2005 Young Naturalist Awards - The World Through a Bat's Ears

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Introduction

People traditionally associate bats with vampires, Dracula, and other unpleasant matters. But watching them fly around in the dark of the Negev Desert in Israel as a young child, I became fascinated by the magical movements of these mysterious creatures. Wherever I visited, wherever I lived, bats were there.

The mouth of a cave

A year ago a scientist I knew told me about some raw material he had from a field expedition to Zimbabwe's Mana Pools National Park that had never been analyzed. So I filled my bedroom with leftover bat food—the wings and legs of katydids, ant lions, moths, and cicadas—and wrote a paper about the results for African Zoology. I wanted, however, to go further—into the field to collect my own data. This past summer I found a course offered by an Ontario university that would allow me to conduct my own research with sophisticated equipment. I leapt at the opportunity.

My interests have not exactly been understood by my friends. When I told a classmate I was going to study the behavioral ecology of bats, she exclaimed, "Eww! Bats are creepy!" But as I began to learn more about bats, I came to appreciate these small, often misunderstood animals.

A bat is a flying mammal.

Bats are the only flying mammals. Their bone structure is like that of humans, and yet they have a special ability to move about and locate their prey in the night skies. Lazzaro Spalanzani said as early as 1794 that bats could "see" with their ears. In 1940 Donald Griffin confirmed Spalanzini's reports and called the phenomenon "echolocation." By echolocation bats produce sounds and use their echoes to determine the location of objects, including prey. According to Griffin, by comparing the original signal with returning echoes, bats use sonar calls to get information about their target, its distance, direction, and nature.

Bats that feed in flight are called aerial-hawkers or aerial-feeding bats. They use echolocation to find, follow, and gather information about their airborne prey. When echolocating bats approach a target, they move in three phases. The first is a "search phase," which is a call with constant features and long intervals between the calls. The second is an "approach phase," and the last phase is called a "feeding buzz." During a feeding buzz the bat greatly shortens the duration of its calls and the intervals between them.

The echolocation calls of bats have two major patterns. The first pattern is called a high duty cycle, in which bats separate the pulse and the echo in frequency. These calls are long, separated by short periods of silence, and dominated by a single frequency. The second pattern is called a low duty cycle, in which bats separate the pulse and the echo in time. These calls are short, separated by long periods of silence, and are not dominated by a single frequency (Fenton and Ratcliffe, p. 612).

Bats can vary features of their echolocation calls. For example, they can change the duration, the frequency, and the cadence or the intervals between calls (inter-pulse intervals).

The features of a bat's call, or its design, affect what a bat hears. The wavelengths of sounds differ between frequencies, lower frequencies having longer wavelengths. Wavelengths affect the level of detail a bat can acquire about its target. Bats can get more detail by using higher frequencies with shorter wavelengths.

The frequencies of the echolocation calls of aerial-hawking bats may be influenced by various factors, such as the body size of the bat, the defenses used by their prey, environmental factors, and atmospheric attenuation (the reduction in the level of sound energy over distance) of bat signals. Biologists have also found that bats change their echolocation calls according to habitat and as a result of social setting.

An echolocating bat receives echoes from more than its flying prey. Echoes are also produced from objects such as trees and walls. These are called "clutter." Clutter can cover up or mask echoes that are coming from prey.

The ability of bats to vary features of their echolocation calls suggests that they will adjust their calls to allow them to operate more successfully in cluttered (closed) environments. Although several researchers have spoken about clutter as perceived by bats, there have been few efforts to quantify either clutter or bats' responses to it. I wanted to know if and how echolocating bats respond to clutter. What can they do to better orientate in cluttered environments?

To find out, I arranged to record the echolocation calls of little brown bats, Myotis lucifugus (Figure 1), as they flew in four situations with different degrees of clutter. After recording, I analyzed selected calls, used statistical analyses to assess variation in the bats' calls, and considered the implications of my results.

M. lucifugus hanging in a tree

M. lucifugus belongs to the Vespertilionidae family and is an aerial-hawking, insectivorous bat. Its echolocation calls are short, separated by long periods of silence, and are not dominated by a single frequency.

During my fieldwork, I had watched little brown bats fly adroitly in the open and in cluttered situations, so I predicted that these bats would adjust their calls according to the setting in which they operated. I hypothesized that in response to the degree of clutter, these bats would adjust the features of their calls, such as duration, frequency, and the intervals between calls, shortening the duration and the intervals between calls and increasing the frequency for shorter wavelengths in response to the need to avoid masking important echoes.

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