The way in which bats use their echolocation signals to track prey has been studied by combining high-speed photography with tape recordings of the normal sequence of airborne interception. These observations are necessarily made under controlled conditions but they have been supplemented by numerous, and increasingly exact, observations made in the field. All the species studied so far follow a fairly standard routine, which can be broadly divided into three stages, known as the search, approach and terminal stages. Much the same sequence is followed whether the bat is intercepting prey, avoiding an obstacle or landing on a perch.
During the search stage, a bat emits sound pulses of a constant, species-specific form at a low repetition rate of about 10 Hz or less, as described in the previous section. As its name implies, the main function of the search stage is to detect potential prey or obstacles. Behavioural tests with bats using FM signals show that they can detect a small sphere (0.5 cm diameter) at almost 3 m and a larger sphere (2 cm diameter) at over 5 m. Calculation of the intensity of echoes returning from targets at these distances suggests that the maximum range of echolocation corresponds with the threshold of hearing in bats.
However, in natural interception, bats do not visibly react to targets at these distances nor do they appear to react to progressively larger targets at progressively greater distances. Instead, the onset of the approach stage, which represents the first visible reaction of the bat to the target, occurs when the bat is between 1 and 2 m away in nearly all cases. Therefore, it is probable that the approach stage begins at a critical distance between the bat and the target and does not necessarily begin when the target is first detected. The transition to the approach stage is marked by the bat turning its head, especially its ears, directly towards the target and by an increase in the repetition rate of the echolocation sounds to a value of about 40 Hz. In bats such as Myotis, which use only FM pulses, the pulses become shorter but the slope of the FM sweep becomes steeper so that the bandwidth of the signal is maintained (see Fig. 6.8c). Species that use long CF pulses for Doppler shift echolocation do not drop the CF component during the search stage but it becomes shorter and the small FM component increases in bandwidth.
There is thus a shift towards brief, FM pulses at the approach stage in all species of echolocating bats. It is probable that bats are taking advantage of the greater information content of broadband signals as they approach the target, especially because the decision about whether or not to catch an item of potential prey is evidently made during the approach stage. When a mixture of living insects and similar-sized plastic discs or spheres is thrown into the air for bats to catch, the bats break off pursuit of the plastic objects at the end of the approach stage. Again, bats trained to discriminate between two targets in flight make their choice, as judged by the orientation of their ears, towards the end of the approach stage. The increase in repetition rate of the echolocation sounds is also understandable as a response to the need for increasingly frequent estimates of range and direction as the bat closes upon the target. When the approach stage goes to completion, it normally takes the bat to within 50 cm of the target.
The transition to the terminal stage is marked by an abrupt increase in pulse repetition rate, which rises to about 100 Hz or even 200 Hz in some cases. This increased rate clearly provides a rapid updating of information about the target's position as the bat makes its final manoeuvres to capture the target. In most species, the pulses emitted during the terminal stage are FM sweeps, often with several harmonics, that are 0.5 ms or less in duration (see Fig. 6.8d). Only in horseshoe bats, and others that exploit the Doppler shift, is a CF component retained during the terminal stage; even then, the CF component is reduced to a length of about 10 ms or less.
Bats do not usually capture flying insects in their mouths but rather use their outspread wing or tail membranes as a scoop for collecting the prey. The accuracy with which this is accomplished is illustrated by experiments in which horseshoe bats are fed on flour-covered mealworms thrown into the air. Flour marks are then found on the wing membrane at the base of the third to fifth digits, in an area with a diameter of 2 to 3 cm (cf. Fig. 6.7). This suggests that the bats are consistently able to localise prey in space to within about 1 cm3.
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