New research suggests that injuries found on the tails of duck-billed dinosaurs may have been caused by mating behavior, offering a potential breakthrough in identifying the sex of these ancient creatures. This discovery, based on a study of fossilized hadrosaur vertebrae, could revolutionize how paleontologists interpret dinosaur biology and behavior.
Paleontologists have long faced the challenge of distinguishing male from female dinosaurs due to the scarcity of direct evidence like reproductive organs or eggs in fossils. Differences in bone structure have often been attributed to species variation or age rather than sex, leaving a significant gap in understanding dinosaur life histories. The new study, published in the journal iScience on November 4, 2025, focuses on hadrosaurs, commonly known as duck-billed dinosaurs, which were widespread herbivores during the Late Cretaceous period.
Researchers led by Dr. Filippo Bertozzo of the Royal Belgian Institute of Natural Sciences analyzed nearly 500 tail vertebrae from hadrosaur fossils across museums in North America, Europe, and Russia. They identified a consistent pattern of healed injuries on the middle part of the tails, specifically on the neural spines—the bony projections atop the vertebrae. These injuries included fractures, tilting, swelling, and in some cases, complete reabsorption of the bone tips, indicating traumatic events that the dinosaurs survived.
The team hypothesized that these injuries resulted from mating, where male hadrosaurs mounted females lying on their sides, applying pressure to the females’ tails and accidentally breaking the neural spines. To test this, they conducted biomechanical simulations exploring alternative causes such as fighting, predation, or daily activities like walking or feeding. None of these scenarios consistently produced the observed injury patterns, strengthening the case for the mating hypothesis.
Evidence supporting this idea includes the widespread occurrence of similar injuries across different hadrosaur species and geographic locations, suggesting it was a common behavior rather than species-specific. Additionally, the presence of healing signs and even secondary injuries in some fossils indicates these were non-fatal events, aligning with known reproductive competition in modern animals like sea lions and birds. This consistency across time and space makes the mating explanation the most plausible based on current data.
While the findings are compelling, some experts urge caution. Dr. Albert Prieto-Márquez of the Autonomous University of Barcelona, who was not involved in the study, praised the creative insight but emphasized that more evidence is needed, such as correlating the injuries with known female indicators like medullary bone or fossilized eggs. Similarly, Dr. Steve Brusatte of the University of Edinburgh noted that while the argument is strong, the inherent uncertainties in interpreting extinct animal behavior mean this should be seen as a step forward rather than definitive proof.
If validated, this research could have far-reaching implications for paleontology, enabling scientists to re-examine other dinosaur fossils for sexual dimorphism—differences in appearance between males and females. For instance, it might help determine if features like skull crests varied by sex, potentially reducing misclassifications of new species. The study also opens avenues for investigating similar patterns in other dinosaur groups, such as sauropods, though initial checks in iguanodons, hadrosaur ancestors, showed no comparable injuries.
Future work will involve expanding the fossil dataset to include specimens from China and South America, as well as using more advanced computer simulations to model tail movements and muscle forces. Dr. Bertozzo and his team hope this marks the beginning of a deeper exploration into dinosaur reproductive behaviors, ultimately shedding light on the intimate lives of these ancient animals and enhancing our understanding of their evolution and ecology.