Scientists say long grooves found in the ground in Colorado were left by dinosaurs engaged in a frenzied mating ritual.
The grooves were etched in the ground by the pawing of clawed feet. Some scrapes are as large as bathtubs, researchers said.
The grooves were found at three sites in western Colorado and another just west of Denver.
"These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior," said Martin Lockley, professor of geology at the University of Colorado Denver.
Scientists believe the grooves were left by two-legged, meat-eating dinosaurs called theropods about 100 million years ago.
Theropods were built roughly like smaller versions of a T. rex. Footprints near the grooves suggest a variety of body lengths, up to about 16 feet from the snout to the tip of the tail.
The dinosaurs, probably males, apparently gathered in groups and "went crazy scraping" with their clawed, three-toed feet to attract mates, Lockley said.
The grooves they carved are up to 6 feet long.
"These animals would have been really frenzied," Lockley said.
The discovery was published Thursday by the journal "Scientific Reports."
The display arenas, also called 'leks' were found in two National Conservation Areas (Dominguez-Escalante and Gunnison Gorge) near Delta, Colorado.
Lockley also discovered evidence of mating areas at Dinosaur Ridge, a National Natural Landmark, just west of Denver.
Until now, scientists could only speculate about dinosaur mating behavior, and assumed it might be similar to that of modern birds.
"The scrape evidence has significant implications," said Lockley. "This is physical evidence of prehistoric foreplay that is very similar to birds today."