Did all bats once have sonar?

Bats are known for their unique way of navigating through the pitch black night: making high-frequency calls in their voiceboxes and listening to them bounce off obstacles in order to create an acoustic map of their surroundings. 

But not all bats actually rely on laryngeal echolocation to get around or to hunt down insects and other flying tasty morsels. Although they do sometimes use tongue clicks to echolocate their way around obstacles, a group of bats, pteropodids or Old World fruit bats, largely rely on their eyes instead.

As the pteropodids have such different sensory systems from their echolocating relatives, scientists thought they were highly diverged lineages. But when geneticists began looking into the animals' DNA, they were surprised to learn that one group of echolocating bats were actually more closely related to the pteropodids than to the other echolocators. 

This kinship has created a longstanding puzzle for bat researchers. How, then, did the night fliers come to use such different sensory systems to get around? Did laryngeal echolocation evolve twice, independently among bats, or did the last common ancestor of all bats echolocate and the trait was lost in the pteropodids?

"This represents a fundamental question," Karen Sears, an evolutionary biologist at the University of Illinois Urbana-Champaign, writes in an email to The Christian Science Monitor. "Laryngeal echolocation is thought to have dramatically shaped the evolutionary success of bats, and enabled the evolutionary explosion of the group such that today 1 out of every 5 mammalian species is a bat."

Now, a team of researchers may have the answer. "Laryngeal echolocation most probably evolved in the common ancestor of bats and was lost in the pteropodids," they report in a paper published Monday in the journal Nature Ecology & Evolution. And this revelation could help unlock other mysteries about the pteropodids.

"There has been lots of attempts to try to resolve this problem," Nancy Simmons, curator-in-charge of the department of mammalogy at the American Museum of Natural History in New York, who was not part of this study, explains in a phone interview with the Monitor. Mostly scientists have looked to the animals' DNA for clues but, she says, "There hasn't been a smoking gun in the genes to show that it was one way or another."

But the scientists that authored the new paper took a different tactic. They looked at the fetal development of different bats' inner ears. 

The size of the cochlea, the bone that houses the inner ear, is known to be correlated with echolocation in bats. The bigger the cochlea relative to the rest of the animal's skull, the better it can pick up on high-frequency sounds. As such, non-laryngeal echolocating bats have relatively smaller cochleas. At least the adults do.

But when the researchers compared the size of the cochlea in echolocating bats and pteropodids in early fetal stages, they saw that they were actually all about the same size – relatively large compared to the width of their skulls. Then, as the bat fetuses grew, the growth of a pteropodid's cochlea slowed down to a crawl so that in adulthood it was the same size relative to its skull as in other mammals that don't echolocate. The researchers suggest that this large fetal cochlea is a vestige of the ancestral echolocation trait.

"I think this is very conclusive evidence that echolocation evolved once," Dr. Simmons says. "I think it's definitely telling us that they were echolocating in their ancestry," particularly because it lines up with the dominant interpretations of the fossil record, where Simmons focuses her research. 

But Dr. Sears, who also wasn't involved in the new research, isn't so quick to call the study conclusive. "Studies such as this assume that ontogeny [development] mirrors, to some extent, the process of evolution, but this is not always the case," she says. "While ontogenetic evidence can produce significant insights into evolution, I believe it is best to view it as another piece of data that helps fill in the larger puzzle, rather than definitive proof of one scenario or another."

John Speakman, a biologist at the University of Aberdeen who also was not involved in the research, agrees in an email to the Monitor. Although he says it seems like "nice data, ... nothing is completely settled. It points us in one direction. Other data may come along showing the opposite. It is best to keep an open mind." 

Dr. Speakman's main hesitation is the sample size used in this study. The researchers looked at just seven species of bats, five with laryngeal echolocation and two pteropodids. But there are over 1,260 species of bat worldwide that fall into 21 families. "It is possible the species they chose have large cochlears during development for other reasons, so it is always safest to have multiple species for each different taxon."

Still, Sears says, "Taken at face value, the results of this study have profound implications for bat evolution. For example, they suggest that large fruit bats lost the ability to perform laryngeal echolocation, which is interesting given the presumed importance of echolocation to bat evolutionary success."

Pteropodids have been quite successful without laryngeal echolocation. There are 186 known species of these large fruit bats.

"It's giving up something that works really well for something really different," Simmons says. And understanding that could help researchers figure out how pteropodids came to have other unique features. 

For example, she says, the Old World fruit bats exploit a different resource than their laryngeally echolocating relatives. These aptly named bats munch on fruit and flower products rather than chasing down insects. The other bats use their unique sensory system to track down their next meal, but the pteropodids don't have a moving target when they're hungry.

Another unique feature of the pteropodids is their large body size. And better understanding whether their ancestors lost laryngeal echolocation might help explain that, too, suggests Simmons. It's thought that there is a body-size constraint for echolocators, as bigger bodies tend to make lower frequency sounds and laryngeal echolocation requires high-frequency sounds.

"This study highlights the potential insights into evolution than can be obtained when researchers use new types of data to address fundamental, outstanding questions in the field," Sears says.