Acoustic flow velocity manipulations affect the flight velocity of free-ranging pipistrelle bats

Echolocating bats moving through natural habitats have to analyse many overlapping echoes that change dynamically in real-time. In increasingly complex natural scenarios, tracking individual echo sources becomes untenable, requiring alternative analytical approaches. Here, we test whether wild free-ranging bats use acoustic flow (analogous to optic flow) velocity for speed control. Acoustic flow presents a viable navigational concept in complex scenarios because indicators of acoustic flow velocity are accessible from single calls as temporal modulations in flight-induced Doppler shift in the habitat echo. Using a conveyor belt system of 8000 acoustic reflectors (artificial leaves) lining 4 m of a natural bat flight corridor, we tracked how bats adjusted their flight velocity to induced changes in acoustic flow velocity. When we increased acoustic flow velocity by moving reflectors against the bats' flight direction, bats flew significantly slower by up to 28% of the induced decrease and faster when reflectors moved in their flight direction. Bats adjusted their flight velocity as they approached the moving panels, matching the spatial pattern of increasing Doppler shift, suggesting this may be the parameter they are responding to. Navigation by Doppler acoustic flow can inspire future autonomous navigation techniques, mirroring optic flow.