Whisking quietly through the night, around buildings, trees and even branches, bats have a keen sense of their surroundings despite darkness. Researchers have known for decades that bats use their sonarlike echolocation to "see" potential obstacles as well as prey. But bats' execution of their airborne acrobatics often got scientists wondering just how they could be so specific—even while moving at high speeds through dense vegetation.

It turns out that the agile mammals emit different frequencies of sound as they fly through more cluttered environments to better detect the nuances of their environment, according to a new study published online March 29 in Proceedings of the National Academy of Sciences.

Researchers monitored big brown bats (Eptesicus fuscus) in a flight chamber using both microphones and thermal-infrared video cameras. They trained the bats to fly among hanging plastic chains, attached in a variety of patterns, to see how the bats would change their flight patterns—and calls. 

The bats were able to sense an echo delay as long as about 30 milliseconds, which translated to about five meters of distance, the researchers, noted in the study. But if an echo had not returned before a bat emitted its next round of sound, returning noise from the first call could get confused and be interpreted as a near-range object coming in from its second call, "causing one or more 'phantom' targets to appear at close range," the scientists wrote. By using more than one frequency for consecutive calls, the bats could better understand which call an echo was coming in from, and therefore, where the object was.

"They've evolved this so they can fly in clutter," James Simmons, a professor of neuroscience at Brown University and coauthor of the study, said in a prepared statement. "Otherwise, they'd bump into trees and branches."

In other bat navigation news, researchers at the Max Planck Institute for Ornithology have discovered that some bats actually use the sun in addition to magnetic cues (which can be irregular) for long-distance traveling, according to a study published online March 29 in Proceedings of the National Academy of Sciences.

To test the idea, the group of researchers caught greater mouse-eared bats (Myotis myotis) and took them 25 kilometers from their home cave. "I was very impressed that the fastest bats arrived back in their cave only two hours after release," Björn Siemers, of the Sensory Ecology Group and coauthor of the study, said in a prepared statement. For journeys out to find prey or between seasonal roosting places, some bats appear to be using faint glows from the set sun to align their internal magnetic compasses.

The researchers were able to back this up by artificially distorting the magnetic field of the earth both at sunset and afterward using a Helmholtz coil. Simmons and his colleagues found that only bats that had experienced a disrupted magnetic field during sunset strayed off course—because their magnetic compasses had been aligned erroneously. The bats "use the position of the sun at sunset as the most reliable indication of direction and calibrated the magnetic field with it to use as a compass later at night," Richard Holland, of the Department of Migration and Immuno-Ecology and lead author of the study, said in a prepared statement. It appears that even a faint glow where the sun has set is enough of a cue for these nocturnal animals to get their bearings.

Video: Seeing the Forest or the Trees

Bats use echolocation to navigate quickly through trees



Video courtesy of James Simmons

Image of a big brown bat flying through the experimental obstacle course, courtesy of James Simmons Lab/Brown University