Whether people are building a flying machine or nature is evolving one, there is pressure to optimize efficiency. A new analysis by biologists, physicists and engineers at Brown University reveals the subtle but important degree to which that pressure has literally shaped the flapping wings of bats. The findings not only help explain why bats and some birds tuck in their wings on the upstroke, but could also help inform human designers of small flapping vehicles. The team’s research is funded by the US Air Force Office of Sponsored Research. “If you have a vehicle that has heavy wings, it would become energetically beneficial to fold the wings on the upstroke,” said Sharon Swartz, professor of ecology and evolutionary biology at Brown. She and Kenneth Breuer, professor of engineering, are senior authors on the paper.