Sue the T. rex may be one of the most famous dinosaur fossils ever discovered.

The skeleton was named after  the person who found it, American explorer Sue Hendrickson. But while this fossil shares her name, if you ask Sue – or the person who runs the  fossil’s social media accounts – they’ll point out that Sue  is actually gender-neutral.

Their media persona is neither male or female, and Sue is referred to using they/them pronouns. That’s because, well, we just don’t know what sex Sue the actual Tyrannosaurus rex was back when they were alive. Which raises the question: Can we ever be sure of the  sex of any fossil creatures?

Well, there are clues to a fossil’s sex, but they might not be what you think. [♪ INTRO] How can you tell whether an  ancient animal was male or female? After all, reproductive organs are usually soft tissue, and that doesn’t preserve well. It’s incredibly rare, but  sometimes we do get lucky and find animals preserved with soft  tissue, like with frozen mammoths.

In those cases you can just, well,  look under the hood, so to speak. In some other cases, an animals’  reproductive equipment does leave clear traces behind even  without their soft tissue. Some male mammals have a baculum, which is a bone that supports the penis.

So if you see one of those, it’s a pretty clear indication that the animal was male. At other times, we’ve been  lucky enough to find fossils that have eggs inside of them, or  which we think might be pregnant. In 2003, scientists announced that  they had found a fossil of a marine reptile called an ichthyosaur, that  had two embryos inside its body.

Based on their skeletal features,  orientation, and lack of trauma, researchers think they really  were embryos, not food. And if there are babies inside a  fossil animal, it’s probably a female. Unless it’s, like, a seahorse, or something.

But what about eggs that have been laid? Dinosaurs and other egg-layers have been found alongside eggs that were already laid, but male animals will also sometimes  incubate or care for nests, so that doesn’t end up telling us too much. Though, put a pin in this, because we’ll come back to something cool about egg layers later.

But maybe we don’t need to  always find babies or eggs to determine an animal’s sex.  Maybe just their bones are enough. And the reason for that is sexual dimorphism. In many species of animal, males  and females look different.

Think of how male peacocks are flashy and colorful and females are more drab. Or how male, silverback gorillas are much bigger than the females, and have, well, silverbacks. Sometimes these differences run so deep that they’re visible in the skeleton.

If you’re looking at a whole  bunch of animals of one species, and there’s two really distinct size categories, you may be looking at sexual dimorphism. And many scientists have  pored over fossil skeletons, looking for any kind of  evidence of sexual dimorphism. In dinosaurs, scientists have looked at things like tail and hip anatomy,  which could be different between males and females  due to the need to lay eggs.

Others have looked at things  like how big the animals were, or looked for differences in the skull crests that could suggest they had a  fancy look to entice the other sex. In 2021, scientists reported  seeing sexual dimorphism in the size of giant ground sloths’ canine teeth, which indicates that one sex would  have bigger fangs than the other. Males of quite a few species of primates will have larger canine teeth than the females do, though our specific human lineage seems to have lost that trait a while back.

We’re also less dimorphic in body size, and researchers are still  trying to figure out exactly how long ago that happened. Scientists have been debating whether species in the hominin genus  Australopithecus were dimorphic, for instance, though recent evidence points towards some size dimorphism in these guys. And, while males and females in the genus Homo have basically equal-sized canines, we’re still not sure if the males and females would have had roughly the same body mass, or had body size dimorphism  more like, say, chimpanzees.

Plus, modern humans have another odd distinction, which is body fat distribution. Human females have a higher  percentage of body fat than males, and we have very little  way of knowing how long ago that particular evolutionary  quirk might have arisen. But whether it’s our own  ancestors or other animals, even if we do see differences in the  sizes or shapes of fossil skeletons, figuring out whether that’s  based on sex can be tricky.

For one thing, individual variation  can be huge, even within a sex. There’s always the possibility  that there’d be overlap in the size difference between the largest  females and the smallest males, or vice versa for species  where females are larger. And in many cases, we just don’t know a lot of the nitty-gritty of what past  species were really like.

We’re often working off of what we know about existing relatives or general  biological rules of thumb. So it can be really hard to tease out which skeletal differences are  due to sex and which are due to individual variation or from living  in different areas or ecosystems. This is made even harder by  the fact that for many species, there just aren’t that many fossils to examine.

T. rex might the most famous dinosaur ever, but we have fewer than 100  fossil specimens of the species. That means paleontologists need  to work off of a small number of individuals who may have  been separated from each other by thousands of miles and  hundreds of thousands of years. In 2017, a Canadian paleobiologist  did statistical tests of fossils from nine different dinosaur species,  including T. rex, allosaurus, and protoceratops, and his model found  no evidence of sexual dimorphism.

There was too much overlap in the sizes, and there weren’t two clear  groups within the species that we could argue were males and females. A 2021 analysis found that some  dinos, including allosaurus, did appear to be dimorphic, but  the results weren’t a slam dunk. In both cases, the analyses were held back by the limited number of fossils.

That said, there have been some finds where we have managed to find enough fossils from one species to make a judgment. We’ve found hundreds of fossils of an early Cretaceous bird called Confuciusornis. It looks like males had two long, pendant-like tail feathers, while females didn’t.

And that lines up with what we see in a lot of modern-day dinosaurs, which  you also know as birds. In 2017, scientists working  with tiny fossil crustaceans were able to analyze over 6,000 individual fossils and they did indeed find some  dimorphism in the population. But there’s one final trick  up our sleeve for figuring out what sex certain fossils,  including dinosaurs, were.

Remember when I said to put a  pin in the egg-laying question? It turns out, one of the most  promising pieces of evidence for figuring out the sex of a fossil  dinosaur may be medullary bone. Medullary bone is a layer of calcium-rich tissue on the inside of birds’ leg bones.

It’s found today in female birds as a way to store calcium needed during egg-laying. So we can look at their skeletons of any egg-laying fossil species to see  if we find that medullary bone. And if we do, we can tell  that individual was a female.

In 2005, researchers found medullary  bone in one T. rex skeleton, and since then it’s also been  reported in other dinosaurs like sauropods, as well as  other critters like pterosaurs. And the good news is we don’t need to break open those leg bones to find it, since we can use techniques  like CT scans to peek inside. Medullary bone only appears in mature, reproductive females, which means that any fossil that doesn’t have it can’t  automatically be considered male.

But it’s still one of the  most sure-fire ways we have to know definitively what  sex an individual fossil was. That said, the sex of many fossils  may continue to be a mystery. We may never know for sure if Sue  the T Rex was a male or a female.

But that doesn’t make their  room at the Field Museum any less crowded with  visitors eager to take a look. Thanks for watching this episode of SciShow. If you liked it, you may want  to go check out PBS Eons, another show made by our  parent company, Complexly.

PBS Eons tells the stories of the past and how we learn about ancient life  from the fossils they leave behind. We really loved their recent episode on past mass extinctions,  and we think you will too. So click the link wherever it  is right now, and check ‘em out! [♪ OUTRO]