Doppler shifts in bat biosonar: The good, the bad, and the unexpected

Abstract

Most models for explaining bat biosonar are based on linear systems theory. A notable exception are frequency shifts due to the Doppler effect that occur whenever there is relative motion between the wave emitter and receiver. Bat species such as horseshoe bats, Old World leaf-nosed bats, and mustached bats have evolved particularly sophisticated biosonar systems that are able to exploit Doppler shifts for detecting prey in clutter. In this case, “good Doppler shifts” induced by the wing beat velocity of an insect prey give the prey echoes unique signatures that stand out among echoes from stationary targets such as reflecting facets in dense vegetation. Since these “good Doppler shifts” are small, bats exploiting them had to evolve specialized biosonar mechanisms to detect them. However, these mechanism are also sensitive to “bad Doppler shifts” that result, e.g,. from a bat's own flight velocity. It has been generally assumed that bats with Doppler-based biosonar will eliminate such nuisance Doppler shifts through compensation behaviors to allow them to detect the good Doppler shifts. However, a complete picture of Doppler shifts in bat biosonar systems may also have to account for other sources of Doppler shifts that do not fall into the above categories.