In a finding that further links dinosaurs and their modern bird descendants, paleontologists report some dinosaurs engaged in mating dances similar to those employed by today's birds.
The evidence for such behavior lies in 100-million-year-old rocks that bear large scrape marks similar to what results from modern birds' "nest scrape displays" or "scrape ceremonies," researchers say.
In such behaviors, males attempt to attract mates by showing off their ability to excavate pseudo nests for potential partners, they explain.
The ancient dinosaur scrapings were found in western Colorado.
"These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior," says Martin Lockley, a geology professor at the University of Colorado Denver.
The new finding, along with previous discoveries of dinosaur head crests and colorful feathers, strengthens speculation that dinosaurs engaged in sophisticated mating displays, the researchers say in a study appearing in Scientific Reports.
More than 50 of the dinosaur scrapings were discovered by Lockley, a noted expert on dinosaur footprints, in an area already known for foot tracks of both herbivorous and carnivorous dinosaurs.
Large theropods, two-legged carnivorous dinosaurs, likely made the scrapes marks in Colorado, the researchers suggest. A leading candidate is Acrocanthosaurus, which could reach 38 feet in length and weigh close to 7 tons.
Paleobiologists have long speculated that dinosaur mating rituals might resemble those of some modern bird descendants like ostriches and Atlantic puffins, both of which conduct energetic dancing displays.
Breeding season would have been a time of great activity and even frenzy among the dinosaurs, Lockley says.
"This is typical of some bird species," he explains. "The extensive scrape evidence suggests much high-energy activity. If small birds get excited when breeding, imagine what big theropods might have done!"
Males would have been the primary dancers and scrapers, he argues, as males are the main "show-offs" in birds today.
However, while some carnivorous dinosaurs evolved into bird descendants retaining display abilities, Lockley acknowledges, "there is no reason to suppose that all theropods developed this behavior, or that all descendants should have inherited it."
Still, he says, the scrapings are strong evidence of dinosaur mating displays and the evolutionary power of "sexual selection," in which female dinosaurs may well have chosen the most impressive male performers as mating partners.
"These huge scrape displays fill in a missing gap in our understanding of dinosaur behavior," he suggests. The scientists made molds of the scrapes and took layered photographs since the scrapes could not be removed without damaging the fossilized markings.