The echolocation sounds of bats

Whereas owls locate prey by listening passively to the noises produced by the prey, insectivorous bats actively interrogate the environment using the technique of echolocation (Fig. 6.7). For this purpose, a flying bat produces a succession of loud calls, each of which consists of a brief pulse of sound. These sounds are often described as ultrasonic because they contain frequencies (20-200 kHz) beyond the range of human hearing. The sound pulses travel out in front of the bat and, when they encounter a target, are reflected back and picked up by the ears. The essence of echolocation lies

The Purpose The Echolocation

Figure 6.7 A greater horseshoe bat (Rhinolophus ferrumequinum) about to capture a moth in flight. The complex structure between the ears and the mouth is the nose-leaf, through which echolocation sounds are emitted in horseshoe bats. Insects are usually captured by being scooped up in the wing membrane rather than being seized in the mouth. (After a photograph by S. Dalton.)

Figure 6.7 A greater horseshoe bat (Rhinolophus ferrumequinum) about to capture a moth in flight. The complex structure between the ears and the mouth is the nose-leaf, through which echolocation sounds are emitted in horseshoe bats. Insects are usually captured by being scooped up in the wing membrane rather than being seized in the mouth. (After a photograph by S. Dalton.)

in the ability of the brain to reconstruct features of the target, most importantly its position in space, by comparing the neural representation of the echo with that of the original signal.

Analysis of the sound pulses used for echolocation reveals what is at first sight a bewildering variety of form as one compares one species with another, but it has become clear that there are basically only two kinds of sound signal used by bats. The first kind are broadband signals, which consist of short pulses, less than 5 ms in duration, that are frequency modulated (FM). An example of this kind of signal is found in the mouse-eared bat (Myotis) in the widespread family Vespertilionidae: each pulse starts at a high frequency and sweeps downwards in frequency during the course of the pulse (Fig. 6.8a, b). At any given instant, the sound within the pulse is a fairly pure tone, corresponding to the fundamental frequency generated by the larynx, with traces of a second harmonic towards the end of the pulse.

Sound Pulse Bats

Figure 6.8 Echolocation sounds of the mouse-eared bat (Myotis myotis). (a) A single frequency-modulated pulse, recorded from a bat in flight (left) and displayed on an oscilloscope (right). (b) Computer-generated sonagram of a single pulse, showing the downward sweep in frequency in more detail. The computer generates the sonagram by plotting curves of the relative intensity of different frequencies at successive intervals of time during the sound pulse. This pulse was emitted by the bat at a distance of 4 m from the target; (c) and (d) are sonagrams of pulses emitted respectively at 36 cm and at 7 cm from the target. (a modified after Sales & Pye, 1974; b-d from Habersetzer & Vogler, 1983.)

Figure 6.8 Echolocation sounds of the mouse-eared bat (Myotis myotis). (a) A single frequency-modulated pulse, recorded from a bat in flight (left) and displayed on an oscilloscope (right). (b) Computer-generated sonagram of a single pulse, showing the downward sweep in frequency in more detail. The computer generates the sonagram by plotting curves of the relative intensity of different frequencies at successive intervals of time during the sound pulse. This pulse was emitted by the bat at a distance of 4 m from the target; (c) and (d) are sonagrams of pulses emitted respectively at 36 cm and at 7 cm from the target. (a modified after Sales & Pye, 1974; b-d from Habersetzer & Vogler, 1983.)

But the downward sweep results in the pulse having a total bandwidth of some 60-70 kHz.

Frequency-modulated pulses appear to have evolved in bat echolocation because the wide range of frequencies makes them suitable for target description and accurate ranging. Behavioural tests show that vespertili-onid bats can discriminate between targets that differ only in distance (range) with an accuracy of 10-15 mm. When these bats are tested with a pair of loudspeakers, each producing an electronically synthesised 'echo' after each echolocation pulse, they can discriminate differences between the speakers down to about 60 ^s, which corresponds to a difference in target range of about 10 mm. Hence, it seems probable that bats estimate target range from the time it takes sound pulses to travel out to the target and return as echoes, just as radar sets do. This is confirmed by studies of the brain, which show that bats take advantage of FM signals for target ranging by making multiple estimates of pulse-echo delay with interneu-rons tuned to different frequencies within the FM sweep (see section 6.8).

The second basic kind of echolocation sound used by bats consists of narrow band signals, in which the sound has a constant frequency (CF). These signals are generally longer, between 10 and 100 ms in duration, and form part of an alternative strategy of echolocation employed by many species. This alternative is particularly well developed in the horseshoe bats, which are members of the specialised family Rhinolophidae (see Fig. 6.7). The echolocation sound of the greater horseshoe bat (Rhinolophus fer-rumequinum) consists mainly of a long component (about 60 ms) with a constant frequency of just over 80 kHz, which is followed by a brief downward frequency-modulated sweep and is often preceded by an even briefer upward sweep (Fig. 6.9).

A long CF signal is unsuitable for target description but is well suited to measuring the Doppler shift, which is the shift in sound frequency experienced by an observer listening to a moving sound source such as a passing train. That horseshoe bats actually perceive the Doppler shifts generated during echolocation is shown clearly by the way they modify their sounds in flight. If a horseshoe bat is trained to fly down a long room to a landing platform, it is observed to alter the sound frequency of its echolocation pulses so as to keep the Doppler-shifted echoes from the landing platform at a constant, species-specific frequency, which is 83 kHz for the greater horseshoe bat.

In human affairs, measurement of the Doppler shift in a radar signal provides an accurate estimate of the relative velocity of a moving target; radar measurement of motorists' speed is perhaps the most familiar example. Similarly, bats using a long CF signal are able to determine the

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Figure 6.9 Echolocation sound of the greater horseshoe bat (Rhinolophus ferrumequinum). The computer-generated sonagram shows the three components of the long pulse: the initial, upward sweep in frequency; part of the long constant-frequency component (note breaks in time axis); and the final, downward sweep in frequency. The faint traces of the fundamental frequency at around 40 kHz indicate that the broadcast frequency is actually the second harmonic. (From Neuweiler, Bruns & Schuller, 1980.)

relative velocity of their prey by comparing the frequency of the outgoing pulse with that of the returning echo. In addition, it has been found that horseshoe bats are able to perceive the relatively small Doppler shifts in echo frequency produced by the beating wings of a flying insect. They use this as a means of detecting insect prey in the face of the extensive echo clutter produced by dense foliage or other background objects (see section 6.9).

Constant frequency and frequency-modulated sound pulses thus represent two different strategies for extracting information from the environment by means of echolocation. It is evident that Rhinolophus depends largely on the former, whereas Myotis employs the latter exclusively. However, there are a number of other genera that employ a mixture of the two strategies and emit pulses in which both CF and FM components are well developed. One example that has been the subject of a detailed neuro-ethological study is the moustached bat (Pteronotus) in the family Mormoopidae.

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