"Seeing" in the dark.

Echo-Location Details.

Although bats have quite good eyesight, they rarely use it for hunting their prey after dark. Instead they use a very sophisticated system of Echo-location, not dissimilar in principle to that used by submarines.

To do this, they emit a strong ultrasonic signal which bounces off any object in their path.The reflected signal travels back to their very sensitive ears. They are then able to build up an 'audio' image of the space surrounding them. Different species have different characteristics to their echo-location signals. This helps us to identify the type of bat we may be observing, much as we may recognise various birds species by their song. If you are close to a group of bats, you may be able to hear their high pitched voices. What you are hearing is not echo-location signals, which are well out of the range of human ears, but social communication within the group.

The emitted signals consist of a series of short pulses which are in the frequency range 20 to 130 kHz. The pulse characteristics vary with the type of bat, and their immediate surroundings. In general, bats which hunt in open spaces tend to emit loud pulses of sound at a lower pulse repetition rate, while bats which fly in confined spaces have a much faster pulse rate, and weaker output. Pulse rates dramatically increase when closing in on prey.   (More information on this later).

In order for us to be able to hear, and analize these ultrasonic signals we need to convert them to much lower frequencies to bring them within the range of human hearing. To do this we use one of a variety of Bat Detectors.

Detailed analysis of echo-location calls.

The diagram shows a variety of echo-location signals in graphical form. Each pulse in a sequence, may be stretched out in time, and would have a form similar to one of those shown.

The graph shows each type of pulse, with frequency (Kilo-Hertz) on the vertical (Y) axis, plotted against time (milli-seconds) on the horizontal (X) axis. They may be characterised as follows:-

Echo Location Signals

It is the frequency and characteristic sound of the the bats' echo-location signals which help us to distinguish one species from another. The process of learning all the different sounds takes a good deal of practice, but like many things, the more you practice, the better you get!

To add to the difficulty, these are not the only frequencies which the bats emit. Although most of the ultra-sound energy may be in the Fundamental frequency of the call, (with exceptions), there will be a considerable amount of energy emitted at twice the fundamental frequency. This is called the 2nd Harmonic. There may even be sound at three times the fundamental, this being 3rd Harmonic energy. It gets worse - some bats, are able to suppress the amount of energy in the Fundamental (1st Harmonic). Confusing isn't it!

Closing in on Prey.

As mentioned earlier on this page, the sound of each bat will vary with its situation. If, for instance, the bat is in a confined space, the pulse repetition rate of its echo location signals will increase to some degree. This is due to the fact that location information fed back to the animal must be up-dated more frequently, to prevent it from colliding with any nearby objects, hence the need for more pulses per second.

This increased pulse rate is probably most noticeable when the bat is homing in on its prey. If the signal is studied closely, a very short burst of fast pulses will be heard each time an attempt to catch prey is made. This gives a characteristic sort of "Zip" noise.

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