Posted on behalf of Matt Kaplan.

You have to walk before you can run, and — if you’re a vertebrate — you have to glide before you can fly. Or so the theory goes. In evolutionary biology, gliding is widely thought to have been a prerequisite for powered flight, a transitional step along the way to full-fledged flapping.

Now, two researchers are challenging that notion with a theory that suggests that early bats used proto-wings to flutter to the ground in a controlled manner that helped them to better capture crawling insects.

Presenting today at the annual meeting of the Society of Vertebrate Paleontology, Kevin Padian at the University of California, Berkeley, and Kenneth Dial at the University of Montana, Missoula, described their idea, which arose from an investigation of the bat family tree. The researchers say that they knew there were no gliding species among bats or even closely related to them, but when they expanded the scope of their search, they found that gliding vertebrates and flying vertebrates were never closely related. The observation eventually led them to explore the possibility that gliding might have had nothing to do with the evolution of bat flight.

“With gliding being a habit that we rarely see in living bats and with the behaviour notably absent amongst their relatives, we really started speculating that something else had to have led to bat flight”, says Padian.

“It is a simple but amazing observation that there are no flying lineages of vertebrates with gliders as a sister group”, says evolutionary biologist Nancy Simmons at the American Museum of Natural History in New York.

Bats first appeared in the Eocene, roughly 50–55 million years ago. When the researchers looked at which modern bats are most like those ancient progenitors, they realized that they are climbing, cave-dwelling, insect feeders rather than tree-dwelling, pollen, blood or fruit eaters. But this presents a mystery, because most insect-feeding bats today are fast and powerful flying animals that rely on echolocation to zero in on their prey. Palaeontological analyses of bat skulls conducted in recent years indicate that early bats lacked the bony structures that would be required for echolocation.

On the basis of this, Padian and Dial theorized that bats started out eating crawling insects that would not have required echolocation to be caught. Yet if such insects were the preferred diet of early bats, they wondered why these bats had evolved to become capable climbers that could hang upside down from cave walls.

An answer came to the two researchers while they were looking at video footage of a baby bat being dropped off a ledge in a lab (onto a pillow). The bat, they noticed, rapidly fluttered its wings to help it control itself as it fell. In the same way, they propose, proto-bats, when dropping down from the ceilings of caves, could use flapping to control their descent and land in the right place to gobble up prey.

“There are a number of bats alive today that drop down from a perch and grab insects that they hear landing on nearby leaves, so this idea of bats evolving as drop-down hunters seems pretty logical”, says Simmons.

Further to this, they argue that bat development in the womb hints that developing a membrane between fingers at the end of the forelimbs would have been a mutation that could have easily emerged in early bats and then, over time, been selected for as controlled descent, and eventually powered flight, became ever more useful. “The reality is we don’t see gliding in extant bats, its doesn’t seem to be present amongst extinct bats, and when baby bats fall, they flutter. Gliding isn’t part of the equation”, says Padian.

Photo credit: Strange Ones, Creative Commons