In 2000, fossil preparator Bob Harmon was  eating his lunch in the Montana badlands   when he spotted a chunk of fossil bone  jutting out from the rocky slope above him.

The bone would turn out to be  the femur of a Tyrannosaurus rex,   and it was just the first  of many fossils to be found.

By the end of the excavation, the team  had recovered almost 40% of the skeleton,   including most of the skull, along  with several vertebrae and limb bones.

This individual was nicknamed “Bob-rex,”  or “B-rex” for short, in Harmon’s honor.

And this find would start to fill a huge  hole in our understanding of dinosaurs.

Because, while we like to think we know a lot  about them – like how they moved and what they   ate – for a long time, we haven’t been able to  ID one seemingly basic thing about their biology… Which dinosaurs are males and which are females?

In the case of B-rex, scientists would have   to crack the fossils open and  look inside to find the answer.

While scientists often give fossils nicknames,  like the famous T. rexes SUE and Stan,   we rarely actually know whether to call  these individuals male or female in sex.

And in this script, we’ll be using  ‘male’ and ‘female’ as shorthand   for sperm-producing and egg-producing individuals.

Being able to recognize these traits from  fossils is actually important for understanding   dinosaurs as living animals, rather than  as movie monsters or museum skeletons, because it can give insights into their behavior.

One way paleontologists have tried to figure  out the mystery of dinosaur sex is by looking   for patterns of features across skeletons of  the same species, to see if they can be easily   split into two groups – one group having  the feature and the other not, for example.

And this is because many of the close living  relatives of dinosaurs are sexually dimorphic,   which means there are differences in the  physical appearances of males versus females.

Like size, for example.

Some species can vary significantly between sexes, with the sperm-producing males often being  larger than the egg-producing females, usually so the males can physically  compete with other males for mates.

Or like the flashy feathers that male peafowl use to attract the attention of  the more drab-colored females.

But sexual dimorphism doesn’t always mean  bigger and more brightly colored males.

In some species, like red-eared slider turtles,   the females are actually bigger,  so their bodies can hold more eggs.

And in spiny-footed lizards,  the adult females are the ones with vibrant red tails that the males  seem to prefer when selecting a mate.

So what might be a trait associated with males in   one species could actually be a trait  associated with females in another.

And an animal’s sex can tell us a lot about  their behavior and reproductive strategies.

Females often make a small number of  large eggs, so they tend to have the most reproductive success when they  are choosy about who they mate with.

Males make a large number of tiny  sperm, so they tend to have greater reproductive success when they stand out  enough to be chosen by females as mates.

So, being able to identify sex in dinosaurs  could reveal a lot about how they lived.

In the 1990s, some paleontologists hypothesized  that female T. rexes could be recognized by the   shape of their pelvis, and that they might be  the bigger sex, like the red-eared sliders.

But they didn’t know for sure.

See, for living animals, one shortcut for  assigning sex to an individual is by their   reproductive organs.

Sperm-producers have  testes, while egg-producers have ovaries.

But for dinosaurs, we don’t have that luxury.

Soft tissues like skin, muscles, and organs   usually decay away long before they can fossilize,  making them pretty rare in the fossil record.

So far, the closest we’ve gotten to  fossilized dinosaur reproductive organs   is a remarkably preserved Psittacosaurus cloaca, which is a single opening used both  for reproduction and expelling waste.

But even this incredible fossil  didn’t preserve the internal anatomy, like testes or ovaries, that we would  use to call this animal male or female.

And it’s possible that some dinosaur species were  sexually dimorphic – because we do see variation.

But without knowing for certain the sex of  any individuals, it’s hard to say whether   particular traits are linked to sex at all.

Because the variations we see could be the  result of sexual dimorphism, or they  could be something else entirely.

Like age-related changes, for example.

Or even just the normal variation  that exists within a species.

Maybe the differences even sometimes mean that  we’re dealing with two dinosaur species entirely.

What paleontologists really needed was  an independent line of evidence that   linked a dinosaur’s skeleton and  its sex – one that didn’t rely on   subjectively interpreting variation in the bones.

What if the difference between male and female  dinosaurs could be found deeper within the bones?

In 2002, when B-rex was being excavated,  the team faced a unique challenge.

See, the site was located in some  pretty rugged terrain with no roads, so the field crew had to transport  the fossils out by helicopter.

And the biggest bundle, containing B-rex’s  legs, ended up weighing over 900 kilograms.

Which was…just over the helicopter’s weight limit.

So the team carefully separated  the block into two sections.

And in this process, a femur from B-rex was split,   resulting in several fragments  of bone coming loose.

Now, a broken dinosaur fossil  might sound like a nightmare,   but to paleontologists who study the  internal structure of fossils, it’s a dream.

Like paleontologist Dr. Mary Schweitzer,  who's made a career of looking past the surface of fossil bones, delving deeper  to examine their complex internal structures.

