Fossils reveal dinosaurs had teen sex 22:00 14 January 2008 NewScientist news service Dinosaurs did not wait to have sex – adolescent females, including Tyrannosaurus rex, started reproducing before they had finished growing to adult size, a new study shows. The findings add to evidence that adolescence was the prime of life for dinosaurs. Age at sexual maturity gives insight into the reproductive strategy and life history of species, but is far from obvious in fossils. Birds, the closest living relatives of dinosaurs, do not reproduce until they reach full adult size, as do small mammals. In contrast, living reptiles and large mammals reproduce while still growing. A previous study discovered that dinosaurs called Oviraptors found brooding on fossil nests were not fully grown – showing that they bred early, like primitive reptiles. (Biology Letters, DOI: 10.1098/rsbl.2007.0254). However, that was only one species of modest size – about 2 metres long and 30 kilograms in weight. Now three larger dinosaurs have been added to the list of early breeders, showing the trait was widespread. Telltale bone The evidence comes from medullary bone – a calcium-rich tissue that female birds form as a calcium reserve during egg-laying, which has been recently identified inside a leg bone of a fossilised Tyrannosaurus rex. And now Sarah Werning of the University of California at Berkeley, US, and colleagues have identified medullary tissue in teenage specimens of two more dinosaurs, the predator Allosaurus and the plant-eating Tenontosaurus. Crucially, all three dinosaurs are known well enough that good data is available on their growth and population distribution. The team reports that the Tenontosaurus was 8 years old, the Allosaurus was 10, and the T. rex was 18 years old. None had reached full adult size. That means dinosaur reproduction differed dramatically from that of their avian descendants. Birds are vulnerable when immature, so they put all their energy into growing to be ready for flight, and do not reproduce until after they reach full adult size, says Werning. Many wait for a year or more before breeding. Extra years In contrast, the big dinosaurs lived only 3 or 4 years after reaching full size. By starting reproduction while still growing, "they are adding a decade for reproduction", Andrew Lee, co-author of the study, now at the Ohio University College of Osteopathic Medicine, told New Scientist. Lee notes that elephants do the same today, with females reaching sexual maturity at 15 years, although they do not reach full size until about the age of 50. By covering a range of dinosaurs, the new study gets closer to the base of the dinosaur family tree, says Kristi Curry Rogers of Macalester College in Minnesota, a coauthor of the oviraptor paper. "It's a nice piece of work and helps create a good picture of the lives of dinosaurs."