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 grew really fast yet matured early, said Sarah Werning, a graduate student in palaeontology 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, reported the finding in 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 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.

The researchers added this onset of sexual maturity to their growth graphs and found that the dinosaurs were reaching sexual maturity in the midst of a teenage growth spurt.

"They are definitely not juveniles, but they are notfully grown yet, and they are also going through a time of really rapid growth... It's very similar to what we call adolescence," Werning said.

The team also confirmed that a Tyrannosaurus rex bone that North Carolina State University palaeontologist Mary Schweitzer found in 2005 contained medullary tissue when the dinosaur died at 18.

All three types of dinosaurs had life spans of about 25 to 30 years, but they didn't reach full adult size until age 20 to 25. Waiting until they were fully grown to reproduce would have been risky, according to Werning. "It makes a lot of sense that (dinosaurs) wouldn't have the same strategy as birds," she said.

BIRDS ARE UNIQUE

The finding complements research published last year which showed that bird like dinosaurs - discovered sitting on their eggs just like birds do - also had sex as teenagers.

"What (Werning) and (Lee) have done in their paper is find exactly the same thing but in a different group of dinosaurs and by using a different marker of reproductive maturity," said Kristi Curry Rogers, a palaeontologist at Macalester College in St. Paul, Minnesota.

Rogers is co-author of the 2007 bird like-dino study with Gregory Erickson, a palaeontologist at Florida State University. She said both studies highlight the uniqueness of sexual maturity in birds.

"(Bird) evolution is a very fascinating mosaic of characteristics that deserves a lot more study," she said. "We're still at the beginning of teasing apart all the fine details."