Bats fly in a completely different way from birds, according to new research that modelled the aerodynamics of their flight.

Important differences between the wings of birds and bats mean that they generate lift and power in quite distinct ways, scientists in the United States have discovered.

An analysis of bat flight conducted at the University of Southern California has revealed that their wings produce a characteristic aerodynamic footprint that challenges previous assumptions about the processes involved.

It shows that while birds generate thrust using only the downstroke of their wings, bats use the upstroke as well.

The differences have arisen because of the separate evolutionary paths by which birds and bats developed flight. The findings, which are published in the journal Science, offer fresh evidence for the way evolution can progress in parallel in different groups of organisms, which find different solutions to filling similar ecological niches.

In the study, a team led by Geoffrey Spedding, an aerospace engineer, looked at the wake left behind in flight by a small species of bat, Pallas’s long-tongued bat or Glossophaga soricina.

Though bats of this size — about 10g to 30g in weight with a wingspan of 25cm to 30cm — have wing structures that are superficially similar to those of birds of similar dimensions, their wings produce very different wakes.

“The tell-tale tracks in the airflow caused by the wing beat have a very different pattern for bats, and this difference can be traced to the peculiar upstroke,” Dr Spedding said. “That, in turn, is likely caused by the collapsible membrane of the bat’s wing, which needs to maintain some degree of tension.

“Instead of feathers projecting back from lightweight, fused arm and hand bones, bats have flexible, elastic membranes that stretch between specially extended, slender bones of the hand. The bones and wing membrane both change shape with every wing beat, flexing in response to the balance between forces applied by the muscles and competing forces due to the air motion around them.”

One of the chief differences arises from the way birds can use their feathers to make the upward stroke of their wings aerodynamically neutral, so they do not produce thrust that would counteract the forward movement of the downstrokes.

Bats do not have feathers, and have had to develop a different upstroke accordingly, which generates positive thrust. “Where birds can feather their wings, opening the feathers like a Venetian blind, bats must do something different,” Dr Spedding said. “Hence, they have developed a twisting wing path that increases the lift during the upstroke.”

The research team now intends to model bat flight more precisely. This could provide insights that could be useful for designing artifical bat wings for aircraft, to improve their aerodynamic peformance.

“Bats are agile hunters, capable of plotting and executing complex maneuvers through cluttered environments,” Dr Spedding said. “These are the traits we’d like our unmanned air vehicles to have because there are so many complex rural and urban environments in which we could use them.

“Bats have relied on very flexible wings for 50 million years to propel and lift themselves into the sky. We still have a lot more to learn about the aerodynamics of bat flight and how their wings allow them to maneuver through incredibly unsteady air flows and turbulent conditions.”

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