From wings to low-density bones to echolocation, the evolution of flight in bats required many radical changes. But the most important change may have been metabolic.
A genetic comparison of dozens of mammal species shows that bats possess highly modified versions of genes responsible for turning food into energy. Improved energy efficiency would have encouraged their ancestors to move from treetop gliding, like modern flying squirrels, to actively flapping their arms.
"Gliding doesn't require huge amounts of energy, but when you start flapping your arms, you start needing more," said David Irwin, a University of Toronto evolutionary biologist. "Changes in energy synthesis need to get well underway before you get sustained flight."
The bat evolution study, published April 26 in the Proceedings of the National Academy of Sciences, grew from lead author Ya-Ping Zhang's interest in avian energy metabolism. In an earlier comparison of flightless and flying birds, the Chinese Academy of Sciences zoologist found that flightless birds had fewer genetic changes in their mitochondria -- the cellular structures that turn oxygen and nutrients into chemical energy. Zhang wondered if mitochondria and flight were tightly linked in mammals, too.
The researchers analyzed mitochondrial genes from four species of bats and 60 other mammal species. By comparing the differences against known evolutionary histories, they extrapolated what mitochondria in a last common ancestor might have looked like.
When they compared modern bat mitochondria to the ancestral animal, they found profound changes in a subset of genes that code for enzymes that break down nutrients -- the fuel cells of the fuel cells, so to speak. In bats, up to 23 percent of these genes show signs of adaptations. Just 2 percent of other genes have changed.
Because bats were fully formed by the time they appear in the fossil record, scientists don't know which adaptations came first. But Irwin thinks the mitochondrial changes must have come early, and are most important. To support their airborne lifestyle, bats require three to five times more energy than other mammals their size.
Irwin next hopes to study the physical structure of enzymes produced by bat mitochondria, with the aim of discovering exactly what makes them so efficient. The insights might eventually be applied to metabolic disorders like obesity and diabetes. "Maybe this will give us a better understanding of how to make our own energy system more efficient," Irwin said.
Image: Jessica Nelson/National Science Foundation.
Citation: "Adaptive evolution of energy metabolism genes and the origin of flight in bats ." By Yong-Yi Shen, Lu Liang, Zhou-Hai Zhu, Wei-Ping Zhou, David M. Irwin and Ya-Ping Zhang. Proceedings of the National Academy of Sciences, Vol. 107. No. 17, April 27, 2010.
Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.
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