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Scientists Fill Blanks on Bat Family Tree

Hillary Mayell
National Geographic News
January 27, 2005
 
Although roughly one in five of all mammal species is a bat, the winged
animal has long been an enigma to scientists.

Despite what Nancy B. Simmons, a scientist at the American Museum of Natural History, characterizes as a "prominent position among mammals," the evolutionary history of bats is largely unknown. Something like 60 percent of the bat's fossil record is still missing.

"With birds we have some great intermediate fossils," said Mark Springer, a biologist at the University of California, Riverside. "With bats it's a lot more of a guess."

Springer is a co-author of a study that fills in some of the blanks in the bat family tree, published in the January 28 issue of the journal Science.

Exploiting the Eocene

The researchers compared genetic sequences from each modern-day bat family in order to reconstruct the evolutionary relationships among small bats (microbats), which use sound waves to find their prey (a process called echolocation), and big bats (megabats), which generally don't.

Microbats are the vast majority of bats and are found all over the world (except for the polar regions). Megabats are found in the tropics where they are sometimes called flying foxes.

The researchers also used fossils to trace the geographic origins of the major bat lineages.

Around 65 million years ago, a mass extinction event occurred, possibly caused by an asteroid collision with the Earth or particularly heavy volcanic activity. Best known for wiping out the dinosaurs, the extinction event elmininated roughly 75 percent of all plants and animals on Earth. One of the survivors was likely a creature that evolved into modern-day bats.

Although the fossil record is notably lacking, scientists believe that bats fully capable of flight and echolocation, or locating prey and other objects through sound waves, emerged during the geologic time period known as the Eocene (about 50 million years ago), probably in North America. This occurred at a time when the planet's temperature had increased by several degrees, and plant and insect diversity was at a peak.

"Bats are a spectacular group of mammals, with a combination of two remarkable specializations that you don't see in any other land mammals: flight and echolocation," Springer said.

Scientists speculate that bats diversified rapidly in response to these environmental changes which created an ecological niche that could be exploited.

"As flying predators capable of capturing prey on the wing, they would have had few competitors for the rich resources of the Eocene night," Simmons wrote in a related commentary.

Microbats and Megabats

Bats display astonishing diversity. "They're one of the most fascinating groups of mammals or vertebrates," Springer said.

Bats come in different body sizes. They have different diets, echolocation strategies, flight styles, and reproductive habits.

Megabats, commonly known in Australia as flying foxes, weigh in at two pounds (one kilogram) and can have wingspans of up to six feet (two meters). Microbats are much smaller, with wingspans of up to five inches (twelve centimeters). The tiniest microbat weighs about as much as a dime.

"Microbats echolocate," Springer said. "They have a highly developed inner ear, which they use the way we would use vision. The megabat relies more on vision, and with a few exceptions, they don't echoclocate."

Flight styles and diet also vary widely. Some species have evolved highly sophisticated skills that allow them to track and catch their meals while flying. Others, like the vampire bat, look for a large, stationary target, like a cow, make a small bite and lick the blood from the wound. Most megabats live in the tropics and are fruit eaters.

"Bats are a very little known group," Springer said. "They're not the kind of animal we see when we're out walking around in nature. But at the same time, they're … a tremendously important group in terms of our habitat—and in terms of [their] adaptation[s], one of the most spectacular."

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