Biosonar interpulse intervals and pulse-echo ambiguity in four species of echolocating bats.

In complex biosonar scenes, the delay of echoes represents the spatial distribution of objects in depth. To avoid overlap of echo streams from successive broadcasts, individual echolocation sounds should only be emitted after all echoes of previous sounds have returned. However, close proximity of obstacles demands rapid pulse updates for steering to avoid collisions, which often means emitting a new sound before all of the previous echoes have returned. When two echo streams overlap, there is ambiguity about assigning echoes to the corresponding broadcasts. In laboratory tests of flight in dense, cluttered scenes, four species of echolocating bats exhibited different patterns of pulse emissions to accommodate potential pulse-echo ambiguity. Miniopterus fuliginosus emitted individual FM pulses only after all echoes of previous pulses had returned, with no alternating between long and short intervals. Pipistrellus abramus and Eptesicus fuscus alternated between emitting long FM pulse intervals to receive all echoes before the next pulse, and short intervals to update the rapidly changing scene while accepting partial overlap of successive echo streams. Rhinolophus ferrumequinum nippon transmitted CF/FM pulses in alternating short and long intervals, usually two to four closely spaced sounds that produced overlapping echo streams, followed by a longer interval that separated echo streams. Rhinolophus f. nippon is a statistical outlier from the three FM species, which are more similar to each other. The repeated overlap of CF/FM echo streams suggests that CF components have a distinct role in rejection of clutter and mitigation of ambiguity.

Echolocating bats perceive natural-size targets as a unitary class using micro-spectral ripples in echoes.

Echolocating big brown bats emit frequency-modulated (FM) sounds covering ultrasonic frequencies in two harmonic sweeps (FM1 from 50-60 kHz sweeping down to 20-25 kHz, FM2 from 100-110 kHz sweeping down to 45 kHz). Using a complex interplay of acoustic cues, the bats perceive object distance from echo delay and object shape from echo spectra. Typical natural targets-flying insects-return discrete reflections, called glints, from prominent body parts (e.g., head, wings). Insect sizes are mostly 0.5 cm to about 3.5 cm, corresponding to reflected time separations of 30 to 210 µs. When closely spaced reflections overlap, they interfere to create a characteristic echo spectrum containing repetitive peaks and nulls. Time spacing of these glints (Δt) is transposed into spectral ripples at frequency intervals (Δf = 1/Δt) from about 30 kHz down to 5 kHz. The bat's perception of rippled echoes as a distinct class was tested in two-alternative forced-choice discrimination experiments with a standard stimulus of 100 µs glint separation (10 kHz ripples), in the middle of natural ripple separations (30 kHz to 5 kHz). Stimuli were electronically generated virtual targets with simulated 2-glint sizes from smaller than 0.5 cm to larger than 3.5 cm. Bats perceived insect-sized virtual objects (glint delays of 36-300 µs) as similar to the 100-µs standard, while smaller or larger virtual objects were perceived as not similar. Insect-sized echoes contained a pattern of microscale spectral ripples that stood apart from the coarser, macroscale spectral features of smaller objects or the temporally separate reflections of larger objects. (PsycINFO Database Record (c) 2019 APA, all rights reserved).