How do scientists know if dinosaur fossils are male or female?

Telling male and female dinosaurs apart is easier said than done. Every once in an extremely long while, researchers might find a skeleton with eggs inside it -- a pretty good clue that one was female. One such find cropped up in Jiangxi Province, China, in 2005. But eggs aren't always the best indicator. It might seem like a skeleton perched atop a nest of eggs belonged to a female, but it's just as likely that male dinosaurs, like male emus and ostriches, were the ones that did the brooding.

Another potential way of dividing males from females is through studying sexual dimorphism. Think of peacocks who display huge, ornate tails in front of nondescript peahens. It could be that differences in neck frills and head crests had to do with a dinosaur's need to attract a mate. The size and proportion of a dinosaur's body may have been sex-related, too. Paleobiologist Phil Senter hypothesizes that the ultralong necks of Diplodocus and Apatosaurus may have evolved through sexual selection, with longer-necked dinosaurs attracting more -- or better -- mates [source: Atkinson].

The only trouble with this theory is that, even if there are sex differences in dinosaurs, we still don't know which sex is which. Did a female Triceratops use its neck frill to attract a mate, or was it the other way around? Were horns and tail spikes for defense against predators or for combat with rival males? Without preserved samples of sex organs or a time-traveling look at dinosaur behavior, it's impossible to say.

Aside from the serendipitous find of a dinosaur fossil filled with eggs, researchers have found one way to figure out if a dinosaur is female, at least if she's pregnant. In 2005, Dr. Mary Schweitzer, Jennifer Wittmeyer and John R. Horner reported the discovery of medullary bone in a Tyrannosaurus rex fossil in the journal Science. In female birds, medullary bone lines the interior, or medullary cavity, of bones. Birds use this bone as a reservoir for the calcium they need to produce eggshells. In all likelihood, the T. rex whose bones the team was studying was both female and pregnant. Unfortunately, researchers can only determine the sex of dinosaurs this way by breaking into the interior of the bone. On top of that, it only works if the dinosaur is pregnant.

With groundbreaking discoveries like Schweitzer's and the well-preserved fossils being unearthed in places like China, it may one day be possible to make a pretty good guess about a dinosaur's sex. But for right now, it's a shot in the dark. To read more about paleontology, dinosaurs and scientific unknowns, read through the links on the next page.