Bats' wing strokes 'defy normal rules of flight'
By Steve Connor, Science Editor
Published: 11 May 2007
Bats use wings to fly very differently to birds, according to a study that has revealed the nocturnal mammals engage in sophisticated aerial acrobatics.

One of the first detailed investigations into the aerodynamics of bat flight shows that they can hover in mid-air by flipping their wings upside down to generate lift on the upstroke as well as the downstroke.

Scientists said that bats often seemed to defy conventional aerodynamic theory because of their ability to twist and turn quickly in mid-flight to avoid predators or catch prey.

"They are a vertebrate version of the bumble bee, which is said to defy scientific logic by being too heavy to fly with such short wings. It's not been known before that bats can generate such high lift forces with their wings," said Anders Hedenström of Lund University in Sweden.

Using a wind tunnel and a fog machine, Dr Hedenström and his colleagues took high-speed video images of a nectar-eating species of bat to study the vortices of air produced by its beating wings. "One of the unusual things we saw was that when the bat flew slowly or hovered it rotated its wings by nearly 180 degrees at the turn of the upstroke and the downstroke. The underside of the wing became the upperside," he said.

A bat's wings are made of flexible membranes stretched over the extended bones of its limbs. This design requires it to keep its wings rigid to produce lift. A bird's wing, however, is naturally more rigid and employs the extra trick of being able to separate its flight feathers on the upstroke so that air can pass through the wing, rather like light passing through an open venetian blind, said Geoffrey Spedding, an aerospace engineer at the University of Southern California, who was part of the research team.

"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," Dr Spedding said. "The bones and wing membranes 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," he said. "Where birds can feather their wings, opening the feathers like a Venetian blind, bats must do something different. Hence they have developed a twisting wing path that increases the lift during the upstroke," he added.

The study, published in the journal Science, found that when bats fly slowly they use the rotating-wing "and this difference can be traced to the peculiar upstroke", Dr Spedding said. "That, in turn, is likely to be caused by the collapsible membrane of the bat's wing."

Bats, a protected species, are the only mammal to have evolved self-powered flight and most species are adapted to eating flying insects, consuming up to their own body weight each night. They employ a sophisticated echo-location system to catch prey as well as avoid collisions