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How did birds get their feathers? Complex feathers might have originally evolved because birds鈥� dinosaur ancestors found them sexually attractive.

Many dinosaurs had simple feathers on at least some parts of their body. Palaeontologists think they know why they first evolved. 鈥淓arly dinosaur feathers really are very hair-like,鈥� says Scott Persons at the Mace Brown Museum of Natural History in South Carolina. Simple feathers were almost certainly for insulation, like mammal fur, he says.

But that makes it difficult to understand why some dinosaurs then developed feathers that were more structurally complex, and that eventually became specialised for flight in birds. These complex feathers are stiffer and don鈥檛 provide much insulation, says Persons. So why did they evolve?

Persons and his colleague Philip Currie at the University of Alberta, Canada, think it wasn鈥檛 natural selection but that drove feather evolution. Complex feathers might have been poor insulators, but if they helped dinosaurs secure more mating opportunities then there would have been a good reason to evolve and retain them.

Biologists already know that sexual selection can shape feathers in dramatic ways, says Persons. Perhaps most famously, many biologists think male peacocks evolved elaborate tail feathers largely to impress peahens.

But Persons and Currie say we have never considered the possibility that sexual selection could act as a bridge that helps link two episodes of natural selection. Under their scenario, dinosaurs like tyrannosaurs evolved simple feathers for insulation. Then, sexual selection encouraged some dinosaurs, like oviraptors, to evolve more complex feathers purely for display. And finally, those feathers reached a level of complexity sufficient for them to help some small dinosaurs to glide. At this point, natural selection kicked in again and shaped feathers for flight in birds.

Persons thinks we might eventually find other situations in which sexual selection serves as a bridge between episodes of natural selection. 鈥淭he implications may go beyond this example of dinosaurs and feathers,鈥� he says. 鈥淚t鈥檚 possible this process has had a similarly impactful role in the evolution of many other organisms for which we don鈥檛 have as clear a fossil record.鈥�

The idea makes sense, says Steve Brusatte at the University of Edinburgh, UK. 鈥淭he more feathered dinosaur fossils that are found, the more it becomes clear that feathers and even wings did not evolve for flight,鈥� he says 鈥淚f I had to cast my bets, I reckon that wings evolved as sexually selected display structures and then were later repurposed into airfoils.鈥�

But to really nail down the concept, says Brusatte, we need to collect more feathered dinosaur fossils. If elaborate feathers were unique to one gender, say, as they are today in peacocks and other ornate birds, it would provide strong evidence that the evolution of elaborate feathers really was shaped by sexual selection.