Big Dinosaurs Had "Teen Sex"

Big dinosaurs, like humans, reached sexual maturity during the messy growth spurts of adolescence, according to a new study.

The reproductive strategy of dinosaurs was unlike that of their reptilian ancestors or their bird descendants, the study concludes.

"They are growing really fast and yet maturing early," said Sarah Werning, a graduate student in paleontology and integrative biology at the University of California, Berkeley.

"Among living animals, the only things that do that are medium- to large-size mammals, including us."

Though reptiles like crocodiles reach sexual maturity before they are fully grown, they grow slowly. Birds grow to their full adult size within a year but delay sex for a year or longer, Werning noted.

She and colleague Andrew Lee, now at Ohio University in Athens, report the finding in tomorrow's issue of the journal Proceedings of the National Academy of Sciences.

Growth Rings

Like tree trunks, dinosaur bones have annual growth rings, Werning said.

The researchers were studying these rings in the bones of the meat-eater Allosaurus and the plant eater Tenontosaurus to determine how fast they grew at different points in their lives.

In a specimen of each type of dinosaur, the team happened upon a type of calcium-rich tissue called medullary bone. Modern-day birds also produce this type of bone prior to laying eggs.

The finding indicates that both the Allosaurus and the Tenontosaurus died shortly before laying eggs—and therefore that they were able to reproduce at the times of their deaths.

"They wouldn't be ovulating if they weren't of reproductive age," Werning noted.