A skilled Colorado dinosaur tracker has unearthed 100 million-year-old dino love nests in Denver’s backyard.

The first evidence of dinosaur dating was discovered by Martin Lockley, a University of Colorado Denver geology professor who stumbled across large scratch marks in Colorado rocks. Initially, the marks had Lockley and his international team stumped.

Taking a cue from birds — relatives to the carnivorous dinos that lived in the area — Lockley said he and his crew started to think the scratches could be a ritual activity many male birds partake in: pseudo-nest-building.

“It’s like they’re showing off to a prospective mate,” Lockley said. “They say, ‘Look, I can make a nest.’ And if a female is watching, they make another and another.”

Dozens of scrapes would send the female dinosaurs swooning until mating took place and a real nest was built.

“When we first realized that they were mating evidence, my first thought was, ‘This is going to be big,’ ” said Lockley, who has been at CU Denver for 35 years. “It’s dinosaurs and sex. What a combo.”

Flowers and a box of chocolates? Hardly.

The scrapes, Lockley said, are very deep, narrow grooves, with a claw mark on the end.

These etchings of courtship, which come in pairs, can be as large as bathtubs.

The markings have been found at Dominguez-Escalante National Conservation Area, Gunnison Gorge National Conservation Area, areas around Montrose, and Dinosaur Ridge, just south of Lakewood, said Harley Armstrong, the Bureau of Land Management’s state and regional paleontologist.

“The reason it’s a big deal is that these kinds of scrapes have never been found ever in the world,” Lockley said, “but that didn’t stop scientists from speculating.”

Many researchers long believed dinosaurs were trying to attract one another, but there was no physical evidence of the prehistoric courtship until Lockley unearthed his two years of research.

“Not only have we found the scrape marks — like dinosaur foreplay,” Lockley said, “but we found 50 or 60 of these things, and these sites are what have been called display arenas where they play out their display activity and then go and nest.”

Because the marks were unable to be removed from the massive rock slabs without being damaged, 3-D images were created to document them. Rubber molds and fiberglass copies of the scrapes are being stored at the Denver Museum of Nature & Science.

The Lockley-led study appears Thursday in the journal Scientific Reports.

“I think it’s wonderful,” Armstrong said. “It’s another feather for Colorado’s fossil cap. Because we have some of the known dinosaur fossils, the world has been coming to our doorsteps since 1877.”

Lockley looks forward to finding more scratches and ones that existed more than 100 million years ago.

“It wouldn’t surprise me at all after publishing this article that there are people in Europe, South America, Asia that go, ‘Oh, we have those. We just didn’t know what they were,’ ” Lockley said.

Elizabeth Hernandez: 303-954-1223, ehernandez @denverpost.com or @ehernandez