Study: Dinosaur Feathers Evolved for Sexual Display

New research, published recently in the journal Evolution, shows that some dinosaurs may have taken flight thanks to sex appeal.

Persons Currie explored how feathers went from dinosaur insulation to enabling flight. Image credit: Zhao Chuang / Martin Kundrát.

Persons Currie explored how feathers went from dinosaur insulation to enabling flight. Image credit: Zhao Chuang / Martin Kundrát.

Modern bird feathers are complicated: the feathers on a bird’s wing each have a central hollow shafted called a rachis, branching filaments called barbs, and even smaller filaments called barbules.

Fossil feathers show us that many dinosaurs were covered only in simple hair-like feathers. Those simple hairy feathers served as insulation and had absolutely nothing to do with flight.

But it has been unclear how simple feathers later developed into complicated feathers on wings.

“The first complex wing feathers show up in tiny raptor dinosaurs that could parachute and glide flying-squirrel-style through the prehistoric tree tops,” said study lead author Dr. Scott Persons, a researcher at the Mace Brown Museum of Natural History.

“We explored how dinosaurs went from staying warm with simple hairy feathers to gliding on complicated wing feathers.”

A feathered tail of a coelurosaur preserved in mid-Cretaceous amber from Kachin State, Myanmar. Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

A feathered tail of a coelurosaur preserved in mid-Cretaceous amber from Kachin State, Myanmar. Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

Dr. Persons and University of Alberta’s Professor Philip Currie found that larger, stiffer, flatter feathers gradually evolved as showy fans on the arms and tails of dinosaurs to be waved and waggled in courtship displays, leading eventually to the evolution of birds.

“The evolution of avian flight is one of the most important transitions in vertebrate evolution,” Dr. Persons said.

“There are now over 10,000 species of birds, and birds were the only group of dinosaurs to survive the mass extinction.”

“The extraordinary success of birds is tied to their ability to fly. But, to evolve into complex flight feathers, traditional natural selection needed a boost from sexual selection. Effectively, sexual selection led to an increased complexity, which bridged the gap from one function to another.”

“Recognizing that critical contribution teaches us something fundamental and important about the evolutionary process. The implications go way beyond the single example of dinosaurs and feather evolution.”

Photomicrographs of feathers: (A) pale ventral feather in transmitted light (arrow indicates rachis apex); (B) dark-field image of (A), highlighting structure and visible color; (C) dark dorsal feather in transmitted light, apex toward bottom of image; (D) base of ventral feather (arrow) with weakly developed rachis; (E) pigment distribution and microstructure of barbules in (C), with white lines pointing to pigmented regions of barbules; (F–H) barbule structure variation and pigmentation, among barbs, and ‘rachis’ with rachidial barbules (near arrows); images from apical, mid-feather, and basal positions respectively. Scale bars - 1 mm in (A), 0.5 mm in (B)–(E), and 0.25 mm in (F)-(H). Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

Photomicrographs of coelurosaur feathers: (A) pale ventral feather in transmitted light (arrow indicates rachis apex); (B) dark-field image of (A), highlighting structure and visible color; (C) dark dorsal feather in transmitted light, apex toward bottom of image; (D) base of ventral feather (arrow) with weakly developed rachis; (E) pigment distribution and microstructure of barbules in (C), with white lines pointing to pigmented regions of barbules; (F–H) barbule structure variation and pigmentation, among barbs, and ‘rachis’ with rachidial barbules (near arrows); images from apical, mid-feather, and basal positions respectively. Scale bars – 1 mm in (A), 0.5 mm in (B)–(E), and 0.25 mm in (F)-(H). Image credit: Lida Xing et al, doi: 10.1016/j.cub.2016.10.008.

While the study sheds new light on the evolutionary steps leading from dinosaurs to birds, there are still rich paleontological mysteries to explore concerning fossil feathers.

“We are still missing clear examples of sexually dimorphic feathers in dinosaurs. Today, it’s easy to tell the sexes of many birds apart based on their feathers,” Dr. Persons explained.

“Male birds tend to have larger, gaudier, and brighter feathers because they are the ones doing the displaying,” he said.

“This was very likely true of feathered dinosaurs, but we haven’t found a definitive example… yet.”

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W. Scott Persons Philip J. Currie. Feather evolution exemplifies sexually selected bridges across the adaptive landscape. Evolution, published online July 30, 2019; doi: 10.1111/evo.13795

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