When it comes to powered animal flight, bats have always seemed to come second to birds. But scientists who flew a plane to track the flight of Brazilian free-tailed bats have clocked these winged mammals flying at speeds rivaling those of the fastest birds.
The findings, described in the journal Royal Society Open Science, reveal that bats might actually be using some of the same aerodynamic techniques as fast birds, and could shed new light on the dynamics of flight.
Birds are fast, efficient and agile, with bodies sculpted over millions of years of evolution to move through the air. Bats, on the other hand, have been a slightly more awkward branch on the flying family tree — a mammal known more for its echolocation skills, whose bony, skin-covered wings seem to make it move erratically, but not quickly.
But this isn't an entirely fair characterization. There's been plenty of research on different aspects of bird flight, but not a whole lot on bats. For example, particle image velocimetry — a common method used to study flying and swimming animals — has been used to study flight in only seven of the more than 1,300 known species of bat.
"First of all, very little is known about what these animals do when aloft. When they go up above a few tens of meters above the ground," said study lead author Gary McCracken, a behavioral ecologist at the University of Tennessee in Knoxville. "Almost all research on the foraging activities of bats, certainly the flight activities of tree flying bats, has been very, very spotty because echolocation is our main window into their behaviors and ultrasound doesn't carry very far."
And keep in mind, not all birds are designed equally. Some have long, thin wings, while others have slightly stubbier ones. Some birds fly with incredible efficiency, crossing the globe during seasonal migrations, while others are masters of speed or of control, able to execute complex maneuvers in the air.
Not all bird species are the same, and neither are all bat species. Moreover, those bats that occupy the same ecological niches as certain birds actually share certain physical characteristics with their plumed peers, McCracken said.
So, to try to see how fast these animals could really fly, McCracken and his colleagues turned to Brazilian free-tailed bats, known formally as Tadarida brasiliensis. Unlike many species that swoop and dive within a forest canopy, Brazilian free-tailed bats actually fly as high as 2 miles above the ground. For seven nights, one of the scientists stood outside the Frio bat cave in Texas and used a hand-net to nab one of the bats as they emerged for their nightly insect hunts. They'd mount a tiny 0.45-gram radio transmitter on the animal's back with surgical glue and then release it. Another team member, flying a Cessna 172, would use track that bat's signal, tracking its speed for several hours, usually until it returned to its cave (or until the plane ran low on gas).
Swifts and swallows are thought to be some of the fastest birds: The common swift can hit speeds of 31.1 meters per second (about 70 miles per hour). But the scientists found that the bats could, at certain points in flight, actually reach speeds of 44.7 meters per second (about 100 miles per hour).
"We did not expect to see these results ... this was sort of a holy cow moment for us," McCracken said.
The researchers also found that the bats seemed to be speeding up when facing headwinds and slowing down when they had tailwinds — a sign that they may be doing something similar to birds that fly harder against the wind and ease up when taking advantage of a helpful wind. They also seemed to use flap-gliding — a technique that alternates powered flapping and gliding flight that is often used by birds.
Much more research on these animals and other species will need to be done, said McCracken, who suspected that this species may not even be the fastest bat out there.
Engineers have been increasingly drawing inspiration from birds when building flying drones — but perhaps they have a thing or two to learn from bats, too.
"There's a real practical and certainly basic science questions interest in how animals achieve this kind of performance," McCracken said. Their ability "informs humans on machine design."
"Evolution is more clever than engineering models may be and we can learn a lot from them," he added.
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