Why It’s So Damn Hard To Tell The Sex Of A Dinosaur

Mama-and-papasaurus

Even some apparent dead giveaways – such as dinosaur skeletons discovered on top of a clutch of fossilized eggs – are unlikely to be reliable indicators of sex. After all, in some modern birds like emus or cassowaries – which are essentially living dinosaurs – it’s the dads who look after the young, and it’s now thought that the enormous Spinosaurus may have been among the dinosaur species to share this pattern.

If your sample size is one, then who the hell can tell what the difference is between male and female?

Dr David Hone

A more robust signal may be the presence of what’s called a medullary bone – but only sometimes. Like female birds, dino mamas developed this temporary structure to act as a calcium store for making eggshells, before the whole thing vanished once the eggs were laid.

“An animal showing this kind of bone is probably about to or in the process of laying eggs, so that tells you with reasonable confidence that you’ve got a female,” says palaeontologist Dr David Hone from Queen Mary University of London. “The problem is, not having it doesn’t tell you that it’s a male. It can be a female that’s too young to breed, or too old to breed, or too ill to breed, or maybe she laid the eggs weeks ago and now is sitting on the eggs,” he tells IFLScience. “So it’s great when you find it, but when you don’t, it’s not very informative.”

What about dinosaur genitals?

A few years ago, researchers discovered the first ever preserved dinosaur cloaca while examining a fossilized Psittacosaurus from China. Like those possessed by some species of bird, reptile, and amphibian, dinosaur cloacas were multipurpose vents that were used for both excretion and mating, depending on the owner’s urges.

Yet as Hone points out, “Having the external genitals of a Psittacosaurus doesn’t tell you if it’s male or female because you can’t see the internal anatomy,” which is much less likely to preserve because it’s “softer and gooier stuff.” Unfortunately, it’s this “internal gubbins” that we need in order to confirm which variety of sexual hardware a dinosaur possessed, as cloacas were present on both males and females, with the actual genitals housed inside. 

Did males and females differ?

With no dino junk for scientists to look at, our best chance of learning to distinguish between males and females is to look for sexual dimorphism, whereby certain physical characteristics are associated with each sex. For instance, in many animals – including humans – males tend to be larger than females on average, and it’s possible that this may have been the case for some dinosaurs too.

However, finding evidence for this is unlikely to be easy. To illustrate these difficulties, Hone and his colleagues looked at sexual dimorphism in gharials, a crocodilian species that may be a good proxy for dinosaurs, and which are easy to tell apart because the males have a fleshy blob called a ghara on the end of their nose, while females don’t.

However, when trying to distinguish between male and female gharials using just their skeletons, the study authors found that the two sexes could not be reliably identified – despite the fact that males are generally larger. In the absence of soft tissue, then, identifying male and female dinosaurs from their bones is clearly going to be highly problematic – especially when one considers that sample sizes for dinosaurs are much smaller than they are for living species like gharials.

There is no dinosaur with some kind of really convincing sexual display or combat gear like armored heads, big frills, splays, spines, plates, or whatever it might be.

Dr David Hone

“Most dinosaurs – and by most, I mean like 70 to 80 percent – are known from one skeleton or one specimen, which may be very incomplete,” says Hone. “So if your sample size is one, then who the hell can tell what the difference is between male and female?”

To make matters worse, Hone explains that dinosaurs are unique in that “they took a very, very, very long time to grow,” which means they didn’t reach full size until fairly late in their lives. “So if you’re then sampling dinosaurs, only a few of them are actually as big as they could be,” all of which makes it impossible to separate males and females based on size alone.

In other words, full-size males might be larger than full-size females, “but an old female is probably still bigger than a young male.”

Obviously, then, body size isn’t very useful – at least, not until we have much, much larger sample sizes. Yet, just like gharials and their funky noses, dinosaurs might have displayed other forms of sexual dimorphism whereby males were adorned with certain unique features with which to allure females or fight with rivals.

According to Hone, however, “there is no dinosaur with some kind of really convincing sexual display or combat gear like armored heads, big frills, splays, spines, plates, or whatever it might be.” Except, possibly, for one small dinosaur known as Khaan mckennai, which Hone says “looked a bit like a weird ostrich”.

About a decade ago, researchers found a pair of these oviraptors that died side-by-side, and which were identical apart from a subtle difference in the tail bones. One of the duo, it seemed, was adapted to fanning its tail up and down, probably to flash its peacock-like tail feathers in a kind of sexual display.

Commenting on this isolated discovery, Hone says, “I try to be very conservative, and you don’t want to over interpret, but that’s really quite a convincing case of [sexual] dimorphism.”

So far, researchers are yet to uncover any other Khaan mating pairs, nor have they found analogous features in other dinosaur species. However, as this extinct flappy-tailed ostrich demonstrates, sexual dimorphism might have existed in dinosaurs, and finding more signals of it is not out of the question.