Mating injuries may lead scientists to identify dinosaurs’ sex

An illustration shows Edmontosaurus annectens, a hadrosaur from the Late Cretaceous in North America. (Credit: Roman Garcia Mora/Stocktrek Images/Getty Images via CNN Newsourc)

Paleontologists have long wrestled with how to differentiate between male and female dinosaurs based on their fossils.

But new research may bring scientists closer to identifying the sex of one group of dinosaur.

Hadrosaurs, also known as duck-billed dinosaurs, were common during the Late Cretaceous Period (100.5 to 66 million years ago), and the bones of these herbivores have been found across multiple continents.

Some hadrosaur fossils show evidence of healing from traumatic bone injuries, all in the same location: on their vertebrae past the base of the tail.

No soft tissues preserving evidence of dinosaur reproductive organs have been found in the fossil record, and differences observed in fossils are often thought to be due to species or age, rather than sex. Additionally, evidence of dinosaurs containing fossilized eggs has been hard to come by.

The authors of a new study published Tuesday in the journal iScience believe the injuries occurred during mating — and they could be used to indicate which fossils belong to female hadrosaurs.

“This will be a game changer since it will enable other questions to be answered about differences between male and female dinosaurs,” said lead study author Dr. Filippo Bertozzo, a researcher at the Royal Belgian Institute of Natural Sciences’ Operational Directorate Earth and History of Life.

Identifying a pattern in fossils

Canadian paleontologist Darren H. Tanke first noticed the injuries on hadrosaur vertebrae on bones he discovered in Alberta’s Dinosaur Provincial Park throughout the 1980s.

Tanke’s original hypothesis was that they were caused by male hadrosaurs mounting females during the mating process, but he based his claims largely on fossils found in Canada.

Bertozzo noticed the same injury while studying a fossil belonging to the hadrosaur species Olorotitan arharensis, during a research trip to Russia in 2019. He was working on his PhD about disease in duckbilled dinosaurs while at Queen’s University Belfast. Bertozzo invited Tanke to join him in conducting new research.

“In order to support behavioral claims, you need to have an extensive dataset able to possibly cover bias from fossilization, preservation, and collection,” Bertozzo said. “Also, the dinosaur biomechanical simulations weren’t as developed as those of today, and Darren couldn’t test his hypothesis.”

Together, the researchers and their collaborators analyzed nearly 500 tail vertebrae across different hadrosaur species kept at museums in North America, Europe and Russia.

The injuries and deformities they discovered on the middle part of the tail across different specimens were strikingly similar.

“At the base of the tail, between the sacrum and the mid-point of the tail, the neural spines (the elongated bars atop the vertebra, what we call the “spinous process” in human anatomy) are broken on their tip, sometimes along the main body of the spine,” Bertozzo said.

The tips were either tilted, swollen, inclined or even missing — meaning the injury caused the tip to break off and become reabsorbed by the body, he added.

The injuries were visible on several vertebrae for each specimen, suggesting the damage was spread along the main line of the tail, Bertozzo said.

The team carried out multiple simulations to see if the injuries could have been sustained through other activities during the dinosaurs’ daily lives, like accidentally stepping on each other’s tails, muscular strain during motion, fighting, being hunted, feeding or walking. But none of the scenarios would have consistently produced the injuries seen on the fossils, according to the study.

Instead, the authors think the males may have mounted the females , who were lying on their sides, and pressed down on the females’ tails during mating, accidentally breaking the neural spines.

“The mating hypothesis is the one that, at the moment, best explains our observations and data,” Bertozzo said. “The best evidence is the fact that we find this condition in many different species in different localities and times, suggesting it wasn’t something species-specific, but rather an event that would be conservative across all of them, happening to all of them, that is mating.”

The injuries also didn’t appear to be fatal because signs of healing, and even evidence of a second injury, were visible.

“Aggressively pursuing a female during reproduction might sound evolutionary disadvantageous for the continuation of the species but we already witness similar occurrences in many modern species, such as in sea lions, turtles, and some species of birds,” said study coauthor Gareth Arnott, professor in the school of biological sciences at Queen’s University Belfast, in a statement. “Reproductive competition is one of the most complicated topics in animal biology, especially for extinct species.”

Dr. Albert Prieto-Márquez, a researcher in the dinosaur ecosystems research group at the Autonomous University of Barcelona’s Miquel Crusafont Catalan Institute of Paleontology, believes the consistent injury pattern of healed middle tail fractures is most likely a genuine signal, rather than coincidence. He was not involved in the study, and applauds the creative insight of the researchers, but believes more evidence is needed.

“Right now, we can’t identify male and female dinosaurs just by looking at tail injuries,” he said. “To be sure, we’d need to find the same pattern in specimens that we know are female, for example with eggs or medullary bone preserved.”

Medullary bone is temporary tissue that forms inside a cavity in long bones of birds and dinosaurs to provide calcium for eggshells, and was previously used in research to identify a pregnant Tyrannosaurus rex in 2016.

For example, in chickens, medullary bone forms one to two weeks before the first egg is laid, and is reabsorbed about three weeks after the last egg is laid, according to the study.

If the same pattern occurred in dinosaurs, any containing this evidence would have died within a time range of about five weeks during the reproduction period, the authors wrote.

Telling female from male

If the hypothesis can be proven, it could open up a whole new way to understand complete hadrosaur fossils that bear such injuries, Bertozzo said.

“This might bring a cascade of implications, such as testing whether skull crests vary between sexes, or identify anatomical features that were previously assigned to new species,” he added.

But more data is required. Although the team compiled a large dataset, they are eager to study fossils from China and South America to continue their comparisons. They also want to use more powerful computer simulations to include tail movements and muscle volumes across different injury scenarios, Bertozzo said.

Bertozzo is also curious to see if the injuries are present in other types of dinosaurs, like long-necked sauropods. He was surprised not to find similar lesions in iguanodons, which were ancestors of hadrosaurs and are also very common in the fossil record.

“The story is just at the beginning, and I hope ours is only one of the initial steps to better understand this aspect of the life of dinosaurs,” Bertozzo said.

Determining male from female dinosaurs based on bones requires a high burden of proof, something that has historically been difficult, said Steve Brusatte, a professor of paleontology and evolution at the University of Edinburgh in Scotland, who did not participate in the study.

“The authors have laid out their case and I think they provide a compelling argument, but as with so many aspects of dinosaur behavior, we just weren’t around tens of millions of years ago to observe them as real animals, so there will always be some degree of doubt, some degree of uncertainty in these interpretations,” he said. “We often just can’t tell the boys from the girls.”

Dr. Yoshitsugu Kobayashi, a professor of vertebrate paleontology at the Hokkaido University Museum in Japan, found the study to be one of the most imaginative he’s seen in recent dinosaur paleobiology and believes the modeling in the study rules out other possible causes. While Kobayashi has discovered new species of hadrosaurs, he was not involved in the new research.

However, while Kobayashi finds the hypothesis intriguing, he regards it as a bold starting point rather than conclusive evidence.

“This study opens a fascinating window into the private lives of dinosaurs,” he wrote in an email. “Even if not all the conclusions hold, it shows that bones can preserve traces of behavior we never thought we’d see. It’s exciting to think that even the scars of these ancient creatures can reveal moments of their most intimate lives, quite literally, the ‘love life’ of dinosaurs written in their bones.”