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Some of the most fascinating animal behavior is really all about romance. That's especially true with birds. They croon or dance or display a brilliant fan of feathers to seduce a reluctant mate. The peacock is a good example. This sort of mating game display apparently has a very long pedigree, going back to the dinosaurs, as NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE, BYLINE: There used to be this kind of dinosaur that was built like a 400-pound ostrich. It lived about 75 million years ago and is called Ornithomimus. Scientists in Canada found the fossilized bones of one in 1995 that looked different from what they'd seen before. It had mysterious markings on the forearms. Two more were found recently with even more pronounced markings. When paleontologist Darla Zelenitsky took a close look at them, she was surprised.

DARLA ZELENITSKY: We're finding these wing-like structures in these relatively primitive dinosaurs.

JOYCE: The markings were the remains of primitive feathers. Now, by no means did this ostrich-on-steroids fly, but Zelenitsky says the feathers would have formed a sort of wing-like appendage on the animal's forearms.

So, why did these and other dinosaurs need feathers? This has been a point of debate among scientists, especially as more fossils from the dinosaur age are found with feathers attached. Did they need them to keep warm, to keep their eggs warm?

Well, Zelenitsky, who's a professor at the University of Calgary, noticed something unusual about these three specimens that suggested something else. The juvenile Ornithomimus had what would have been a thin, downy coat and stripy. The two adults sported the real thing: bigger, showy feathers with quills.

ZELENITSKY: So because we're only finding them in the adult individuals, this suggests that the wings were used for purposes later in life, like reproductive activities, for example, such as display or courtship.

JOYCE: Now, the idea that these dinosaurs used feathers for a kind of sexy fan dance is a scientifically educated guess. After all, these things went extinct tens of millions of years ago. But Richard Prum, an expert on birds present and past at Yale University, says recent discoveries do suggest that feathers were some kind of signal.

RICHARD PRUM: The idea is that these were for communication. And that's fascinating, because we recently have new evidence that the feathers of dinosaurs were pigmented, and perhaps pigmented very boldly. So that already implied that there was a communication function for early feathers.

JOYCE: In fact, Prum says the need for dinosaurs to look hot had some important consequences for today's birds.

PRUM: The evolution of attractiveness or beautiful traits may have had an important role in the origin and early diversification of feathers.

JOYCE: This latest discovery is described in the journal Science. Prum says he expects it will encourage scientists to look harder for signs of feathers.

PRUM: These are the kinds of things that people probably would have been overlooked years ago.

JOYCE: That's partly because traces of feathers are easily destroyed during excavation. And even if you're looking for them, they're easy to miss. But these are the first feathered dinosaurs found in North America. So clearly, they're buried here somewhere, waiting for another chance to show off.

Christopher Joyce, NPR News.

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