A bat under field conditions however first reacts to a flying insect at a much lesser range, although this may represent a critical reaction distance rather than a detection threshold. Experiments performed on Eptesicus fuscus suggest it can detect a 2cm diameter sphere at a distance of 5m 4. The search phase is characterised by a slow repetition rate (“low duty cycle”) of about 10s -1. Three phases have been identified in the standard routine of bats during prey interception, obstacle avoidance and landing: search, approach and terminal. Signal design is thus tailored to the local ecology and information requirements of the hunting bat. CF bats can however compensate in cluttered situations by relying on the motion of a moth through successive echoes 5. The greater detection range of CF pulses is only an asset in open areas, whilst the target-discrimination ability of FM sweeps is preferable in crowded, acoustically cluttered environments, such as around vegetation or the ground. The bandwidth of FM sweeps (especially with harmonics) thus aids target description and accurate ranging, whilst the forte of CF tones is in assessing relative velocities and initial prey detection, both through its greater detection range and sensitivity to periodic intensity fluctuation and Doppler shift. FM sweeps also allow many estimates of the pulse-echo delay, and hence target range, from the same pulse. Surface textures imprint on the echo a characteristic “colour”, and so FM sweeps are potentially very good at target description. During the beat cycle the wing surfaces regularly approach and recede from the bat, producing a frequency up-shift and down-shift respectively, as well as intensity variations the periodicity of which is neurally computed. The former allows the bat to adjust its attacking run or gently approach a perch, whilst the later aids discrimination of insect echoes from inedible clutter such as leaves. They are also suitable for measuring the Doppler shift of the returned echo which can potentially provide two pieces of information about the target: the approach speed (steady shift), and in the case of insect prey, the frequency of wing flapping (periodic shift). CF signals are well suited for long-range detection of objects, as pure tones carry more energy than FM sweeps and consequently propagate further before attenuating below the auditory threshold. The most prominent frequency and the signal signature varies between species, and may consist of either an FM or CF pulse, or a combination of the two.
(b) FM/long CF/FM echolocation signature of Rhinolophus hipposideros. Two basic acoustic forms are used: broadband frequency modulated (FM) sweeps of about 5ms duration and longer narrowband constant frequency (CF) signals, as seen in Figure 1.įigure 1. (a) downward FM sweep with strong harmonics of Pipistrellus pipistrellus. The second half deals with the question of a bat-moth co-evolutionary arms race.īat echolocation functions by emitting ultrasonic-frequency (20-200KHz 4) pulses focused, often by specialised nasal structures, into a highly directional, narrow (60°) cone. This review covers firstly bat call design and hunting behaviour, and the neural basis of their capabilities. Much research has been conducted into the neurophysiology of echolocation with its superb acoustic discrimination capabilities, including very fine temporal and frequency resolution.
Most are insectivorous and hunt with aerial-hawking or gleaning tactics, although some, such as Noctilio leporinus, predate on fish and use echolocation to detect localised surface ripples. The suborder Microchiroptera contains approximately 800 species of bats with a sophisticated biosonar system used to actively interrogate the local environment and analyse the retuned echoes to extract information on obstacles and potential prey in the flight path.
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