How did dinosaurs mate? Scientists solve mystery thanks to fossil discovery

BRISTOL, England — Scientists have discovered how dinosaurs had sex — by reconstructing their genitals for the first time. The discovery comes from the remains of a psittacosaurus, a primitive relative of the triceratops. They were so well preserved even that the cloaca, an opening used for mating, was visible.

Resembling a giant turkey, the spectacular psittacosaurus had a huge beak, horns on either side of its face, and was about six-and-a-half feet long. The organ in question was rich in the natural pigment melanin, which colors the skin, eyes and hair of animals. This would have provided the “vent” with a function in display and signaling, similar to baboons and salamanders. It’s believed large lobes on either side harbored musky scent glands, as seen in crocodiles today.

Dinosaur mating
Close up of the preserved cloacal vent in Psittacosaurus and the authors’ reconstruction of it. (Credit: University of Bristol)

“I noticed the cloaca several years ago after we had reconstructed the color patterns of this dinosaur using a remarkable fossil which clearly preserves its skin and color patterns,” explains lead author Dr. Jakob Vinther, of the University of Bristol, in a statement. “It took a long while before we got around to finish it off because no one has ever cared about comparing the exterior of cloacal openings of living animals, so it was largely unchartered territory.”

The complete skeleton is housed at the Senckenberg Museum of Natural History in Frankfurt, Germany. It was dug up at a fossil site in northwestern China called the Jehol Biota.

Cloaca discovery solves mystery — thanks to birds

Birds, the descendants of dinosaurs, have a cloaca. Usually covered by feathers, it is found under the body. Crocodiles, the closest living relatives of the biggest animals to walk the planet, also have one. In males, the penis is hidden inside during the resting state.

It is believed this applied to psittacosaurus while the females laid eggs through it. Most birds don’t have a penis.

“Indeed, cloacas are pretty nondescript,” adds co-author Dr. Diane Kelly, an expert on copulatory systems, from the University of Massachusetts Amherst. “We found the vent does look different in many different groups of tetrapods, but in most cases it doesn’t tell you much about an animal’s sex. Those distinguishing features are tucked inside the cloaca, and unfortunately, they’re not preserved in this fossil.”

Pronounced “klo-A-ca,” it comes from a Latin word meaning “to cleanse” as it performs urinary, intestinal and reproductive acts.

Psittacosaurus
Psittacosaurus specimen from Senckenberg museum of Natural History, preserving skin and pigmentation patterns and the first, and only known, cloacal vent.

When birds mate, they touch their cloacas for a few seconds. This brief “cloacal kiss” is just enough time to transfer semen to the female. Now Dr. Vinther and colleagues have identified the anatomy part in detail in a dinosaur. Less than an inch long (2 cms), it is surrounded by dark tissue.

We know dinosaurs came in all shapes and sizes, ranging from colorful feathered varieties to scaly, horned creatures. But very little is known about their sexual organs because soft tissue is rarely preserved in fossils.

The study in Current Biology finds that the cloaca was unique in its appearance. But it exhibits features reminiscent to living alligators and crocodiles. It isn’t possible to tell the sex of the particular animal, but the cloaca’s resemblance to those of crocodiles suggests males had a penis.

Birds are one of the few vertebrate groups that occasionally exhibit visual signaling with the cloaca. The elaborate mating ritual can now be extended back to their ancestors from the age of the dinosaurs.

“As a palaeoartist, it has been absolutely amazing to have an opportunity to reconstruct one of the last remaining features we didn’t know anything about in dinosaurs,” says Robert Nicholls. “Knowing that at least some dinosaurs were signaling to each other gives palaeoartists exciting freedom to speculate on a whole variety of now plausible interactions during dinosaur courtship. It is a game changer.”

Psittacosaurus was early ceratopsian related to triceratops and other horned dinosaurs. It has been on public display in Germany for over a decade. Despite the discovery of its cloaca, it was not possible to establish the individual’s gender as none of the internal anatomy had survived.

Sex between dinosaurs has puzzled scientists for decades. It would have been very difficult for large species to mate without having a very long penis. The cloaca means a male’s genitals would not be visible as he walked along. Instead it could be seen as just a slit beneath the tail.

A few male birds, such as the waterfowl, have a penis and, interestingly, their lineages all originate near the base of the family tree. If ancient birds had penises that were lost later in evolution, dinosaurs almost certainly did too. Very long, flexible penises could explain how huge or spiky dinosaurs got close and personal, without one of them being killed.

SWNS writer Mark Waghorn contributed to this report.