Using high-speed video equipment that has become affordable for scientists in the past five years, Breuer and Swartz created the movies as a research tool to investigate the flight mechanics of bats. They coax the bats to fly inside wind tunnels using food rewards.
Lesser dog-faced fruit bat.
The videos reveal the intricacies of how bats use their wings.
“One thing we found is that the movements of the wing are extremely complex compared to birds and insects,” said Swartz. “They have many, many joints they can control independently.”
Their complex wings enable bats to make sharp turns that birds can’t manage.
“They don’t necessarily turn fast,” said Swartz, “but they can turn on a dime.”
Bat makes a sharp turn.
Bat wings are also made out of soft skin – which Swartz calls “compliant.” This gives bats yet another advantage over birds and insects.
“If your wing is made of compliant skin,” she said, “then it billows and that gives you extra lift.”
Not all bat wings are the same, though. Mexican free-tailed bats, which are abundant in California’s Central Valley, have stiffer wings than species like the lesser dog-faced fruit bat found in Southeast Asia.
“As we’ve studied the Mexican free-tailed bats, it turns out that their wing motions look very much like birds’,” said Swartz. “The wing stays pretty rigid, even though they have the same joints than the fruit bats.”
Mexican free-tailed bat. Credit: Nick Hristov and Tatjana Hubel, Brown University
Swartz believes the differences in the wing capabilities of Mexican free-tailed bats and lesser dog-faced fruit bats are related to the differences in their diets. The Mexican free-tailed bats eat mainly insects, while the lesser dog-faced fruit bats eat, well, mainly fruit. Fruit-eating bats need to move through their habitat slower than insect-eating bats, Swartz said. And because they’re moving more slowly, it’s harder for them to maintain lift.
“It might be that the animals that need to fly more slowly need the three-dimensional complexity to make up for that,” she said.