Pterosaurs Flew Long, Slow

The ancient reptiles of the skies were champion loiterers, circling lazily above the tropics more economically than modern raptors do.

THE GIST

Pterosaurs were less efficient than today's birds, but compensated with slow takeoffs and landings.

Models of pterosaur wings tested in a wind tunnel revealed their likely ability to vary the curvature of their wings.

Slow flight would have helped them search more effectively for prey.

How were pterosaurs able to cruise the skies of the ancient world between 220-65 million years ago? Prehistoric flight palaeontologists have moved a step closer to understanding just that.

The new research, published today in the Proceedings of the Royal Society B, has found that pterosaurs did best in gentle tropical breezes rather than the stormy winds typical of today's southern oceans.

Study author Colin Palmer, an engineering and paleontology expert from the University of Bristol, made models of pterosaur wings using resin, carbon fiber and latex rubber and tested them in a wind tunnel.

Palmer was inspired to do the tests after discovering that previous research on pterosaur flight was only based on pre-1950s data on aircraft wing behavior. He says that research had overestimated the efficiency -- that is, the ratio of lift to drag -- of pterosaurs in flight.

His research by comparison also factored in the wing bones, which produce additional drag.

Palmer found that while pterosaurs -- vast flying reptiles weighing up to 200 kilograms -- are less efficient flyers than many birds today, they made up for this by being able to fly and land very slowly. This is quite a different flying style to the albatrosses they are often compared with.

"I suspect that pterosaurs were able to vary the camber (curvature) of their wing sections. An increase in the camber allows the wing to generate more lift," he says. This would have enabled them to stay airborne in even gentle breezes.

"It's the same effect as the flaps you see coming down on airliner wings as they come in to land. The drag acts as a brake and reduces speed in a controlled manner," he says.

Pterosaur expert Assistant Professor Mike Habib from Chatham University, Pennsylvania, says being such slow flyers had other advantages.

"Flying slowly ... would be an impressive addition to their behavioral arsenal," he says.

"Slow flight is good for searching an area thoroughly for prey. Staying in one area on the wing is called loitering, and modern vultures and eagles are champion loitering flyers. If Palmer is correct, then pterosaurs may have been even better at slow, searching soaring flight than a living eagle."

Such methods of flight means they could not have lived in regions with strong winds such as present day northern Europe, says Palmer.

"Instead they needed conditions more like today's tropics, with gentle breezes and regular sunshine to generate thermal lift. Such climatic conditions were widespread in the period when they lived and many fossils have been found in locations that experienced these conditions."

Habib does not believe pterosaurs were as sensitive to sudden gusts as Palmer has calculated. "The sheer inertia of such a large species could give them reasonable penetration in high winds," he says.

If we want to imagine how the pterosaur flew, we can look to the modern day frigatebird, says Palmer. This large black seabird lives on tropical Pacific and Atlantic islands and is best known for its vivid red chest, which it puffs out to attract a mate.

Palmer says frigatebirds also fly relatively slowly and make use of updrafts and thermal lifts over land and sea.

"Common seagulls also use updrafts ... but frigatebirds are true masters of this craft," he said.