British Bats
Going in for the kill
How do bats use echolocation in practice to hunt? When a bat is travelling echolocation may not be used much, often there is enough light for a bat to orientates itself even at night. If it is travelling from its roost to its hunting ground it probably knows its route well and recognizes objects along the way, orientating itself with the use of hedgerows or other large immovable structures. Echolocation calls will be emitted at only a slow rate. As echolocation is only really useful at close range, it is a waste of time and energy producing calls to try to see further a field. When the bat starts hunting for prey the echolocation calls begin to be emitted more frequently and regularly, at a rate of 5 to 20 per second. This does not give the bat a very detailed picture of its surroundings but it does enable it to detect and search for prey. In bats the breathing, wing beating and emission of echolocation calls are all linked. Echolocation calls are expensive to produce, but if it produces them when it exhales it is much more efficient.
When the bat detects a possibly suitable prey insect it changes its call and enters what is known as the approach phase. In the approach phase the echolocation calls become shorter and much more frequent. This allows the bat to build up a very detailed picture of the insect it wants to catch, the bat is trying to gain as much information at this stage, the more the bat knows about its prey, the easier it will be to catch. In the terminal phase close to capture of the insect, the bat will be emitting echolocations at a rate of 200 per second. Then it swoops and catches the insect, sometimes in its mouth, but more usually in its tail membrane, where the bat can easily retrieve it by bending down. In many species of bats the frequency of calls the point of capture stop, this produces a 'feeding buzz', which can sometimes be heard by people, and is noticeable with a bat detector. Once the prey item has been eaten the bat returns to its hunting emitting calls regularly again.
Call intensity
The frequency of the calls is not their only property. They can also have a high intensity and be very loud or have a low intensity and be quiet. The intensity of bat echolocation calls varies between the species, but is usually quite loud. If the echolocation calls of bats were audible to man the night sky would be full of sounds somewhat similar to road drills! Maybe it is better we are unable to hear bat calls and at least have some peace and quiet! One exception to this is the long eared bats whose calls are more of a whisper and are much quieter.
CF and FM calls
There are basically two categories of echolocation calls. However most bats species actually use a mixture of both. Frequency modulated calls are short, but encompass many frequencies; they start at a relatively high frequency and run down to a lower one. FM calls provide detailed information both about potential targets, and are useful in complex cluttered environments. FM calls enable bats to discriminate between close objects well. The second types of ultrasonic call are constant frequency pulses. These are relatively long calls that have a constant or static range of frequencies. They do not normally occur alone but have frequency modulated components, normally tagged on to the end. When a bat, which uses this type of echolocation call, captures an insect the length of the call shortens, the CF part of the call, and the appended frequency modulated component may sweep through more frequencies. This provides the bat with the most information about the prey animal before it is captured. Constant frequency calls are probably used because it is more economical to put energy into a single frequency rather than into several. Constant frequency calls can also be used to exploit Doppler shift effect that allows target speed and direction to be discovered.
Different species of bats have different echolocation calls
Different species
of bat hunt in different kinds of way. Because of this, different species
have different kinds and types of echolocation calls. Certain echolocation
calls with different frequencies, intensities etc. are most suitable
for certain situations, and bats have evolved to use the call, which
will give them the most success in the habitat they utilize. For example
the Noctule bat is a fast flyer that hunts high above the tree canopy
away from obstructions and clutter. Its echolocation calls are low in
frequency, about 20 kHz and for the most part constant frequency. As
the Noctule uses low frequency calls, its calls travel further, which
is useful. If you are a fast flyer like the Noctule you want to be able
to 'see' well ahead. Because the call is constant frequency then the
Noctule can use special techniques so that it receives more information
from its calls, for example it can use Doppler shift. This is useful
because the more information you can gain about the insects you are
going to catch the more successful you will ultimately be at catching
them. Another species such as the Bechstein's Bat has calls with higher
frequencies than the Noctules. These calls are more FM calls, and sweep
down a long range of frequencies. The Bechstein's Bat hunts amongst
cluttered vegetation and the insects it hunts will mostly be at close
range. It therefore used the type of call most suitable for this habitat.
The high frequency gives the bat a high resolution at close distances,
and the FM sweep gives the bat much information about its immediate
environment. Ideal if you need to detect an insect on a leaf. However
bat calls should not be thought of as having a static structure in each
species. Bats can adapt and change their calls to a certain extent to
suit the conditions they are in at.