Dinosaurs engaged in mating behavior similar to modern birds, leaving fossil evidence behind in 100-million year old rocks, according to new research.

Paleontologist Martin Lockley of the University of Colorado Denver led a research team that discovered what he called large “scrapes” in prehistoric Dakota sandstone in western Colorado.

The team described the scrapes as similar to those left behind from modern birds’ “nest scrape display” or “scrape ceremonies.” in these, males show off their ability to provide by digging pretend nests for potential mates.

“These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior,” Lockley said. “These huge scrape displays fill in a missing gap in our understanding of dinosaur behavior.”

The study was published Jan. 7 in the journal Scientific Reports.

Lockley, an expert on dinosaur footprints, reported evidence of more than 50 scrapes, some as large as bathtubs, in an area where tracks of both meat-eating and plant-eating dinosaurs have also been confirmed.

The researchers said they found display arenas, also called leks, in two National Conservation Areas Dominguez-Escalante and Gunnison Gorge on property permitted by the U.S. Bureau of Land Management near Delta, Colo. Lockley also described evidence of mating areas at Dinosaur Ridge, a National Natural Landmark, just west of Denver.

The new evidence supports theories about the nature of dinosaur mating displays and an evolutionary force known as sexual selection, Lockley and colleagues argued. Since prehistoric times, males looking for mates, have driven off weaker rivals. Females, meanwhile, have chosen the most impressive male performers as consorts.

Similar sexual selection behaviors are common in mammals and birds. But until now scientists could only speculate about dinosaur mating behavior, assuming it might be similar to that of their modern relatives, the birds.

“The scrape evidence has significant implications,” said Lockley. “This is physical evidence of pre-historic foreplay that is very similar to birds today. Modern birds using scrape ceremony courtship usually do so near their final nesting sites. So the fossil scrape evidence offers a tantalizing clue that dinosaurs in “heat” may have gathered here millions of years ago to breed and then nest nearby.”