When Archaeopteryx first took flight at the end of the Jurassic, dinosaurs had already been clad in feathers for millions of years. Even later throughout the Cretaceous, theropod dinosaurs like T. rex and velociraptor, who never had the ability to fly, were covered in feathers. If these dinosaurs weren’t using their feathers for flight, why did they exist? A new study suggests that they played on the dinosaurs’ excellent color vision for communication and sexual selection purposes. The paper was published in Science, with Marie-Claire Koschowitz of the University of Bonn in Germany serving as the lead author.
Though a relationship between dinosaurs and modern birds was first suspected 150 years ago during Charles Darwin’s heyday, evidence of the existence of feathered dinosaurs did not appear until the late 1990s. Subsequent studies found that dozens of non-avian dinosaurs were sporting plumage. It has not been clear as to why feathers were so widespread among these creatures, which has led to a series of variable hypotheses.
"Up until now, the evolution of feathers was mainly considered to be an adaptation related to flight or to warm-bloodedness, seasoned with a few speculations about display capabilities" Koschowitz said in a press release. "I was never really convinced by any of these theories. There has to be some particularly important feature attached to feathers that makes them so unique and caused them to spread so rapidly amongst the ancestors of the birds we know today.”
Koschowitz’s team wondered if the color of the feathers played a role in their prevalence among dinosaurs. Without dinosaur DNA to determine what color receptors they might have had, the team had to study the evolutionary relationship between crocodiles and birds, which are the dinosaurs’ closest living relatives. They were able to determine that in addition to the three color receptors that humans have, dinosaurs had an additional receptor that allowed them to see some UV light.
"Based on the phylogenetic relationships and the presence of tetrachromacy in recent tetrapods, it is most likely that the stem species of all terrestrial vertebrates had photo receptors to detect blue, green, red and UV," added co-author Christian Fischer from the University of Göttingen.
Birds and reptiles are well-known for using their colors for communication and mate-selection purposes, so it is likely that dinosaurs used a similar system. However, the small, pre-avian dinosaurs also needed a means of retaining body heat. The short, hair-like feathers that provided insulation were not very colorful, robbing the animal of that important communication ability. Large, flat feathers covered the shorter hair needed for warmth and were also great at refracting light, making them more visible to others.
"Feathers enable a much more noticeable optical signaling than fur would allow. Iridescent birds of paradise and hummingbirds are just two among a wealth of examples," elaborated Koschowitz.
Early mammals were nocturnal, so color signaling was not important. This allowed for the growth of hair, which isn’t as flashy, but provides excellent insulation. Dinosaurs evolving two kinds of feathers would have given them the best of both worlds: warmth and the ability to communicate with light and color.
[Header image credit Ruth Hartnup via flickr]