When she looked at fragments of  B-rex’s femur under the microscope,   she recognized something that  could help solve the question   of how to tell a male dinosaur from a  female – at least for this individual.

What she saw was a type of bone tissue  called medullary bone, which is a unique,   spongy tissue found inside  the bones of modern birds.

It’s produced when they’re getting ready  to lay eggs, and it partially fills in the air pockets found in the center of bird  bones that make them light enough to fly.

It acts as a readily-available source  of calcium for building eggshells.

Now, Schweitzer had studied bird  biology throughout her research   to better understand the connection between  birds and their non-avian dinosaur ancestors,   so she was very familiar with medullary bone.

So to try to determine whether what she was  seeing in B-rex’s femur was really medullary bone,   she compared it to bones from ostriches and emus  that had died during their egg-laying cycles.

And the porous, irregular structure of the ostrich   and emu samples was nearly identical to  the structure she saw in B-rex samples.

This was potentially a really  big deal – it seemed like a   sure-fire way to tell the sex of a dinosaur.

Or, at least, this dinosaur.

Because if Schweitzer was correct,  then “Bob-rex” was actually a female.

And not just a female, but one that  had been pregnant when she died.

While many had suspected that  theropod dinosaurs like T. rex   produced medullary bone just like their avian  descendants, no one had ever found it before.

And this made for a pretty heated debate that  isn’t even fully resolved today, with a number of   paleontologists proposing and testing alternative  hypotheses to explain the funky-looking bone.

Like in 2009, researchers  described samples of diseased   bone tissue from a turkey vulture and a  Cretaceous dinosaur from Transylvania.

And they noted striking similarities in appearances  between these samples and those of B-rex.

They suggested that what Schweitzer saw  in B-rex was possibly the result of a   bone disease called osteopetrosis,  which affects many modern birds.

These researchers also argued that  big dinosaurs like T. rex may not   have needed medullary bone in the first place.

See, it’s thought that medullary bone  in birds may have first evolved as a   way to compensate for their thin-walled bones.

Because pulling calcium from the already light,   delicate bones could cause  those bones to easily fracture.

So medullary bone provides them with  a source of calcium for egg-laying   without putting their bone structure at risk.

But this isn’t necessary for egg-layers  with thick bones, like crocodiles,   who pull the calcium for their eggshells directly  from their skeletons without any ill-effects.

And since the bones of T.rex were fairly thick,   the researchers suggested that these dinos  may not have had medullary bone at all.

Then, in 2014, a new aspect of the  debate emerged when a study reported   finding another example of medullary bone-like  tissue in the lower jaws of several pterosaurs.

Now, pterosaurs are thought to have laid eggs  with very thin, leathery shells, which don’t   need much calcium.

So this study challenged the  link between medullary bone and egg production.

At the time, it was also believed that medullary   bone was produced almost  exclusively in limb bones.

So if medullary bone-like tissue  could be found in pterosaur jaws,   the researchers suggested, it may have  had another purpose beyond egg-laying.

If this was true, it would mean that it couldn’t  be reliably used to identify pregnant individuals.

So, the debate raged on.

Did  B-rex really have medullary bone?

And do we have a way to  recognize female dinos or not?

To make a stronger case for the B-rex  medullary bone being the real deal,   Schweitzer and her team had some more work to do.

And in 2016, they published a study that  analyzed the B-rex bone at the molecular   level, directly comparing it to the unique  chemical signature of bird medullary bone.

To do this, they applied stains that react   with certain types of molecules  found only in medullary bone.

And the samples of supposed B-rex medullary  bone did react, becoming stained in dark   brown and blue – just like the medullary  bone samples from a hen and an ostrich.

They also applied the same stains to  samples of chicken bones with osteopetrosis.

These samples didn’t react, suggesting that the   B-rex samples wouldn’t have reacted  either if the bone was just diseased.

And in 2019, Schweitzer collaborated  on another paper that revealed that   birds in their egg-laying cycle  can actually produce medullary   bone in almost all areas of their  skeletons – including the lower jaw!

Which means that we can’t yet rule  out the role that medullary bone   plays in reproduction based solely on  where in the skeleton it’s discovered.

So where does the debate stand now?

Well, these analyses did persuade many scientists  of the usefulness of medullary bone for   potentially identifying mature female dinosaurs,  but some paleontologists still aren’t convinced.

And a new paper came out while we  were working on this episode that   found variations in a sample of 61  ornithomimosaurs from the same herd   that all died together that looks  consistent with sexual dimorphism.

So there are other approaches researchers  can take to try to ID the sex of dinosaurs,   if they have just the right sample.

What it all comes down to is that understanding  certain aspects of dinosaur biology is more   complicated than it seems, even for  dinosaurs as well-known as T. rex.

And with every hypothesis we test, we come closer   and closer to understanding what life  would’ve been like for these dinosaurs.