Abstract
Bats of the Rhinolophidae and Hipposideridae families, and Pteronotus parnellii, compensate for Doppler shifts generated by their own flight movement. They adjust their call frequency such that the frequency of echoes coming from ahead fall in a specialized frequency range of the hearing system, the auditory fovea, to evaluate amplitude and frequency modulations in echoes from fluttering prey. Some studies in hipposiderids have suggested a less sophisticated or incomplete Doppler shift compensation. To investigate the precision of Doppler shift compensation in Hipposideros armiger, we recorded the echolocation and flight behaviour of bats flying to a grid, reconstructed the flight path, measured the flight speed, calculated the echo frequency, and compared it with the resting frequency prior to each flight. Within each flight, the average echo frequency was kept constant with a standard deviation of 110 Hz, independent of the flight speed. The resting and reference frequency were coupled with an offset of 80 Hz; however, they varied slightly from flight to flight. The precision of Doppler shift compensation and the offset were similar to that seen in Rhinolophidae and P. parnellii. The described frequency variations may explain why it has been assumed that Doppler shift compensation in hipposiderids is incomplete.
Highlights
During the course of evolution, the echolocation systems of bats have adapted to deliver information necessary to successfully perform species-specific tasks
88% (HA 1) and 91% (HA 2) of the total signal duration was determined by the constant frequency (CF) component
Frest is determined by the average frequency of the CF component of the CF-frequency modulated (FM) signals in stationary bats, and Fref is measured in bats performing Doppler shift compensation (DSC) as the average frequency of the CF component in the echoes returning from ahead
Summary
During the course of evolution, the echolocation systems of bats have adapted to deliver information necessary to successfully perform species-specific tasks. Flutter detecting foragers have evolved a highly specialized echolocation system to find fluttering insect prey between background targets They emit signals which consist of a long constant frequency (CF) component followed by a shorter frequency modulated (FM) part (Fig. 1b). The afferent projections from the enlarged area on the basilar membrane lead to foveal areas in the brain with an overrepresentation of sharply tuned neurons with best frequencies around Fref (P. parnellii[31], R. ferrumequinum[32], H. speoris[33], H. armiger[34]) These neurons are very sensitive to amplitude and frequency modulations contained in the echoes from fluttering insects. Long CF-FM signals emitted at a high duty cycle, DSC, and an auditory fovea are adaptations for the detection and evaluation of echoes containing specific flutter information within unmodulated background echoes (reviewed in4)[35,36,37,38,39]
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