For ancient duck-billed dinosaurs, mating might have been a bone-jarring, even bone-breaking, affair.
A new study describes a peculiar pattern of tailbone fractures in hadrosaurids (duck-billed dinosaurs) across North America, Europe, and Russia.
This research supports a long-standing but previously unproven hypothesis that these injuries were sustained during mating.
“The weight of the male could have crushed the female’s back. These injuries may help us identify female dinosaurs,” said Filippo Bertozzo, first author from the Institute of Natural Sciences in Belgium.
It sheds new light on the intimate lives of these massive herbivores that roamed the Earth during the Late Cretaceous Period.
Peculiar injury marks
Life for hadrosaurid dinosaurs was brutal, as evidenced by their skeletons showing various pathologies like trauma, infections, and tumors.
In 2019, Bertozzo observed a puzzling pattern of healing fractures on upper tail vertebrae of the hadrosaur Olorotitan arharensis in Russia.
What struck him was the concentration of these injuries. Most of the injuries were clustered in the upper part of the tail, near what would have been the dinosaur’s cloaca (a multi-purpose opening for waste and reproduction), but did not extend to the tail’s tip.
This specific injury pattern was unusual, as previous observations were limited to isolated bones.
Extensive analysis validated a “mating hypothesis” first proposed in 1989 by co-author Darren H. Tanke, who suggested that these fractures were caused during mating.
Tanke’s original idea lacked sufficient evidence until Bertozzo found the same consistent pattern in specimens outside North America.
For this new study, the team analyzed roughly 500 damaged tail vertebrae from various hadrosaurs spanning North America, Europe, and Russia.
Interpretation of dinosaur’s sex
The team confirmed that the backbone injuries were strikingly similar across different hadrosaur species, appearing as vertical-to-oblique fractures caused by vertical pressure on the tip of the vertebral spines
To test this mechanically, they combined statistical analysis with Finite Element Analysis (FEA), an engineering simulation.
The FEA results demonstrated that the fractures were consistent with an external pressing force acting from above at an angle of 30 to 60 degrees, confirming that vertical pressure caused the stress fractures on the elongated tail vertebrae.
Alternative explanations for the tail injuries, including predation, muscular stress, herding accidents, and behaviors like mud wallowing, were ruled out.
The mating hypothesis was the best correlation, where the male’s weight caused bone stress fractures on the female’s upper tail.
Notably, these injuries were not fatal, as evidenced by signs of healing and, in some specimens, even repeated injuries, indicating that the females survived and likely mated again.
“Aggressively pursuing a female during reproduction might sound evolutionary disadvantageous for the continuation of the species”, said Prof. Gareth Arnott, co-author.
“But we already witness similar occurrences in many modern species, such as in sea lions, turtles, and some species of birds. Reproductive competition is one of the most complicated topics in animal biology, especially for extinct species,” Arnott added.
The primary significance is that the mating hypothesis provides a potential method for sex identification: an injured individual can likely be inferred as female
To gain a deeper understanding of the lives of long-dead dinosaurs, the authors suggest further analyzing a more diverse range of species and utilizing more advanced computer modeling techniques.