WASHINGTON Bats and birds, the only two vertebrate fliers on Earth, use their wings very differently, according to scientists who observed small, nectar-feeding bats flying through fog in a wind tunnel.
Writing on Thursday in the journal Science, the researchers described aerodynamic differences between bats and birds. They both fly by flapping their wings, but use the upstroke of the flap in different ways, with bats flicking their wings upward and backward unlike birds to gain lift.
"Bats seem to be mostly specialized for agile and maneuverable flight in complex environments," Geoffrey Spedding, a University of Southern California professor of aerospace and mechanical engineering and one of the study's authors, said by e-mail.
"In broad generalities, bats are characterized by a darting, sharply turning and maneuvering flight. This can be seen as they wheel about catching insects, or flit from flower to flower," Spedding added.
Pallas's long-tongued bat, which has the scientific name Glossophaga soricina, served as the test pilot for the research team from Sweden, Germany and the United States.
The bats flew in a wind tunnel filled with fog from a fog machine. The scientists tracked the movement of fog particles in the wake left by the flying bats to understand the aerodynamics of each wing beat.
Scientists had studied birds flying in similar wind tunnels previously.
Powered flight has evolved three times among vertebrates in the history of life on Earth.
The first to achieve the feat were flying reptiles known as pterosaurs. They first appeared roughly 220 million years ago but died along with the dinosaurs 65 million years ago when an asteroid slammed into Earth.
Scientists think birds descended from small, feathered dinosaurs roughly 150 million years ago. They survived the asteroid impact.
Bats, which generally are nocturnal, are the only mammals to have developed powered flight -- some like flying squirrels glide but do not fly. Bats arose about 50 million years ago.
The wing structure of bats and birds differs. Birds have feathers projecting back from lightweight, fused arm and hand bones. Bats have flexible, relatively short wings with membranes stretched between elongated fingers.
Spedding said while birds can open their feathers like a Venetian blind, bats have developed a twisting wing path that increases the lift during the upstroke.
There are about 1,000 species of bats, accounting for about 20 percent of mammal species. Most catch insects.
"It is certainly true that our comparative ignorance of bats is based on the rather arbitrary point that we do not see them because they come out at night. This ignorance sometimes comes out as misplaced fear, and so bats are the source of all kinds of rather peculiar myths and legends," Spedding said.