Echolocation is the art which is utilized by many animals in the detection of their pray and for navigational purposes. These animals are generally not equipped with eyes that are functionally advanced to the extent that makes them a good predator in their habitat. Common examples of such animals are Bats, dolphins, whales etc. Such animals emit an ultrasonic signal into the environment and sense the signal that returns back after striking from the surroundings. This enables them to appreciate what waits ahead of them, its range, size etc. In short echolocation works exactly like modern day sonar in which the animal makes an estimate of the range of the pray by calculating the time interval between its own emission of the sound and the echoes that return back.
Bats use echolocation signals that range in the frequency from 14kHz to 1000khz (Neuweiler 245) .These signals are thus well out of the range of human ear to hear which is capable to decipher audio signals falling between the frequency of 20 Hz -20,000 Hz .. Studies have indicated that bats have a very characteristic pattern of hunting by echolocation that can be divided into four phases namely search phase, approach phase, terminal buzz and the post buzz phase. All these phases differ in their band width and inter pulse duration. When a bat is in flying smoothly in search of it’s pray it is said to be in the search phase which is characterized by pulses which have a comparatively low pulse repetition rate, a higher pulse duration and inter-pulse interval .
When the pray is detected the signal is changed to the approach phase in which the interval between the pulses decreases to about 33 ms. Just before the capture of the pray most of the echo locating bats emit a terminal buzz, which has further decreased inter-pulse interval. The terminal buzz phase can also be divided into a Buzz I and Buzz II phase. Irrespective of the fact that a prey capture attempt is successful or not the same pattern of phases is repeated again during the course a bat is flying in search of its pray. To tell more simply the usual signals in the search phase are well separated and have a lesser rate/sec as compared to the signals which are released when the bat has spotted and is just approaching its target. The pattern of the phases are however different which would be discussed later in the same article.
One may be wondering what makes these animals utilize such advanced mechanisms to hunt for their pray. The answer lies in the fact that bats are provided with a highly sensitive inner ear (The part of the ear that detects and transmits hearing impulses to higher centers in the nervous system). In the same way bats have a very advanced auditory cortex in order to interpret these signals (Grinnell 25).
Numerous studies have been conducted on the echolocation pattern of the bats; below it the summaries of some selected studies which would enable the readers in a better understanding of the topic.
Although it is an extremely vast topic yet every effort is made to provide the summaries of recent five researches
1) Effects of successful capture attempt versus unsuccessful capture attempt on post-buzz period:
Adam et al in their study demonstrated the effect of a successful catch over an unsuccessful attempt on the post-Buzz period. Post Buzz period is the interval that follows the terminal buzz till the usual pattern of the pulses, search phase is restarted. According to this study the post-buzz phase was significantly greater after successful attempts as compared to unsuccessful attempts.206±112.3ms for successful attempts and 106.2± 83.3 ms for unsuccessful attempts. This increase in the post-buzz period has been attributed to time that is required in the handling of the pray. Similar studies have indicated increase in post-buzz time upon the successful capture of the pray (Kalko 225).
However the post-buzz time was shorter in this study than that found in that found by Adam et al possibly due to the use of smaller insects. So the pray size is also a determining factor in the duration of post-buzz interval. Similar results were obtained by a study done by Annemarie, Vibeke and Jakob Tougaard from the University of Southern Denmark. In addition to the calculation of post-buzz time for successful and unsuccessful attempts they also calculated the post-buzz periods for touches also, i.e. when a bat is unsuccessful in capturing the prey yet its body parts slightly touch the prey in flight. Similar to the previous studies the post-buzz period was greatest in successful catches 272.2 ms and shortest in unsuccessful attempts, 117.9 ms and almost a middle value in case of touches i.e. 181.2 ms.
2. Effect of the size of prey on the bat’s response time
One might think that it makes sense if we say that larger prays are detected from a longer distance as compared to prays of smaller size. But in actual studies this belief has been negated. One of the studies indicated that there was no significant effect of the size of the pray on the reaction time of bats. The mean time from detection till attempted capture was 604±141 ms (Britton 1796). Similar results were found by Acharya and Fenton in their study titled “Echolocation behavior of vespertilionid bats attacking airborne targets including arctiid moths”
3. Echolocation frequency of bat species and its relation with geographical habitat:
In a lot number of studies the interesting phemenon of the relationship between specific bat frequencies to their habitat was demonstrated. Gareth Jones and Sofie M. V Parijs demonstrated the presence of both high frequency and low frequency Pipistrelus bats in Britain. The echolocation signals of the low frequency bats were never found to exceed 52 KHz while that of the high frequency bats were usually above 52 KHz (Jones 123). They attributed this frequency difference to their geographical habitats. In recordings from Scotland the high frequency bats which were found to be 71% clearly dominated the low frequency bats. Thus it was concluded that in a particular geographical area different phenotypic bats having nearly similar echolocation frequencies of the same specie can exist and the change in frequency is not exhibited by the same phenotype which change its frequency according to ecological conditions.
4) Echolocation studies in Dolphins:
Dolphins appear to have a lot more advanced echolocation system as compared to bats. They listen to their pray and then use echolocation signals to get more information about the pray (Harley 62). There is also a possibility that dolphins use echolocation signals for communication purposes as well this was proposed by Brownlee.
Keeping in view the diversified studies that have been done on the echolocation patterns of bats one finds himself amazed to see what is known and highly curious about what is yet to be found out. Future works can target coding bat species according to their echolocation frequencies. Although this is not going to be an easy job but it is going to open windows to new dimensions of research. Through this coding an attribution to the geographical location of the bats can also be made.