Palaeontologists have been trying to identify female and male dinosaurs for decades. Finding out the sex of a dinosaur can provide us with important information on how these prehistoric animals lived, and can help us gain further insights into their anatomy, biology, behaviour, and evolution.
However, spotting the differences between dinosaur sexes is very difficult and has long been a source of debate for scientists.
Let’s start by looking at modern-day dinosaurs – birds! The appearance of many birds varies depending on their observed sex. This includes differences in size, weight, colour and markings. This phenomenon is known as ‘sexual dimorphism’.
Let’s look at pheasants for example. Male pheasants are typically larger than female pheasants, and possess colourful, decorative plumage and longer tails. Female pheasants usually have a more subtle appearance with uniform sandy-coloured feathering.
Sexual dimorphism is actually a common phenomenon in most animals around today. However, when it comes to sexual dimorphism in dinosaurs, things get a little trickier.
Sexing a dinosaur isn’t easy
When new dinosaur bones are discovered, palaeontologists carefully examine and compare them with bones already in museum collections.
This allows them to determine any potential differences, and the reasons behind these variations.
However, significant differences may not necessarily be due to sexual dimorphism. They could instead mean that the new bones belong to a new species, or that they appear different because the dinosaur is of a different age to those previously found. In reality, it’s rather difficult for palaeontologists to pinpoint which one of these reasons is correct.
This is because for many dinosaur species we only have a limited number of incomplete skeletons. This means that we are unable to accurately compare them with other specimens.
In short, telling male and female dinosaurs apart is easier said than done! However, researchers have tried to sex dinosaur fossils through a variety of methods, including taking a more detailed look at their bones.
The medullary bone method
In 2005, molecular biologist Mary Schweitzer and her colleagues reported on a well-preserved, small thigh bone of a Tyrannosaurus rex. The bone was discovered from the Hell Creek Formation (approx. 70 million years old) in North America.
Examination of this bone revealed the presence of a special tissue inside the main cavity, known as the medullary bone. The medullary bone can be found in modern-day female birds. They use it as a reservoir for the calcium needed to produce hard eggshells.
This bone is unique to female birds. Scientists thought that if this tissue was found in dinosaur bones, it would suggest that a female dinosaur died as it was preparing to lay its eggs.
However, not all palaeontologists agreed with this. Some thought that the bone tissue could have been from a disease previously found in the jaws of pterosaurs, a group of flying reptiles that are the sister group to the dinosaurs. Therefore, it wasn’t seen as a reliable indicator that the dinosaur was pregnant or female.
In 2016 Schweitzer and her colleagues re-examined the structure and chemical composition of the tissue within the original T. rex thigh bone. The team concluded that the tissue was definitely medullary bone as it contained more mineral content than the surrounding tissue. They therefore concluded that this T. rex really was pregnant when she died.
Feathered females
Many dinosaurs were feathered, just like modern-day birds. Could it be possible that some dinosaur species used feathers and colour to differentiate themselves? A 2013 study highlighted a rare example of an extinct dinosaur, albeit a bird, that showed sexual dimorphism in its feathers.
A team of researchers examined hundreds of specimens of the crow-sized bird Confuciusornis sanctus. They were discovered from the Early Cretaceous (127-121 million years ago) of north-eastern China. These fossils were amazingly well-preserved, including the preservation of delicate soft tissue and feathers.
These fossils had significant differences in their size and feathers. Some had two long, central ornamental feathers extending from their tails, while others did not. The team noted that one of the fossils preserved without long ornamental tail feathers also possessed the medullary bone. As we learnt from the work of Mary Schweitzer and her colleagues, the medullary bone only forms when the female was either preparing to or laying eggs.
This discovery provided definitive evidence that individuals of Confuciusornis sanctus without long tail feathers were females. It was the first example of gender identification in a Mesozoic bird!
Unfortunately, in dinosaurs, techniques using the medullary bone can only be used to identify females who are pregnant and ready to lay eggs. Detecting male dinosaurs or non-reproducing females is much more difficult.
However, with advancements in ground-breaking scientific techniques and continued discoveries of well-preserved fossils being unearthed worldwide, it may one day be possible for palaeontologists to accurately determine the sex of a dinosaur.