Echolocating bats produce sounds above the human range of hearing and use the echoes of these ultrasonic, high frequency calls to locate objects and prey.

The human ear is capable of hearing sounds ranging up to 20 kHz, while most bats use a broader range (e.g. in Europe from 18-115 kHz). With the aid of a bat detector it is possible to hear them echolocate Nyctalus noctula (detector set to 25kHz) and with some practise it is possible to hear differences between species or families.

Myotis
( Myotis daubentonii detector set to 45kHz)

Pipistellus
(Pipistellus pipistrellus/pygmaeus detector set to 55kHz)

Rhinolophus
( Rhinolophus hipposideros detector set to 105kHz)

Echolocation, forms the sonar "sight" of bats, it is also called "biosonar". Not only bats but even some birds, toothed whales, porpoises and some species of shrews developed this sense.

When a bat begins to echolocate it usually produces short millisecond long pulses of sonar and listens to the returning echoes. These echo pulses are not just reflected back 'as is', there is some compression of the sound waves when it bounces off an object, which causes them to reflect back at a higher frequency. Based on the energy and frequency reflected from the pulse, bats get information about the position, distance, size, shape, surface structure and velocity of the target.

If prey is detected by the bat, it will generally fly towards the source of the echo continuing to emit sounds and focus in more accurately on the prey. As the bat gets closer and closer to the target, the sonar pulses are emitted faster with a shorter duration, also called, a "feeding buzz" (N. noctula sonogram 1).

The overview pictures in this programme show a fascinating variation of ear and nostril design. They illustrate the differences in sound-collecting ability of different species and therefore the variety of hunting and eating habits mentioned earlier.

Some species of bats emit calls from the mouth and other species through their nostrils. Nasal emitters fly with their mouths closed, oral emitters fly with their mouths open, photographs of these latter species showing the sharp teeth, while they are echolocating, wrongly give them a fierce and aggressive look.

Hipposideridae and Rhinolophus bats are very specialised nasal emitters who use calls with a constant frequency (CF) (R. ferrumequinum sonogram 1)
and are able to detect differences between prey and background using the Doppler effect (a phenomenon produced as objects move towards or away from each other - as a train nears the tone rises, and lowers when it passes).

Most other bat calls usually start at one frequency and sweep down to another (frequency modulated, or FM calls). In some cases, bat calls have both a CF and an FM component. The FM portion of a call provides a bat with information about the texture of an insect target and its position in horizontal and vertical space, while the CF component relays information about the insect's velocity. Harmonics or overtones, which are multiples of the sound frequencies used by the bat, further assist in pinpointing the insect's location (N. noctula harmonics).

(after Fenton, 1992, 1995)

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"The recordings were made in Europe not India so we don't know for certain that they reflect the echolocation pulses of Indian individuals!"

Although little to date has been studied of the echolocation calls of Indian bats it is a field of research that offers immense possibilities for the non-invasive study of bat diversity, distribution, behaviour and population status.

Jones et al., 1994; Roberts, 1975; Rydell and Arlettaz, 1994; Surlykke et al., 1993.
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Recordings used in this topic, courtesy of:
-The Bat Conservation Trust, London.
http://www.bats.org.uk

Sonograms were made of time expanded recordings, courtesy of the:
- Bat Ecology & Bioacoustic Laboratory, School of Biological Sciences, University of Bristol.
http://www.bio.bris.ac.uk/research/bats/batpage.htm

See their Web pages for more information on echolocation.