Mating Scars in Duck‑Billed Dinosaurs: Tail Injuries May Reveal Hadrosaur Sexes

Healed tail injuries in hadrosaurs could be a clue to identifying female dinosaurs

Paleontologists have long struggled to distinguish male from female dinosaurs using bones alone. A new study led by Dr. Filippo Bertozzo and collaborators suggests a consistent pattern of healed fractures in the base and mid‑section of hadrosaur tails may record mating-related trauma — and could provide a reliable indicator of sex for this group of duck‑billed dinosaurs.

What the researchers found

The team examined nearly 500 hadrosaur tail vertebrae from museum collections across North America, Europe and Russia. They documented a recurring lesion pattern: damage to the neural spines (the upward projections on vertebrae) located between the sacrum and the mid‑tail. In many specimens the tips of those spines were tilted, swollen, missing, or showed evidence of healed breakage across multiple adjacent vertebrae.

How they tested explanations

To evaluate causes, the authors ran biomechanical and behavioral simulations and considered alternative scenarios such as accidental trampling, predation, fighting, strenuous movement, and feeding. According to the paper, none of those scenarios consistently produced the specific, repeated pattern of injuries observed across different species and locations. The team therefore favors a behavioral explanation: males mounting females that were lying on their sides, forcing load onto the tail base and breaking neural spines.

“The mating hypothesis is the one that, at the moment, best explains our observations and data,” said Dr. Filippo Bertozzo. He notes the pattern appears across species, localities and geological times, suggesting a widespread behavioral cause rather than a species‑specific pathology.

Evidence the injuries were nonfatal

Most damaged specimens show signs of healing, and a few even show evidence of a second healed injury, indicating these were survivable traumas. That parallels observations in several modern animals — for example, certain sea lions, turtles and birds — where aggressive mating behavior can produce nonfatal injuries.

Caveats and independent views

Several outside experts praised the creativity and argued the pattern is compelling, but urged caution. Dr. Albert Prieto‑Márquez emphasized that to conclusively link these lesions to sex, researchers would ideally find the same injury pattern in specimens independently known to be female — for example, fossils preserving eggs or medullary bone. Medullary bone is a temporary calcium reservoir used in egg production and has been used previously to identify pregnant dinosaurs (notably a 2016 study of a Tyrannosaurus rex).

Other commentators, including Steve Brusatte and Dr. Yoshitsugu Kobayashi, described the study as imaginative and promising while noting intrinsic uncertainty when inferring behavior from fossils. Brusatte summarized the broader challenge: “We often just can’t tell the boys from the girls.”

Implications and next steps

If confirmed, the mating‑injury hypothesis would open new avenues for interpreting hadrosaur fossils. Researchers could test whether skull crests and other anatomical features were sexually dimorphic, reassess characters previously used to define separate species, and investigate whether similar lesions occur in other dinosaur groups such as sauropods.

The authors plan to expand the dataset to include fossils from China and South America and to run more powerful biomechanical models that incorporate tail movement and muscle volumes. They also noted surprise that related iguanodonts did not show the same lesion pattern, a question they hope to explore further.

Conclusion

This study provides a well‑argued, testable hypothesis that mating behavior left a durable signature on hadrosaur tails. While not yet definitive, the work highlights how careful anatomical surveys combined with biomechanical testing can reveal aspects of dinosaur behavior previously thought impossible to detect from bones alone.

Key facts: nearly 500 vertebrae examined; lesions concentrated between sacrum and mid‑tail; injuries show healing; alternative causes tested and considered unlikely; further data and medullary‑bone associations needed for confirmation.