Powered flight in nature has only evolved through four stages. The earliest stage, the pterosaur, was a flying reptile that is now extinct. Today, insects, birds and bats represent the remaining evolutionary stages. But as far as wing structure goes, bats have the upper hand. Compared to their fellow flyers, bats are the most energy-efficient flappers.
While birds flap with outstretched wings, the hand-like structure of bat wings allows bats to fold their wings during the upstroke. This strategy reduces the total energy expended during flight by 35 percent, according to a study published April 11 in the journal Proceedings of the Royal Society B.
But since each energy-saving wing includes an elbow, wrist, three digits and thumb, bat wings are relatively heavy, accounting for up to 30 percent of the bat's total weight. The bats' flapping strategy compensates for the heaviness of their wings, said Attila Bergou, postdoctoral research associate and co-lead author of the study. The other lead author, Daniel Riskin, completed his postdoctoral research at Brown in 2010 and is currently the co-host and producer of the television show "Daily Planet."
The study was funded by the U.S. Air Force. "There is growing interest in the energy cost of flight," said Professor of Engineering Kenneth Breuer, one of two senior authors on the study. Sharon Swartz, associate professor of ecology and evolutionary biology, is the other senior author.
Breuer and Swartz have been studying bats for over a decade and use a robotic bat to measure the force of bat flaps. They work with a range of experts, including biologists, engineers, physicists and computer scientists. Breuer described the interdisciplinary nature of his work as a "mixing of cultures that makes our life fun."
Understanding bat flight may hold the key to the future of small, unmanned flying vehicles.
"Here's how one organism solves the problem of heavy wings in flight in nature, and it seems to be a pretty good way of solving it," Bergou said. "It gives a hint at what might be a good idea to mimic."
It is much easier to mimic the elastic membrane of bat wings than the feathers of bird wings, said Geoff Spedding, professor of aerospace and mechanical engineering at the University of Southern California. But the efficiency of flapping wings decreases as scale increases, explaining why there are no airplane-sized bats in the sky.
The researchers focused on data collected eight years ago from six species of bats, including video footage of bats flapping in a wind tunnel. Though the data were not initially collected with the current study in mind, "that's how science goes," Breuer said.
The growing interest in the energy cost of flight inspired the team to use the footage to create a model of the flapping pattern. With this model, they determined the energy cost of folding wings and compared it to the cost of flapping with outstretched wings.
"Evolution has devised strategies to save energy, and this is one that is particularly appropriate for an animal like a bat," Breuer said.
Both Bergou and Breuer said working with bats is not an easy task. Each bat has a distinct personality, Bergou said. Some are very clever, grasping within hours what researchers want them to do. Others are stubborn, refusing to cooperate.
But the fickle nature of their test subjects will not deter the researchers from continuing to study bat flight. Bergou is currently studying bat maneuverability, while Breuer plans to study the interactions of these energy-efficient flyers.
"Nature has a lot to teach us," Bergou said.