From the beautifully-crested Dilophosaurus to the lavishly-ornamented Pentaceratops to the flamboyantly-frilled Protoceratops, paleontologists have long debated the purpose of the elaborate, and often bizarre, ornaments sported by many prehistoric dinosaurs.
Were they used as weapons, temperature regulators, warning signals?
In fact, these attention-grabbing horns, crests, and frills were most likely used to woo mates and intimidate rivals in a peacock-like prehistoric mating show, according to a new study.
Scientists have long suspected decorative formations on dinosaurs served as sexual signals, but a new study published in the journal Palaeontologia Electronica marks the first time they have been able to link them to sexual selection.
"It's the first time we've been able to show that ornamental structures in dinosaurs may have been used to attract mates or assert social dominance, with a numerical study in any dinosaur that includes multiple different age groups," David Hone, a lecturer in zoology from Queen Mary University of London, and a lead author in the study, tells the Christian Science Monitor. "We've long suspected these kinds of ornaments showed these patterns but showing it convincingly has been very hard."
First proposed by Charles Darwin, sexual selection is a mode of natural selection in which traits, often ornamental ones, evolve as a result of members of one sex choosing their mates. It explains the emergence of extreme features, such as the peacock's plumage and the lion's mane, that are not otherwise clearly tied to an organism's survival.
For this study, scientists examined fossils and photographs of Protoceratopsspecimens, a small, horned dinosaur that lived in what is now Mongolia during the Cretaceous Period. About the size of a sheep, this prehistoric creature sported a large, bony frill that extended from the back of its head over its neck.
In past, scientists have proposed a series of explanations for the elaborate frills, bumps, and crests sported by many prehistoric animals.
The frills served as built-in thermostats, heating or cooling a creature as needed, they proposed. Or perhaps the bony horns, spikes, and plates served as defenses against aggressive predators. Were colorful crests merely a dino ID badge, identifying creatures as members of the same species, or did they act as warning signals to other dinosaurs?
While some of these uses may still be possibilities, in most situations they can be ruled out, leaving sexual signaling as the most plausible explanation, according to the study.
Researchers favor the sexual signaling hypothesis over other explanations for a number of reasons.
Because they disappeared from Earth some 66 million years ago, dinosaur behavior is difficult to reconstruct. But scientists often look to modern animals for analogies.
The ornamental features prehistoric dinosaurs flaunted are similar to those of modern animals – such as chameleons, hornbills, rhinos, and cassowaries – that use their special structures in sexual selection.
What's more, the growth rate of some of these ornamental features suggest they played a role in attracting mates.
After studying 37 Protoceratops specimens ranging from babies to adults, including actual fossils found in the Djadochta Formation in the Gobi desert, researchers noticed that the young quadrupeds were lacking the distinctive frill, which suddenly grew as the animal reached maturity, suggesting it was used for sexual attraction and selection.
"Palaeontologists have long suspected that many of the strange features we see in dinosaurs were linked to sexual display and social dominance but this is very hard to show," Dr. Hone said in a statement. "The growth pattern we see in Protoceratops matches that seen for signalling structures in numerous different living species and forms a coherent pattern from very young animals right through to large adults."
While the new study examined only Protoceratops, Hone says the conclusion can be extended to other animals. "I think we can make a decent case for quite a few dinosaurs and indeed a fair few other extinct animals, and indeed this has been much discussed in the past – this isn't a new idea – the difference here is we can confidently demonstrate it well thanks to the excellent set of data available.
"The findings may help scientists better understand the role sexual selection plays in animal diversity and evolution, says Rob Knell, an evolutionary ecologist from Queen Mary University of London.
"Biologists are increasingly realizing that sexual selection is a massively important force in shaping biodiversity both now and in the past," Dr. Knell said in a statement. "Not only does sexual selection account for most of the stranger, prettier and more impressive features that we see in the animal kingdom, it also seems to play a part in determining how new species arise, and there is increasing evidence that it also has effects on extinction rates and on the ways by which animals are able to adapt to changing environments."