Sheep sized dinosaur likely developed neck frill as a result of sexual selection

Sexual selection is the idea that certain traits in animals are favoured by members of the opposite sex, and so in time can become more elaborate. Today we famously see this in the extraordinary feathers of the birds of paradise and the elaborate enlarged mandibles of the stag beetle.

About the size of a large sheep, protoceratops is a fairly common and well-studied species of ceratopsian dinosaur that would have lived in what is now Mongolia's Gobi Desert. The species presents a great opportunity to study the unusual skulls of ceratopsians because there are so many of them held in collections giving a better understanding of how the morphology varies within a population.

Dr Andrew Knapp, a postdoctoral researcher at the Natural History Museum, has been applying modern technology and analysis to the skulls of protoceratops. He wanted to see if sexual selection could explain the evolution of its frills.

Andrew explains, 'In many fossil animals we have unusual structures and traits which aren't really seen in living animals today. Protoceratops didn't have any horns but they still had a huge frill.’

‘Several theories have previously been suggested for the emergence of these neck frills. Some have suggested that they were used for protection, others that they played a role in cooling the large herbivores down, or that they allowed individuals to recognise different members of their own species.’

Sexual selection can help explain the existence of features that may not help the animal survive but instead are favoured by the opposite sex and so persists often leading to difference in the appearance of males and females of the same species, this is known as sexual dimorphism.

'People have come to realise that in actual fact, sexual selection is quite often rather complicated.' Andrew adds.

'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.'

‘In a species of bird known as the crested auklet, both males and females have an impressive plume of feathers curling from the tops of their heads. This is because the plume is used by each sex to signal their health. So while the plume of feathers is a sexually selected trait, it has not lead to sexual dimorphism.’

'It looks like things are a bit more complex than the bigger the male trait the more successful it is.'

This sentiment is backed up by what he found looking by at Protoceratops fossils.

Andrew and his collegues 3D scanned 30 complete skulls of Protoceratops, which makes this the largest complete set of 3D data for any one dinosaur. These ranged in size from tiny day-old hatchlings to fully grown adults, giving him a full growth series of the species.

This allowed them to compare not only how different regions of the skull vary to each other, but also how the skull changed in shape as the animal grew. From this, it was possible to see if there were any indications that the shapes of the frills were a result of sexual selection.

In living creatures, certain features show a distinct pattern of growth, known as allometry. When the allometry is positive, in that they show a much greater rate of change in growth when compared to other traits on the animals, it is almost always a sexually selected feature, such as the antlers of a deer.

When looking at the frills of Protoceratops, they found that the growth patterns matched this, indicating that the frills were very possibly sexually selected. They were also able to show that the frills were able to evolve and change independently, which is also indicative of this.

What they were unable to show, however, was that there was any sexual dimorphism among Protoceratops.

'That could be because there was none, or that the differences were very minimal,' explains Andrew.

'There almost certainly were differences between males and females but quite often differences are in body size, so males will be bigger than females or vice versa. It could also have been through something else like colouration, which doesn't preserve in fossils.'

The team agreed that frills of dinosaurs like ceratopsians were likely down to sexual selection. As, however, there is no way to definitively prove that the individuals of extinct species with particular traits were more successful and had more offspring. For this reason they concluded that the frills were the result of the more general socio-sexual selection, a term that includes other social behaviour associated with such intricate ornaments, such as defending food or territory.

The study Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill is published in the journal Proceedings B.

Notes to editors

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