Bats’ great plan B for when they can’t hear

When bats cannot hear, new research shows that these hearing-dependent animals employ a remarkable compensation strategy.

They adapt immediately and powerfully, suggesting for the first time that bats’ brains are equipped to launch a plan B in times of reduced hearing.

The work of Johns Hopkins University, recently published in Current biologyraises questions about whether other animals and even humans might be capable of such agile adaptations.

“Bats have amazingly flexible adaptive behavior that they can use at any time,” says senior author Cynthia F. Mossa Johns Hopkins neuroscientist who studies bats. “Other mammals and humans also have these adaptive circuits that they can use to make decisions and navigate their environment, but what’s striking here is that it’s very fast, almost automatic.”

All animals adapt in different ways in response to sensory deprivation. People at a loud bar may lean forward to better hear what someone is saying. A dog may tilt its head to a muffled sound.

Here, researchers wondered how hearing-dependent echolocating bats might adapt if a key auditory region of the brain was disabled.

They trained bats to fly from a platform, down a hallway and through a window to get a treat. Researchers then had the same bats repeat the task, but with a critical auditory pathway in the midbrain temporarily blocked. Switching off this brain area is not the same as plugging your ears; it prevents most auditory signals from reaching the deep brain. The drug-induced technique is reversible and takes approximately 90 minutes.

Image credit: Johns Hopkins University

Because their hearing was blocked, bats were able to follow the course surprisingly well, even on the first try. They weren’t as agile and bumped into things, but every bat tested compensated immediately and effectively.

“They struggled but they succeeded,” Moss said.

The bats changed their flight path and vocalizations. They flew lower, oriented themselves along walls and increased both the number and length of their calls, increasing the power of the echo signals they use for navigation.

“Echolocation works like strobes, so they essentially took more snapshots to help them get the missing information,” says co-author Clarice A. Diebolda former Johns Hopkins graduate student who is now a graduate student at Washington University in St. Louis. “We also found that they increased the bandwidth of these calls. These adjustments are very interesting because we usually see them when bats compensate for external noise, but this is an internal processing deficit.”

Although the team repeated the experiments, the bats’ compensatory skills did not improve over time. This means that the bats’ adaptive behavior is not learned; they were innate, latent, and hardwired into the bats’ brain circuits.

“It highlights how robust the brain is against manipulation and external noise,” says co-author Jennifer Lawlora postdoctoral fellow at Johns Hopkins.

The team was surprised that the bats could hear at all with this part of their brain disabled. They believe that bats either relied on a previously unknown auditory pathway, or that unaffected neurons could support hearing in previously unknown ways.

“You would think an animal wouldn’t be able to hear at all,” Moss said. “But it suggests that there may be multiple routes by which sound can travel to the auditory cortex.”

The team then wants to determine to what extent the findings apply to other animals and people.

“Can this work tell us something about auditory processing and adaptive responses in humans?” Mos said. “Since no one has done this, we don’t know. The findings raise important questions that will be exciting to pursue in other research models.”

Authors include Kathryne Allen, Grace Capshaw, Megan G. Humphrey, Diego Cintron-De Leon, and Kishore V. Kuchibhotla, all of Johns Hopkins.