Early Bats Flew First, Developed "Sonar" Later
for National Geographic News
|February 13, 2008|
Bats learned to fly before they developed their internal "sonar" to navigate and catch insects, the most primitive bat fossil ever found shows.
"This new bat [fossil] is clearly a flying animal, but it lacks the features in the skull that we'd expect to see in an echolocating bat," said Nancy Simmons, chair of Vertebrate Zoology at the American Museum of Natural History in New York and co-author of a new study on the fossil.
Echolocation is the radar-like ability of some animals to emit high-pitched sounds, then detect obstacles or prey by listening to the sounds bounce back.
Bats are thought to have evolved from terrestrial mammals, and scientists have long pondered whether they took to the air before or after they could echolocate.
Previously the most primitive bats known were from the early Eocene, about 50 million years ago, and were fully capable of flapping flight.
They also had physical adaptations for echolocation, and a few fossils even have preserved stomach contents that reveal meals of flying insects.
"So we know they were flying animals and [were] probably echolocating and catching flying insects," Simmons said.
But the new species found in Wyoming's fossil-rich Green River formation, Onychonycteris finneyi, is some 52.5 million years old.
Onychonycteris had fully developed, flight-capable wings, but its ear structure shows that it would not have been able to employ modern bats' famous sonar.
(Related news: "Scientists Fill Blanks on Bat Family Tree" [January 27, 2005].)
Which Came First?
The new study, which appears tomorrow in the journal Nature, may put to rest the idea that echolocation evolved first.
That theory suggests that small tree-dwelling mammals developed echolocation to snatch their insect prey in midair.
This hunting method could have gradually led to evolutionary selection for longer arms and digits that enabled the animals to leap—and later glide—after their prey, until they eventually developed full-flapping flight.
"We don't know for sure, but what makes the most sense in terms of the evolution of flight is that bats evolved from a gliding ancestor, something similar to a gliding squirrel," Simmons said.
Gliding has evolved several times in tree-dwelling mammals as they gradually acquired membranes to enable gliding through canopies.
"The hypothesis for bat flight is that it evolved as a means to expand their range and maneuverability to find food and escape predators," Simmons added.
Onychonycteris also has morphological features that suggest it was an expert climber. Its limb proportions and claw-tipped fingers resemble characteristics of both modern bats and non-flying animals, such as sloths, that hang under branches.
"Probably what it was doing was flying to get from place to place, landing in some vegetation, and then climbing around looking for food," Simmons explained.
"As a bat that can't echolocate, it probably wasn't chasing food in the air but listening for prey-generated sounds [like a beetle hitting a leaf].
"That's the way that some living bats hunt, even using echolocation for navigation."
(See a picture of a bizarre new bat.)
While Onychonycteris makes it clear that ancient bats could fly without the ability to sense prey, scientists are still wondering if the mammals flew in the dark.
John Speakman, the chair of zoology at the University of Aberdeen in the United Kingdom who wrote a commentary on the study, has a theory.
"The possibility is there that bats were originally [awake during the day] and were forced into the nocturnal niche by the appearance of avian predator species some 50 to 60 million years ago," Speakman said.
Both birds and mammals increased dramatically during this time period, which followed the extinction of the dinosaurs at the end of the Cretaceous, 65 million years ago.
New nocturnal lifestyles might have led some bats to develop echolocation, while others may have relied on increased night vision to get around.
Unfortunately existing Onychonycteris fossils will shed no light on this question.
"The eye sockets were crushed, so it can't be determined if they were enlarged as in other nocturnal, non-echolocating animals," Speakman explained.
Despite this setback, however, the fossils represent a breakthrough in the understanding of bat evolution.
"This kind of puts the icing on the cake," Speakman said.
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