Dinosaurs Were the Original Love Birds

Question: Were dinosaurs hot, as in hot lovers?
Answer: probably yes, when they were “in heat.”
This is a serious scientific question. We know some dinosaurs had flamboyant head gear: horns, frills and crests. Add the evidence that many had feathers and were related to birds, and a whole new vista of possibilities opens regarding their courtship, mating and display activities. Like birds, some of their closest cousins, the carnivorous 'theropod' dinosaurs may have engaged in colorful display. They say: “if you've got it flaunt it.” We may think of carnivores like Allosaurus and T. rex as cold-blooded killers, but they may have been amorous as well. And the evidence? Some is indirect, based on theropod family tree relationships to birds, those masters of showy displays arising to emotional peaks in the breeding season, but some evidence is more direct. It is just this direct evidence, reported in the prestigious journal Scientific Reports, (Jan 7th) that has allowed Martin Lockley, dinosaur tracking professor of geology at the University of Colorado Denver to lead an international team of trackers to unearth actual physical evidence of dinosaur love fests right here in Colorado.

When asked what this evidence looked like Lockley said “we've found several large areas where tracks of carnivorous dinosaurs are found with dozens of large scrapes some the size of bath tubs.” He quickly added that “many modern bird species engage in a behavior known as 'nest scrape' display or 'scrape ceremonies' where males show off to females by excavating “pseudo nests.” The areas where they do this are called “display arenas” or “leks.” Lockley said the largest arenas, found in two National Conservation Areas (Dominguez-Escalante and Gunnison Gorge), in the Uncompahgre District, near Delta, Colorado, have more than 50 scrapes covering areas that probably extended for several acres. “These are also the first sites with evidence of dinosaur display rituals ever discovered, and the first physical evidence of courtship behavior.” Such evidence supports paleontological ideas about dinosaur mating display, that were previously based only on purely speculative comparisons with modern bird behavior.

Lockley's 15-strong tracker team is truly international with six Colorado residents, three Koreans, representing the National Research Institute of Cultural Heritage, which partly funded a three year joint dinosaur tracking project in Colorado and eastern Utah. The team also includes two Canadians, a Chinese colleague, and two Polish paleontologists, who like Lockley, are associated with Moab Giants, a new museum devoted to tracks in eastern Utah, where the display arena story is being worked into the exhibits. The team also includes three Bureau of Land Management (BLM) employees who have worked closely with the team for several years assisting in state-of-the-art 3D photogrammetric documentation of the sites at the locations on BLM-administered land in western Colorado.

The rock formation in which the scrapes occur, is the Dakota Sandstone, also found all along the Colorado Front Range, including Dinosaur Ridge. In fact one of the traces described comes from this area. This 100 million-year-old rock unit has yielded the tracks of both carnivorous and herbivorous dinosaurs, crocodiles, turtles, pterosaurs, horseshoe crabs and other invertebrates. “The best candidate for the amorous scrape-making dinosaurs is the probably the carnivore known as Acrocanthosaurus” said Lockley. We can imagine these large dinosaurs, with feet up to 18 inches long, and bodies 20-25 feet, scraping sandy substrates near the shores of the famous Cretaceous gulf known as the Western Interior Seaway. They were probably not only excited but impressively vocal. It was perhaps like Spring Break in the Cretaceous: courtship has a very long history. Team members pointed out that one cannot remove such large scrape marks by removing the gigantic slabs of rock in which they occur. According to BLM Wyoming State Office Regional Paleontologist Brent Breithaupt and Neffra Matthews, a BLM paleontologist at the National Operations Center in Denver , “ because the traces cannot be removed without damage, and as required by BLM permit stipulations and the Paleontological Resources Preservation Act, the traces were documented using 3D digital photogrammetry.” Both BLM staff and their Canadian colleagues, creating a digital 3D data set for interpreting and monitoring the site over time. This data was also used in the scientific publication on the sites. In addition, several of the scrape patterns have been replicated by making rubber molds and fiberglass hard copies, held at the Denver Museum of Nature and Science. “These huge trace fossils tell a fascinating story” said curator Joseph Sertich, adding that “they fill a gap in our understanding of dinosaur behavior.”

Lockley cautioned that some paleontologists have asked if there might be other explanations for the scraping. “We explored other possibilities” he said, explaining that digging for water or food by carnivores was very unlikely, as water was abundant in these environments. Likewise, these would not be actual nest sites because there are no eggshells or bones of hatchlings, and in any case sitting in a nest for long periods would smudge out the very clear scratch marks. Aggressive male theropods might have used suitable sites to make territorial displays, but these, say the team, would be difficult to distinguish from display arenas. “The display arena hypothesis is extremely convincing” said Richard McCrea and Lisa Buckley, team members and directors of the Peace Region Paleontological Research Center, in British Columbia.

The scrape evidence has several significant implications. Modern birds using scrape ceremony courtship, usually do so near their final nesting sites. This means that finding the right type of scrapes could mean a nearby nest colony. No one has ever found one in the Dakota Sandstone, where acid conditions have removed most calcium carbonate shells, and probably eggshells also. But the scrape evidence is a clue that when “hot” (in heat) dinosaurs congregated here in the spring breeding season and may have then nested nearby. Ornithologists have long been fascinated by the courtship displays of birds, especially the peacock and birds of paradise. They have interpreted such behavior, which we might call showing off, as a driver of evolution, through the process Darwin called “sexual selection.” Males either compete for females by driving off weaker rivals, or females select the most impressive male performers as mates. Similar sexual selection behaviors are well known in mammals whether it be in the rutting of elk or human posturing in the disco or sports arena. So, even in the Cretaceous, male and female dinosaurs evidently made considered choices about partners.

There may perhaps be 'minor' problems with all this talk of dinosaur love fests and sexual selection. Will it make the next text book editions a little too risqué? Will children need to avert their eyes when viewing dinosaur exhibits? Perhaps. But we need to remember that birds do it and bees do it, so dinosaurs did it also, and without their ancestral antics, the colorful bird world might look quite different today. Our language might even lack the emotional impact of such heart-warming expressions as “love birds.” Were dinosaurs the first love birds, and did the first dinosaur romances take place on ancient beaches that are today preserved as sandstones at Dinosaur Ridge and Colorado's western slope? The scientific evidence points strongly in this direction.