February 13, 2012
Figuring out how dinosaurs mated is a frustrating task. There is relatively little that can be gleaned from the fossil record, and much of what paleontologists suspect about behavior and soft tissue anatomy comes from comparisons to birds (specialized, living dinosaurs) and crocodylians (the closest living relatives to the dinosauria). Even worse, exactly how to tell male and female dinosaurs apart from one another has puzzled scientists have decades. If we even can’t sort the females and the males, how can we accurately envision dinosaurian sex?
For a time, it seemed like the skeletal construction of dinosaurs might hold the answer. The clue paleontologists were looking for was sexual dimorphism. This is a difference between males and females of the same species as expressed in secondary characteristics—not the fiddly bits actually used during mating, but traits like size, bizarre ornamentation, coloration and similar features. Detecting such differences in dinosaurs requires a large sample of individuals of the same species which are about the same age and come from the same time and place (the more closely a paleontologist can approximate a population in a sample, the better). If such a group can be separated out into two distinct types—say, with one being larger than the other and with a larger crest—then there is a possibility that those two forms represent females and males.
Paleontologists have hypothesized sexual dimorphism for multiple dinosaur species, from Protoceratops to Tyrannosaurus. None of the proposed cases is especially well supported. What might seem to be a split between robust and gracile forms of a species—often taken as males and females, respectively—might actually represent different growth stages of the same dinosaur, different species of dinosaur, or individual variation in a small sample size.
The case of Lambeosaurus is a good example of the difficulties involved in distinguishing the dinosaur sexes. In 1975, paleontologist Peter Dodson undertook a review of the many, many species of hadrosaur described from the roughly 77-million-year-old strata of Alberta, Canada’s Oldman Formation. Paleontologists had named three genera and twelve species of crested hadrosaurs from this area, but after comparing the skulls of these dinosaurs, Dodson concluded that only the dinosaurs Corythosaurus casuarius, Lambeosaurus lambei and Lambeosaurus magnicristatus were present. More than that, Dodson proposed that he had discovered sexual dimorphism in each of these species, with the anatomy of these dinosaur’s crests being the primary way to tell females from males.
But paleontologists David Evans and Robert Reisz found a different pattern when they re-examined the sample of Lambeosaurus from Alberta. In the case of Lambeosaurus magnicristatus, in particular, a combination of a small sample size and an incomplete fossil had caused the confusion. Dodson included only two individuals of the hadrosaur species in the study, and since the crest of one individual was larger than that of the other, concluded that the two skulls represented the two sexes. As Evans and Reisz pointed out, the crest of the specimen Dodson regarded as female had been broken and so seemed superficially smaller. If the missing part had been in place, the difference between the two individuals would have disappeared.
Other paleontologists suggested different dimorphic schemes. James Hopson proposed that individuals grouped under the species Lambeosaurus lambei were females, and the species Lambeosaurus magnicristatus were males, and Kenneth Carpenter advocated a similar lumping. The problem with such a scenario is that the two species are not found at the same stratigraphic level. Evans and Reisz pointed out that Lambeosaurus lambei is found in greater numbers at a lower geologic level than the much rarer Lambeosaurus magnicristatus. The species did not overlap and so cannot represent different sexes of the same species.
Other attempts to set sex differences for dinosaurs have met similar frustrations. For a time, it was thought that male and female Tyrannosaurus could be distinguished on the basis of a tiny bone at the base of the tail. A row of small, spike-like bones called chevrons runs beneath much of the tail in dinosaurs, and it was thought that the first chevron in female Tyrannosaurus—the one closest to the hips—was reduced in size so that eggs could more easily pass out of the body. A similar observation had been reported before in crocodylians, and the fact that the trait seemed to be associated with larger specimens of Tyrannosaurus appeared to indicate that female tyrants were more robust than males of the same age. But this turned out to be a mistake. Crocodylian expert Gregory Erickson and colleagues discovered that the report on the reduced chevron in crocodylians was in error, and the fact that a complete chevron was found in the huge Tyrannosaurus “Sue” further eliminated the connection between the bone and sex identification.
