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Figure 35. Rhinolophus capensis in pursuit of a moth. The dashed lines depict the echoes perceived by the hunting bat.

      Not all vocalisations produced by bats are echolocation calls. For example, the audible squeaks bats make in their roosts, the calls that mother and young make to one another, or the calls flying bats make to defend their foraging territories, are not echolocation calls. Instead, these are usually referred to as social calls and are less well understood than echolocation calls.

      The echolocation frequencies of most bat species are ultrasonic (i.e. above the range of human hearing), and peak echolocation frequencies (i.e. the frequencies with the highest intensity) usually fall within 20–60 kHz (Fenton 1990). This may be due to the frequency-dependent effects of atmospheric attenuation and target strength (Jones and Rydell 2004). In contrast, many social calls are audible to humans.

      Echolocation coupled with flight enables bats to capture nocturnal flying insects in a variety of habitats. This ability probably explains how the radiation of bat species has manifested into the highest trophic diversity among mammals (Patterson et al. 2004, Roemer et al. 2019). Nevertheless, not all bats echolocate (e.g. fruit bats from the family Pteropodidae, except Rousettus species), nor do all echolocating bats use the same type of echolocation.

ECHOLOCATION SYSTEMS

      Two different echolocation systems – high and low duty-cycle echolocation – evolved independently in the Chiroptera (Eick et al. 2005). Low duty-cycle echolocation bats emit narrowband or broadband sound pulses separated by inter-pulse intervals that are much longer than the duration of the emitted pulses. Such species therefore separate the emitted pulse from the returning echo in time (Fenton 1990).

      High duty-cycle bats emit long, narrowband pulses that are separated by much shorter inter-pulse intervals. Consequently, the emitted signals often overlap with the returning echo. However, the overlap does not produce masking effects (Schnitzler and Kalko 2001), because Doppler-shift compensation keeps the target echo in the range of the neurons of the acoustic fovea – a group of neurons sharply tuned to a very narrow frequency band, a few kHz higher than the peak frequency of the emitted signal (Neuweiler 1990). The Doppler effect is the change in frequency of a sound wave as perceived by a listener moving relative to the source of the sound. For example, the frequency of the siren of a passing emergency vehicle will start out high, slide down as it passes by the listener, and continue lower as it recedes into the distance. In other words, high duty-cycle bats separate the emitted pulse from the returning echo in frequency rather than time (Fenton 1990).

TYPES OF BAT ECHOLOCATION PULSES

      Broadband, low duty-cycle, frequency-modulated (LD-FM) echolocation pulses typically sweep downward through as much as an octave for a short duration of time (Fenton 1990, Schnitzler and Kalko 2001) (Figure 36). At the same time, the bat senses with increasing precision the range and position of the object in space – its localisation (Simmons and Stein 1980). This is because LD-FM signals sweep rapidly through the corresponding neural filters, and are therefore reliable time markers to determine the range of the target from the foraging bat (Moss and Schnitzler 1995). In addition, the neuronal filters are activated across a broad frequency range, which increases the reliability of the monaural and binaural cues the bat uses to localise the target in space (Schnitzler and Kalko 2001). Nevertheless, LD-FM signals are less suited for the detection of distant and/or weak echoes, because the neuronal filters are activated for only a short time (Schnitzler and Kalko 2001).