The below mentioned article provides a study note on echolocation of bats.
Some kinds of bats are able to fly and feed in the dark because of their famous ‘radar’ system which allows them to avoid all obstacles. They obtain the major part of their information through the ear and reflection of the sound waves that they emit at the moment of flight.
Bats have a highly developed apparatus by which they emit supersonic sounds which — as soon as they strike an object — are echoed back and received by the bat so that it follows an unhampered path.
At the moment of flight they emit ultrasonic sound whose frequency is 100000 cycles/sec and acts as ‘radar’ system. Mammals can hear sound commonly over the wide range of 20-20000 hertz. A list of hearing frequencies of various mammals is given in Table 44.
Lazzaro Spallanzani, an Italian naturalist and physiologist, first conducted experiments, in between 1793 and 1799, on the contribution of ears of bats to avoid the obstacles in their pathways during flight. He captured the bats from the belfry of a cathedral and released the bats after removing the eyes. After 4 days it was seen that some bats were at the same roost.
The stomach of these blinded bats was full of insects, that indicated the use of some sense other than vision. Unfortunately this type of modern experiment was unknown to the zoologists for a long time.
In the middle of 20th century, D. R. Griffin and R. Galambos (1940, 1941) at Harvard and S. Dijkgraff at Utrecht discovered independently that bats produce ultrasonic sounds and are reflected back by any obstacle in the pathway and ears are sensitive to these sound frequencies.
Griffin and Galambos also showed the use of same echolocation for navigation and capturing the insects. Bats are broadly divided into fruit-eating (Megachiroptera) and insect-eating (Microchiroptera) species. Insect-eating bats, whose diet mainly includes insects, apply echolocation system to locate their prey even in broad daylight.
Some special features are observed in bats by which they may be active in the dark and live in deep caves and catch their food in the dark. Their inferior collicula (concerned with hearing) and cerebellum are large but cerebral hemispheres are small, and olfactory portions become reduced. The eyes are moderately large and are used in twilight.
The retina contains mainly rods. The insectivorous bats emit high discrete pulses from the mouth and nose whose frequency is up to 150 kilohertz. These are produced by large larynx whose cartilaginous rings are ossified and forms a strong framework.
The cricothyroid muscles are strong that create great tension on the vocal cords. In some Horse-shoe bats (Rhinolophus) special resonating chambers and the face is modified like a nose-leaf to beam the sound forwards (Fig. 10.65).
There are 800 species of micro chiropteran bats in the world and probably all use echolocation. These species live in various habitats and vary in their behaviours and physical characteristics. They use various types of signals to obtain information about their environment including position, distance, path, speed and nature of the potential prey (Simmons and Stein, 1980).
The bio-sonar pulses of micropteran species may differ among the different species within the same genus and these pulses are classified into 3-types. They are constant frequency (CF), frequency modulated (FM) and combined CF-FM.
CF pulses consist of a single frequency or tone, and are found among rhinolopids and some other rhinolophoid bats. Constant frequency pulses are longer (40 – 1000 ms duration) than frequency modulated pulses and are emitted through the nostrils. CF pulse repetition rates are less than 10 per second. FM pulses sweep downward and sound like chirps, observed among vespertilionids and some other bats. These pulses are short (1-5 ms duration) and are emitted through the mouth.
The pulse repetition rate varies from less than 10 per second at rest and above 100 per second when hunting. Combined CF-FM pulses consist of a long constant tone followed by a downward chirp. In many bats the tones are not pure but consist of a fundamental or first harmonic and several higher harmonics.
Except information about the target the bio-sonar signals can provide varied information with some remarkable details (Fig. 10.66). Doppler Shifts (changes in the frequency of the echo relative to the original signal) convey information not only about the relative velocity of a flying insect but also about its wing beat (Fig. 10.67).
The amplitude of the echo, combined with the delay, indicates the size of the target. The amplitudes of the component frequencies correspond to the size of various features of the target (Fig. 10.66).
Pinnae vary in shape among different micro chiropteran bats and play a major role in echolocation. The small-eared bats fly fast and emit loud sounds. The large-eared bats prey ground insects or on the vegetation using faint pulses.
The hunting strategies and behaviours of a bat species are directly related to the characteristics of its bio-sonar. The main components of the bio-sonar are reflected in the functional organisation of its auditory system. About sixty years ago the bio-sonar of bat was established first by D. R. Griffin and Galambos.
Then several neuroethologists have worked on the auditory system of several bat species but most of all in the little brown bat (Myotis lucifuqus), the mustached bat (Pteronotus parnellii) and the horse-shoe bat (Rhinolophus ferrumequinum).
Each species produces different bio-sonar pulses. The auditory mechanisms of the mustached bats have been thoroughly studied by A. Novick and his co-workers in 1964 and 1972 first, respectively.
A flying mustached bat detects the relative velocity of objects by the Doppler shift in the echoes. When a bat flies toward a stationary object, the pulses that strike and are reflected by it become compressed or Doppler shifted.
The echo received by the bat is, therefore, uniformly higher in frequency than the emitted pulse. When the bat flies toward a flying insect, the insects beating wings introduce oscillating frequency shifts, which are superposed on the overall Doppler shift.
Some specializations are observed in the bat’s inner ear or cochlea for this mechanism. The cochlea contains the basilar membrane, a thin elongated sheet curled up like a snail. When sound waves vibrate the eardrum, this vibration is conducted to the basilar membrane, stimulating tiny hair cells on the membrane. The excitation is transmitted, via the spiral ganglion cells, along the auditory nerve fibres to the brain.