Bats use their sense of hearing to hunt at night.

On Barro Colorado Island in Panama, the night sky is full of voices too high for humans to hear. Unless, like Alan in "Echoes in the Night," you're accompanied by researcher Elisabeth Kalko and what she calls her "bat detector," a high-tech box that instantly lowers the pitch of the bat sounds so humans can listen in.

To Kalko's trained ear, each tick and trill, screech and squeal indicates a different behavior. The sounds are part of the bats' echolocation system, one Kalko has been investigating for ten years. Bats use echolocation to "see" in the dark. They can produce up to 170 calls per second with their vocal cords - similar to how humans produce speech. The sounds then bounce off the bats' surroundings, including individual prey targets. The way the echoes return to the bats tells them everything they need to know - what objects are out there, how far away they are, how fast they're moving and even what species they are. Kalko has found that bats use different calls for different purposes: searching, targeting and identifying prey.

Kalko uses flash photography to match bat behavior to calls.

Kalko has been able to correlate the varied calls with their distinct purposes through years of recording and flash photographing the bats of Panama. After a night of observations, Kalko calibrates her sound recordings to the flash photography. In this way, she has learned to discern among bat species and determine the different sounds made by an individual searching for prey versus those made by an individual swooping in for the kill. Thanks to her work, bat sounds are among the best understood of all calls of the wild.

For more on this topic, see the web feature:
Seeing With Our Ears