one has yet found an unequivocal case.
Since 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 the rare oviraptorosaur specimen—is one way to pinpoint a female dinosaur. But there is another option.
In 2000, a special specimen of T. rex finally yielded a way to unmask female dinosaurs. The Museum of the Rockies field crew, directed by the museum’s curator of paleontology, Jack Horner of Montana State University, hit the fossil jackpot: in the course of a summer excavating Montana’s Hell Creek Formation, they found five Tyrannosaurus specimens, including one nicknamed “B. rex” after its discoverer, the crew chief Bob Harmon. The position of this dinosaur twenty feet up a steep cliff made the excavation process especially difficult, and when the skeleton was finally dug out, the dinosaur’s jacketed femur was too heavy for a hired helicopter to fly out of the remote field site. The paleontologists reluctantly broke the thigh bone in two for transport. This decision ended up making all the difference.
The frustrating field circumstance turned into a lucky break for one of Horner’s former students, Mary Schweitzer. Schweitzer is an expert in the microscropic study of dinosaur tissues, and at the time of the B. rex discovery, she was searching for fossils that hadn’t been doused with Vinac, glue, and the various other adhesives paleontologists apply to keep fossils together in the field. Fresh, untampered-with fossil surfaces had the potential to reveal some aspects of dinosaur biochemistry. Fortuitously, Horner sent Schweitzer, who had just started teaching at North Carolina State, some fragments that had fallen from the freshly broken femur. The gift offered her an opportunity to carefully analyze the unadulterated fossil bone of T. rex.
Schweitzer was shocked by what she found. The Tyrannosaurus had been pregnant when it died. A peculiar type of bone tissue inside the femur gave the girl away. 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, and it was what Schweitzer observed in B. rex. Not only did the discovery mean that this physiological response to pregnancy evolved in non-avian dinosaurs first—Tyrannosaurus was a distant cousin of early birds, and so the characteristic must have evolved deeper within the dinosaur family tree—but Schweitzer and her collaborator Jennifer Wittmeyer discovered a subtle way to identify some female dinosaurs. The technique wouldn’t work for all females—the dinosaurian hens would have to be laying eggs for the tissue to form—but medullary bone was still a way to pick at least a few females out of the vast collection of dinosaurs that paleontologists have accumulated.
Graduate students at the University of California, Berkeley, Andrew Lee and Sarah Werning ran with this discovery to investigate the tempo of dinosaur sex lives. While the discovery of medullary bone was new, paleontologists had been tracking dinosaur growth through bone microstructure for years. Previous research had established that dinosaurs have rings in their bones which can be used to estimate the age at which they died. These bands, called lines of arrested growth (or simply referred to as LAGs), most likely represent a regular slowdown in growth during especially tough times, such as a dry season when water and food are scarce. This works to the benefit of paleontologists. These rings, combined with studies of bone tissue types and reconstructions of dinosaur growth curves, indicated that many dinosaurs grew very rapidly during their early lives, and eventually grew more slowly as they approached skeletal maturity.
In addition to investigating B. rex, Lee and Werning found traces of medullary bone in two other kinds of dinosaurs—a beaked Early Cretaceous herbivore called Tenontosaurus and the Jurassic carnivore Allosaurus. All the dinosaurs were young moms. Lee and Werning estimated that the Tenontosaurus died at eight years of age, the Allosaurus at ten, and the Tyrannosaurus at eighteen. All of these dinosaurs were still growing when they died—their skeletons had not yet developed to full maturity. Based on these findings, it seemed that dinosaurs in general might have started reproducing even earlier. Regarding each of the dinosaurs in the study, the cases of medullary bone in each indicated only the latest date at which each kind of dinosaur started having sex.
