Over 900 species of bats exist and majority of them use active biosonar system to detect their prey through echolocation (Braun, 2001). A high frequency sound is emitted by these bats through their vocal apparatus. These pulses and sounds emitted by the bats interact with the nearby objects and some of these pulses or sounds are reflected when they hit the object. This echo pulse will be then detected by the bat if the reflected sounds contain enough sound energy. These echoes are used by the bats to navigate through the environment and to locate and identify their prey (Moiseff, 2001).
Observations and experiments conducted by the scientists in the past with echolocation of bats have suggested that the bats use solely biosonar systems to perceive a detailed acoustic three-dimensional image of their prey. For example, little brown bats have shown the capability to pursue a moth into dense vegetation and capture it. At the same time, the bat is also able to avoid the vegetation successfully using its active biosonar system. This suggests that bats are also able to detect and create three dimensional acoustic images of the local environment too.
Bats extract information about the objects elevation and distance with the help of returning echoes (Moiseff, 2001). Distance Determination Time delay between the return of the echo and the emission of the pulse is used by the bats in order to extract distance information. The sonar system used by bats depends on the fact that in a given medium, the sound waves travel at a fixed speed. Therefore the sound pulse emitted by the bat travels through the air and then is reflected off an object. This sound pulse is then echoed back to the bat.
The speed of sound and the distance the sound travels are the determinants of the total time that the sound wave will take to return. The delay between the emission of the sound and the detection of the echo is directly proportional to the distance of the bat from the reflect object. Closer objects are signified by shorter time delays between the emission and detection of the sound waves whereas the objects which are distant are signified by larger time delays. This is because the speed of sound is constant and if the time delay between emission and detection of echo is known, the distance from the reflected object can be calculated.
Therefore to find out the distance between the bat and the object, the total distance is divided by two. The bat emits repetitive constant-frequency signals but before the signals are completely emitted, the bat starts receiving the echoes back. Therefore the bat receives a mixture of echoes and the emitted signals. During the climax when the bat is very close to capture its prey, the repetition rates of receiving echoes can reach to 200 pulses per second. The distance approximation of the bats can be as accurate as 330 micrometers or about three times the width of a human hair (Moiseff, 2001).
Characteristics of Sounds The physical properties of sound pulses are the major determinants of how the bat perceives the surrounding environments. The bats which locate the preys through echolocation, they produce relatively loud sound pulses which range between 100 and 120 decibels. These sounds, however, cannot be heard by human ears as they are above the range of human hearing. The reason behind loud sounds is that the objects small in size do not reflect back low frequency sound pulses, whereas, the loud sounds are reflected back efficiently.
Small objects in size do not reflect back low frequency sound pulses because not much sound energy is reflected back. Hence, in order to detect and locate small insects and obstacles, the bats need to use high frequency sound signals. The bats using echolocation can be grouped into two broad categories based on their echolocation pulses frequency composition. Frequency Modulating Bats The frequency modulating bats produce a band of pulses with different frequencies.
An example to consider is about the Eptesicus fuscus bats which emit sound waves ranging over the frequencies of 110, 000 Hz to 25, 000 Hz. However, some bats are ever sensitive to 150 KHz of frequencies (Forsythe, 2001). The number of different frequency channels may convey different information about the object which is reflecting back the pulses. This is a great advantage of producing sound pulses containing different frequencies. Therefore by using a broadband of frequencies in echolocation, the bat may be able to perceive more information about the shape, weight and density of a physical object.
The situation is very similar to using a single color or frequency to illuminate an object as opposed to using a broadband of frequencies, which is white light. Constant Frequency Bats The sound pulses produced by the constant frequency modulating bats are confined to a single frequency. It would be more accurate to say that these bats produce signals which have most of the sound energy concentrated on a specific frequency or a small range of frequencies. An example is the mustached bat which emits pulses that are concentrated on 63, 300 Hz pulses.
The advantage these bats have over the bats emitting broadband of frequencies is that the bats having a constant frequency while emitting pulses can have a very specialized ear. This ear can be specialized as it will be sensitive to only the small range of frequencies that are emitted. Hence the bat could detect even the weak echoes that are reflected back from distant objects and environment. Moreover, the chances of the pulses being reflected are higher as most of the energy is concentrated to a confined frequency. Specialized Features in Bats for Detecting Echoes
The audio cortex (AC) of bats has been examined extensively by scientists and biologists in the past. The neurons of bats are sensitively paired to stimuli which simulate pulse-echo pairs. In all of the bats studied, there is a set of neurons in the AC that are sensitive to the pair of sounds that are separated by specific time delays. These cells only produce action when they are given a stimulus by some bat specific frequency sounds. This stimulus only works when the bat encounters sonar signals produced by the same species of bat.
These neurons only work within a specific time period between the emission of pulse by the bat and the receiving of the echo. Therefore these neurons will not respond to all the sound signals that are received by the audio cortex even if these sounds are of the frequency that is detectable by the ear (Moiseff, 2001). The organization and distribution of these neurons within the audio cortex vary with species. The audio cortex of bats show tonotopic organization of neurons and it can be divided into non-tonotopic and tonotopic regions.
In the tonotopic region the neurons that are sensitive to low frequency echoes are found in groups with the sensitivity of these neurons increasing as one goes towards the non-topotopic area (Moiseff, 2001). This organization allows the bats to create a neural diagram of the range of the target. Studies have shown that if the neurons would not have been organized according to their sensitivity to sonar echoes, then the bats would not have had the capability of carrying out fine-range discrimination. Conclusion
Bats are gifted mammals that only use echolocation to detect the environment and their prey in the environment. It is difficult for us humans to understand the complex organization and working of the audio cortex, the emitting organs and neurons of bats. However, the scientists have been successful in understanding the organization of neurons around the cortical areas of the bats using echolocation. The neurons and the detectors of sonar pulses in bats play a vital role in extracting important information about the prey.
The bats are successful in avoiding the obstacles in the environment and catching their prey through analyzing the echoes that bounce off the prey and other objects in the local environment. Bibliography Braun, C. (2001, June 20). Sensory Systems in Vertebrates: General Overview. Encyclopedia of Life Sciences . Forsythe, I. (2001, July 25). Auditory Processing. Encyclopedia of Life Sciences . Moiseff, A. (2001, April 25). Prey Detection by Bats and Owls. Encyclopedia of Life Sciences , pp. 1-7.