Can We Determine the Sex of a Dinosaur?

On August 12, 1990, the most complete Tyrannosaurus rex skeleton ever discovered was unearthed in the badlands of South Dakota by paleontologists Peter Larson and Susan Hendrickson. The animal was nicknamed “Sue” in honor of the latter scientist and, after a lengthy legal battle, her remains were purchased for the hefty sum of $8.4 million by the Chicago Field Museum. Today, she's easily become the world's most famous dinosaur. But what if “Sue” wasn't actually a female?

“We'd given the T. rex that name and always referred to it as 'she' or 'her',” said Larson, “But even at the site, people were asking me 'What if she's a male?' I'd say that we could always point to that Johnny Cash song 'A Boy Named Sue.'”

Country classics notwithstanding, can paleontologists accurately determine the sex of a long-dead dinosaur?

The short answer is yes ... and no.

While Sue's gender remains very much in question, comparatively recent research has shown that at least some dinosaur specimens may be sexed with reasonable certainty by modern scientists. The riddle's solution may very well lie in the bones of present-day dinos—also known as “birds."

Producing eggs is a very taxing process for expectant avian mothers, as the calcium in their shells is biologically expensive to build. When eggs start forming in the womb of a gravid bird, her body releases a heightened level of estrogen. This triggers the growth of what's known as “medullary bone” inside of her hollow leg and arm bones. The specialized medullary bone tissue provides much-needed calcium for the growing eggshells and lingers until their completion, at which point it disappears.

In 2005, paleontologists at North Carolina State University and Montana State University announced that they'd discovered medullary tissue within the limb bones of a T. rex specimen, suggesting that the animal was A) female and B) gravid at the time of its death.

The approach is hardly fool-proof: Only the bones of pregnant female dinosaurs would show any sign of medullary tissue, rendering the technique useless for deducing the sex of the vast majority of dinosaur skeletons. Nevertheless, the findings have excited a number of paleontologists, including one international team which sought to deduce the link between gender and this tell-tale tissue in a group of well-preserved fossilized birds this past February.