Damaged bones allow paleontologists to approach dinosaur mating habits—and their consequences—a little more closely. Painful-looking punctures on the skulls of large theropod dinosaurs such as Gorgosaurus, Sinraptor and others indicate these dinosaurs bit each other on the face during combat, according to Darren Tanke and Philip Curie. These fights were likely over mates or the territory through which prospective mates might pass. Tanke, Andrew Farke and Ewan Wolff also detected patterns of bone damage on the skulls of the horned dinosaurs Triceratops and Centrosaurus. The wounds on Triceratops, in particular, matched what Farke had predicted with models of the famous horned dinosaurs: They literally locked horns. The confrontations that left these wounds could have happened anytime, but during the mating season is the likeliest bet. Ceratopsian dinosaurs have a wide array of horn arrangements and frill shapes, and some scientists suspect these ornaments are attributable to sexual selection.
These notions are difficult to test—how can we tell whether female Styracosaurus preferred males with extra-gaudy racks of horns, or whether male Giganotosaurus duked it out with each other over mating opportunities? But an unexpected discovery gives us a rare window into how some dinosaurs courted. For decades, conventional wisdom held that we would never know what color dinosaurs were. This is no longer true. Paleontologists have found more than 20 species of dinosaurs that clearly sported feathers, and these feathers hold the secrets of dinosaur color.
Dinosaur feathers contained tiny structures called melanosomes, some of which have been preserved in microscopic detail in fossils. These structures are also seen in the plumage of living birds, and they are responsible for colors ranging from black to gray to brown to red. As long as a dinosaur specimen has well-preserved feathers, we can compare its arrangements of melanosomes with those of living birds to determine the feather’s palette, and one study last year did this for the small, feathered dinosaur Anchiornis. It looked like a modern-day woodpecker, the analysis showed: mostly black with fringes of white along the wings and a splash of red on the head.
So far only one specimen of Anchiornis has been restored in full color, but so many additional specimens have been found that paleontologists will be able to determine the variation in color within the species, specifically looking for whether there was a difference between males and females or whether the flashy red color might be mating plumage. Through the discovery of dinosaur color, we may be able to understand what was sexy to an Anchiornis.
So where does all this leave the mystery of Stegosaurus mating? With all that elaborate and pointy ornamentation, we can imagine male Stegosaurus lowering their heads and waggling their spiked tails in the air to try to intimidate each other, with the victor controlling territory and showing off his prowess. Not all females will be impressed—female choice determines ornamentation as much as competition between males does—but those that are will mate with the dominant male. All the bellowing, swaying, and posturing allows females to weed out the most fit males from the sick, weak or undesirable, and after all this romantic theater there comes the act itself.
Figuring out how Stegosaurus even could have mated is a prickly subject. Females were just as well-armored as males, and it is unlikely that males mounted the females from the back. A different technique was necessary. Perhaps they angled so that they faced belly to belly, some have guessed, or maybe, as suggested by Timothy Isles in a recent paper, males faced away from standing females and backed up (a rather tricky maneuver!). The simplest technique yet proposed is that the female lay down on her side and the male approached standing up, thereby avoiding all those plates and spikes. However the Stegosaurus pair accomplished the feat, though, it was most likely brief—only as long as was needed for the exchange of genetic material. All that energy and effort, from growing ornaments to impressing a prospective mate, just for a few fleeting moments to continue the life of the species.
Brian Switek blogs at Dinosaur Tracking and is the author of Written in Stone: Evolution, the Fossil Record, and Our Place in Nature.