Dinosaurs lived fast and died young. Rapid growth and early reproduction, Lee and Werning suggested, might be a sign of difficult, dangerous lives in which mating early was required for a dinosaur to pass along its genes to the next generation. This would have been especially important for the biggest dinosaurs. If an eighty-foot dinosaur such as Apatosaurus took decades to grow to sexual maturity, there would be very few of them around to breed. Instead, Lee and Werning estimated, these dinosaurs probably started copulating long before they reached maximum size, probably by nineteen years of age. Teenagers will be teenagers, after all.
Paleontologists are starting to outline the biology of dinosaur sex. But the mating behavior of dinosaurs remains mysterious. How did dinosaurs pick their mates and show off? Did brightly colored Ceratosaurus strut through the mating grounds like John Travolta? Did Apatosaurus nuzzle each other’s neck to give the proceedings a sense of romance? Paleontologists have had their fun with this topic when they have waded into paleo-fiction. In his children’s book The Year of the Dinosaur, the late paleontologist Edwin Colbert avoided detail to present only a perfunctory view of dinosaur courtship among a group of “Brontosaurus.” “[A]s their desires reached a peak, copulation took place,” Colbert blandly described. Not exactly the most tender affair. Although, in artist William Stout’s lavishly illustrated The Dinosaurs, William Service added a little more color to the proceedings and imagined a Parasaurolophus breeding ground where males uttered sonorous notes “[r]icher, deeper, thicker than any other animal tone heard in the air before or since” in their attempts to attract females to their little patches of wetland.
Whether dinosaurs were fans of love bites or preferred to cuddle is forever lost to time. Nevertheless, paleontologists have wondered whether many of the bizarre features of dinosaurs might be related to their sex lives. The connection between dinosaur ornamentation and mating is an application of one of Charles Darwin’s most important theoretical frameworks—sexual selection. Creatures not only struggle to survive, Darwin recognized, but also to reproduce. In some mating systems, males compete with other males of their species for access to fertile females. Females also have different interests than the males, especially since they often take on the brunt of reproductive responsibility: a male has to invest only a little sperm, while a female contributes far more resources to a developing embryo. And this is to say nothing of conflicts among females themselves. The competition within and between the sexes can produce complicated signals and bluffs, from the artistic architecture employed by male bowerbirds to the deep-throated roars and powerful displays of American alligators during the mating season. It all comes down to sex. The best package of survival traits is meaningless if the individual animal in possession of those characteristics never breeds.
Since we can’t observe the behavior of non-avian dinosaurs d
irectly, their skeletons are all we have to gauge their sex lives. Crests, spikes, plates, horns, and feathers can all act as prominent display structures, and maybe such ornaments acted as indicators of a dinosaur’s health or dominance. As the paleontologist and popular host of the kids’ show Dinosaur Train Scott Sampson has suggested, the strange adornments were so dazzling that even we cherish dinosaurs for them—sexy structures enthrall us just as they may have attracted amorous dinosaurs.
But picking out the influence of sexual selection in the fossil record is extremely difficult, especially when its most classic sign—sexual dimorphism—is absent among dinosaurs. The long, elegant necks of Apatosaurus, Brachiosaurus, and their sauropod kin were recently at the center of a debate over whether their strange shapes betrayed the influence of sexual selection. Giraffes inspired the hypothesis.
The classic story of how the giraffe got its neck is that the gradual extension of the mammal’s cervical vertebrae allowed the herbivore to reach food above the heads of competitors. But in 1996 the zoologists Robert Simmons and Lue Scheepers proposed something different. Male giraffes frequently fight each other in competitions called “necking.” It’s not as nice as it sounds. The fighting males swing their long necks to batter each other with the stout ossicones on the tops of their heads. These contests establish dominance; presumably, more dominant males that control territories end up with more mating opportunities than the losers (provided the losers survive the sometimes lethal ordeal). Therefore, Simmons and Scheepers proposed, such competitions drove the evolution of giraffe necks. Starting with a relatively short-necked ancestor, males who had necks just a little bit longer—and had a more powerful swing—would win more often and therefore mate more frequently. Over time, this selection for longer and longer necks would result in the giraffe as we know it today. Their idea became known as the “necks for sex” hypothesis.