Dr. Richard Carter makes new discovery in anatomical study of bats

JOHNSON CITY – While the birds heard singing during the daytime sleep in their nests at night, bats flutter about in the dark, taking advantage of food sources only available at that time and avoiding large daytime predators. And in the darkness, they find their way around using echolocation, or a form of sonar, projecting soundwaves to form images of their surroundings. But what assists these flying mammals in doing this so effectively?

That is what an East Tennessee State University researcher is looking at in a study, “Evolution of the windpipe and larynx in echolocating bats,” funded by a $10,000 grant from the university’s Research Development Committee.

Dr. Richard Carter, an assistant professor in the ETSU Department of Biological Sciences, first developed an interest in bats through a research project as a graduate student after being more interested in his youth with amphibians, like snakes, lizards and chameleons. 

“Bats are kind of mysterious,” he said. “We don’t see them very often – they’re flying around at nighttime, they’re little.  They are sort of an enigma.

“I started reading about their echolocation and realized how really complicated it is. And ever since graduate school, I’ve been looking at the evolution and the anatomy of echolocation in bats, and flight, to a certain extent.  It turns out that flight and echolocation are actually linked in terms of behavior.”

In studying the anatomy of bats, Carter was intrigued by some patterns of reinforcement that he noticed in the larynx.  He says this finding was not unexpected but had not been described in the scientific literature, and so he started looking deeper into how the windpipe, larynx and lungs work together to deal with the “high demand of echolocation” as they produce various levels of screaming sounds.

“Some bats are quiet and do not scream very loudly,” Carter said, “while others are very loud echolocators and put a lot of energy into the sound they produce. Some bats emit really short little noises, and others emit these long sorts of squeaks, so there’s a huge variety in how different bat groups use sonar.

“The goal of this project was to look at these different echolocating groups and see what their larynges look like. Does a larynx that has to produce a very loud noise look a certain way? Does a larynx that has to produce a very soft noise look a certain way?”

To accomplish this, Carter spent three days at the American Museum of Natural History in New York, which, he says, has the best collection of bat specimens in the world. He selected the specimens he wished to study and produced images of them using micro-computer tomography, or micro-CT. He then returned to his laboratory at ETSU, where he used a special computer purchased with the grant funding to study these images, looking at the formation and mineralization of the bats’ larynges and how the cartilage responds to increased mechanical loads.

“When there’s a stress or a strain put on a piece of cartilage, you often have calcium being dumped into that cartilage to make it stronger,” Carter said. “So we looked at the patterns of the reinforcement in the larynges of these different bat groups, and then tied those to echolocation types – those that scream loudly, those that scream softly, and so forth.”

As expected, Carter says, he and the undergraduate biology students participating with him in the project found that the larynges of the louder bats were more highly reinforced than those of quieter bats.

However, they found something new in looking further at the other parts of the anatomy that come into play as the bats generate sound – and fly.

“We also looked at the rest of the windpipes, or the trachea, which takes air from the lungs past the larynx and out of the mouth,” Carter said, “and we found that most bats also have reinforcement of the tracheal rings, which hasn’t been described yet. We think it has to do with flight – bats also use their flight muscles to actually pump air in and out of their lungs, which basically makes breathing much easier. This reinforcement of the entire trachea was an unusual finding, one that I think is new to bat biology.”

Carter has submitted an article on these findings for publication, and is also using the data from this study to support the application for a major research instrumentation grant through the National Science Foundation, for which he is a co-investigator with ETSU scientists and staff from the Museum of Natural History at the Gray Fossil Site and the Reece Museum. If approved, this grant would support the purchase of a micro-CT scanner that could be used for many purposes by multiple ETSU departments and other regional entities.

Carter, who holds his Ph.D. from the University of Northern Colorado and B.S. and M.S. degrees from Ohio University, joined the ETSU faculty in 2018.