Pre-flight check.
At 80 days, the three forelimb digits that will eventually support the bat's wing have already begun to elongate significantly.

Scott Weatherbee/Memorial Sloan Kettering Cancer Center

How Bats Got Off the Ground

Bats are shy creatures, especially during the daytime, when they hide in caves, trees, and even under bridges. Yet when it comes to Darwinian success, they are anything but low-profile: Bats make up one out of every five mammalian species. And they are innovative as well, being the only mammals to fly under their own power. A new study pins this airy achievement on a bone growth gene, whose boosted expression may have literally helped bats sprout wings and take off.

A team of developmental biologists led by Karen Sears and Lee Niswander of the University of Colorado Health Sciences Center in Aurora, set out to unveil the hidden origins of bat flight. The researchers compared the wings of the earliest known bat fossil, a 50-million-year-old specimen housed at the American Museum of Natural History in New York City, with those of three other species of extinct bats and 10 species of modern bats. The membranous bat wing is supported by the animal's highly elongated third, fourth, and fifth forelimb digits. As the team reports this week in the Proceedings of the National Academy of Sciences, the length of the three digits, relative to body size, has not changed over course of bat evolution. This suggests that the bat wing evolved suddenly and very quickly.

Because bat experts think that the ancestor of all bats was a small mammal with short forelimbs like that of the mouse, the researchers next compared bat and mouse embryonic development. During early development, bat and mouse forelimb digits grew at about the same rate relative to their overall gestation periods of 120 and 20 days, respectively. But roughly halfway through gestation, the bat's third, fourth, and fifth forelimb digits abruptly took off on a major growth spurt. Meanwhile, the same digits in the mouse continued at their slower rate, as did the bat's hindlimb digits.

The team suspects the difference may be due to the expression of a gene that codes for a growth factor called bone morphogenetic protein 2 (Bmp2); expression was 30% higher in the developing forelimbs of bats than it was in mice. Just such an upregulation of Bmp2 might have powered the elongation of the three digits in an ancestor to the bat 50 million or more years ago, helping to make winged flight possible.

"This is an excellent paper," says Emma Teeling, an evolutionary biologist at University College Dublin. "Without a doubt flight is mainly responsible" for the evolutionary success of bats, she adds. Molecular biologist Sean Carroll of the University of Wisconsin, Madison, says the study is an important contribution to the new field of evolutionary developmental biology, also known as "evo devo." The new work, Carroll says, "helps us to understand how evolutionary transformations are achieved by tinkering with the development of individual structures--in this case, the digits."

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