Cries in the Night

We think of bats as mysterious creatures, fluttering silently on ghostly wings. You may be surprised to learn that bats are not so quiet. You learned in biology that bats use echolocation to navigate and catch prey in the dark, but have you ever thought about "echolocation" and what it really means?

We think of bats as mysterious creatures, fluttering silently on ghostly wings. You may be surprised to learn that bats are not so quiet. You learned in biology that bats use echolocation to navigate and catch prey in the dark, but have you ever thought about "echolocation" and what it really means?

In simple terms, bats have voices; they aren't silent at all! What's more, they use their voices for a variety of things, much as we do, but for much more than we can do. We cannot hear them because they are "speaking" in frequencies that are above the level of sensitivity of our ears, about 20 kilohertz (cycles per second). If we listen in, there is much to learn about bats and their voices. The best known use for these ultrasonic cries is navigation. Using the echoes of their own voices bats can fly through caves or into the darkest night, avoid obstacles, including each other, and find their way back again. And while they're at it they can locate and capture, in the air, flying insects for their food.

The only way I can visualize this for myself is to imagine that the bat's brain takes the information from the echoing cries and creates a mental scene that must be much like vision. Maybe it's like an arcade or computer game - images in black and white and shades of gray. But I do know this, a bat can learn a great deal about what's out there just from those echoes. It can tell whether an object is large or small, hard or soft, moving or stationary, something to eat, or something that might eat it.

What is echolocation, and how does it work? Most bats, and all bats in Missouri, emit ultrasonic cries through their mouths. These cries range from about 20 khz to 100 khz; in Missouri they don't rise much above 80 khz. Most bats have a mechanism for turning off their hearing at the moment they emit a cry, presumably to avoid being deafened by the outgoing sound. Their ears pick up the returning echoes and the information is sent to the brain.

Scientists have learned that different areas of the bat's brain are stimulated by the various qualities of the returning echoes. This must be the way in which the bat "sees" what is around it.

By recording calls from the various species of bats we have devised ways of recognizing and telling species apart. The frequency of a call helps tell us which species it is, as well as whether a call is constant or sweeps through a range of frequencies. Some bats utter a single tone while others include harmonics in their calls.

The duration of a single cry (and we're talking about milliseconds here) is diagnostic as well. Using electronic wizardry and an oscilloscope, researchers can view the frequency change of a call over time. This creates a vocal signature that is characteristic for each species.

Not only are bat cries different from species to species, but different indi viduals can recognize their own voices, and those of other bats. Even when flying with thousands of other bats, an individual bat can recognize the returning echoes of its own voice and avoid colliding with colony mates or the wall of the cave. What's more, a mother bat in a nursery cave with thousands or even millions of other mothers and their pups can recognize the voice of her youngster, and it hers, when she returns from feeding and comes to nurse it.

Bats typically utter a series of cries with distinctly different phases. While simply flying and navigating or looking for potential prey, the calls will be relatively slow and well-spaced - the "search phase." An "FM" bat, one that modulates the frequency of his calls, will give a broad frequency sweep during this phase. When heard on an audible bat detector these cries sound almost musical and can be called "chirps."

Upon detecting a potential prey item, let's say a juicy moth, the bat will begin to home in by shortening both the duration of its cry and its frequency sweep - the "approach phase." The terminal phase is called the "feeding buzz" and is a rapid fire, staccato burst of calls that helps the bat zero in and catch its prey.

Now, lest you think that the bat has all the advantages in this aerial combat, be aware that some moths themselves have ears. It is no coincidence that these organs are tuned in to the frequency of bat cries. Upon hearing a sound in the range of a bat, these moths fold their wings and plummet to the ground. Better to crash land, I suppose, than to be an entree. Some moths even "jam" bats' sonar by emitting high frequency clicks of their own.

This game of cat and mouse, or bat and moth, gets even more interesting. There are high intensity (loud) bats and low intensity (quiet) bats. Are the quiet bats sneaking up on moths?

Besides navigating and catching dinner, bats use their voices for socializing. As mentioned above, mothers and their pups recognize each other by voice. They also use smell - the mother rubs her own scent on her youngster. Thus bats have a system of recognition that works both from a distance and "up close and personal."

Upon returning to the cave, the mother bat flies to the vicinity of the place where she left her youngster. She will hover nearby and call, listening for the voice of her pup amid the tumult of cries from the assembled gaggle of other young that have huddled together while their mothers were off feeding. Once she recognizes her pup, the mother squeezes into the squirming mass and begins nursing it.

In the fall of the year, when thoughts turn to bat romance, the voice again comes into play. Male bats take up stations spaced out on the ceiling of the cave where they're going to hibernate. There they hang and utter distinctive calls at the females who fly in and survey the gathered suitors, from whom they select their mates. Does a female select a particularly smooth-voiced crooner? Perhaps we'll never know. But there is no doubt that voice plays a role.

So, take a walk some warm Missouri evening. And watch along a tree-lined avenue, in a park, at the forest's edge or over a stream. Those silent, darting figures you see outlined against the luminescent sky aren't so silent after all. If you had ears to hear, the night would be full of their cries.