As paleontologists Kevin Padian and Jack Horner pointed out in a Journal of Zoology paper published last year, sexual dimorphism “has never been conclusively established in dinosaurs.” Yet there is a way to identify at least one of the dinosaur sexes. The clues can’t be seen in the gross anatomy of skeletons or flashy ornaments, but in the structure of dinosaur bones.
In 2005, researchers Mary Schweitzer, Jennifer Wittmeyer and Jack Horner reported that they had found “gender-specific reproductive tissue” in a Tyrannosaurus specimen given the name “B-rex.” The specific type of tissue, called medullary bone, indicated that the particular dinosaur was female. Comparison to modern birds provided the key to this puzzle. Medullary tissue forms inside the long bones as a source of calcium when female birds are laying eggs. The same tissue is not naturally found in males. While there is no methodology to identify male dinosaurs in a similar way, the presence of medullary tissue inside dinosaur limb bones can be used to identify egg-laying females.
Paleontologists Andrew Lee and Sarah Werning ran with this finding to investigate how dinosaurs reached sexual maturity. Not only did Lee and Werning find medullary bone in two other dinosaurs—the ornithischian herbivore Tenontosaurus and the theropod Allosaurus—but, by combining these findings with evidence of dinosaur growth, they found that dinosaurs began reproducing when they were still actively growing. Tenontosaurus, Allosaurus and Tyrannosaurus had the dinosaurian equivalents of teen pregnancies, and this finding fit with the idea that dinosaurs lived fast and died young. Dinosaurs started having sex before they were skeletally mature, which corresponds to a lifestyle of rapid growth and a high likelihood of death before reaching maximum body size.
With any luck, future discoveries and studies of medullary bone will help us better understand when and how dinosaurs reproduced. Perhaps, paired with analyses of dinosaur skeletal anatomy, this peculiar type of bone may even help test ideas about sexual dimorphism in dinosaurs. If you can identify at least some female dinosaurs in a sample, you can then look to see if that subgroup contains any particular skeletal features that set them apart. The trouble is that medullary bone only works for identifying egg-laying females—males or females that are not reproducing cannot be distinguished this way. Still, the fact that paleontologists are able to pick out even a few female dinosaurs is a wonderful discovery that has the potential to show us previously unknown aspects of dinosaur biology. We are only just beginning to learn the more intimate secrets of dinosaur lives.
This post is the third in a short series of articles on dinosaur reproduction that will run through Valentine’s Day. Because nothing spells romance like dinosaur sex.
Dodson, P. 1975. Taxonomic implications of relative growth in lambeosaurine hadrosaurs. Systematic Zoology, 24 (1), 37-54
Erickson, G., Kristopher Lappin, A., & Larson, P. (2005). Androgynous rex – The utility of chevrons for determining the sex of crocodilians and non-avian dinosaurs Zoology, 108 (4), 277-286 DOI: 10.1016/j.zool.2005.08.001
Evans, D., & Reisz, R. (2007). Anatomy and Relationships of Lambeosaurus magnicristatus, a crested hadrosaurid dinosaur (Ornithischia) from the Dinosaur Park Formation, Alberta Journal of Vertebrate Paleontology, 27 (2), 373-393 DOI: 10.1671/0272-4634(2007)27[373:AAROLM]2.0.CO;2
Lee, A., & Werning, S. (2008). From the Cover: Sexual maturity in growing dinosaurs does not fit reptilian growth models Proceedings of the National Academy of Sciences, 105 (2), 582-587 DOI: 10.1073/pnas.0708903105
Padian, K., & Horner, J. (2011). The evolution of ‘bizarre structures’ in dinosaurs: biomechanics, sexual selection, social selection or species recognition? Journal of Zoology, 283 (1), 3-17 DOI: 10.1111/j.1469-7998.2010.00719.x
Schweitzer, M., Wittemeyer, J., Horner, J. (2005). Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex Science, 308 (5727), 1456-1460 DOI: 10.1126/science.1112158
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