It helps to be looking your best when an opportunity to get lucky arises, and it seems dressing up also improved the mating potential of a certain ceratopsian dinosaur that sported a dashing neck frill in the Upper Cretaceous. A study published in the journal Proceedings of the Royal Society B used modern kit to investigate the anatomy of Protoceratops and found that its neck frill likely developed as the result of sexual selection.
Sexual selection is seen across many species alive today. It’s most enjoyably seen in the absurd mating rituals of New Guinea’s birds of paradise, as competing males have developed increasingly bizarre routines as a means of outcompeting their rivals. It can go so far as to make an animal less well adapted to survive (ever seen a peacock flying?) but the hindrance only further proves their strength and favorable genes as evidenced by the fact they’ve not yet been eaten. It’s not just about feathers, too, as seen in Attenborough’s fan throated lizard (yes, named after that Attenborough).
Protoceratops was roughly the size of a large sheep when it roamed what’s now known as Mongolia’s Gobi Desert. They are well documented in scientific collections, making them a favorable dinosaur group to study with so much reference material to work from. The study was actually able to use the largest complete set of 3D data for any one dinosaur ever collated to assess how the ceratopsians put their fancy frills to use.
Dr Andrew Knapp, a postdoctoral researcher at the Natural History Museum, London wanted to use modern technology and analysis to see if sexual selection could explain the evolution of neck frills among Protoceratops. “In many fossil animals we have unusual structures and traits which aren't really seen in living animals today,” he told the Museum. “Protoceratops didn't have any horns but they still had a huge frill."
There are several theories on the emergence of neck frills in ceratopsian dinosaurs, which also have characteristic "beak" bones. Some argue the frills were used for defense, while others suggest they were for identification. Some have even suggested they may have helped regulate the temperatures of large herbivores.
Three-dimensional scans of 30 complete skulls of Protoceratops allowed Knapp and colleagues to compare how different regions of the skull varied specimen to specimen. Those scanned ranged from day-old hatchlings to grown adults, meaning they could also observe how the skull changed in shape as the animal grew.
When looking at the scans of Protoceratops, they found that its neck frill growth followed a pattern known as allometry, which looks at how certain features of living things change in relation to body size. When allometry is positive, it shows a trait has a much greater rate of growth than other characteristics in the animal, which usually indicates it's a sexually selected feature, like antlers or horns.
The Protoceratops skulls indicated this positive allometry, suggesting the frill is most likely sexually selected. However, the remains revealed little evidence of sexual dimorphism between male and female skull size and frill morphology. The researchers note that sex-dependent differences may have been lost in tissue or feathers that weren't preserved as part of the fossil.
“While there are quite a few examples in living animals where usually females select males based on the size of their tail feathers or calls, it is quite often overlooked that males do the same thing with females as well,” said Knapp. “Sexual selection is a bit more complex than the bigger the male trait the more successful it is.”
It's important to note though, that while the fancy neck frills of these dinosaurs were likely down to sexual selection, it's also possible they had other functions that include social behaviors, which are difficult to identify from the skull of an extinct animal. So, Knapp and colleagues suggest the frills were the result of socio-sexual selection.
"In reality, many 'sexually selected' traits perform additional functions such as establishing dominance in a population, which can improve an individual's access to resources like food, water, and territory," Knapp said, speaking to the Natural History Museum. "So really, the boundaries between sexual and social selection are quite blurred, and social selection will quite often be an important factor too."