Echo-acoustic flow affects flight in bats

Abstract

Flying animals need to react fast to rapid changes in their environment. Visually guided animals use optic flow, generated by their movement through structured environments. Nocturnal bats cannot make use of optic flow, but rely mostly on echolocation. Here, we show that bats exploit echo-acoustic flow to negotiate flight through narrow passages. Specifically, bats' flight between lateral structures is significantly affected by the echo-acoustic salience of those structures, independent of their physical distance. This is true even though echolocation, unlike vision, provides explicit distance cues. Moreover, the bats reduced the echolocation sound levels in stronger flow, probably to compensate for the increased summary target strength of the lateral reflectors. However, bats did not reduce flight velocity under stronger echo-acoustic flow. Our results demonstrate that sensory flow is a ubiquitous principle for flight guidance, independent of the fundamentally different peripheral representation of flow across the senses of vision and echolocation.

Echo-acoustic and optic flow interact in bats

“…Previous work (Kugler et al, 2016) examined the effect of echoacoustic flow on the flight and echolocation behaviour in free-flying bats of the species P. discolor. In the present study, we extended the experimental paradigm to quantify the behavioural response to optic flow and the interactions with echo-acoustic flow in the same bat species.…”

“…In our previous study (Kugler et al, 2016), bats were exposed to echo-acoustic flow created by structured wooden side walls, while flying from one end of a 5 m long and 1 m wide flight tunnel to the other in darkness. Each side wall could be set to create stronger (vertical orientation of ridges in the side wall) or weaker (horizontal ridge orientation) echo-acoustic flow (see Fig.…”

“…Experimental and theoretical work has provided evidence that bats respond to echo-acoustic flow (Kugler et al, 2016;Lee et al, 1992;Müller and Schnitzler, 1999;Warnecke et al, 2018aWarnecke et al, , 2016. Although explicit information about the distance of objects is provided by echolocation through the delay between call emission and echo arrival time (Simmons, 1971), echo-acoustic flow is used to adjust the lateral distance along structured surfaces (Kugler et al, 2016;Warnecke et al, 2016). This behaviour is often observed, for example, in bats following the edges of vegetation in commuting flight (Holderied et al, 2006).…”

“…For this experiment, the black walls were equipped with ridges that elicit echo-acoustic flow and perpendicularly oriented white stripes that elicit optic flow. Data from an earlier experiment (Kugler et al, 2016) were re-analysed to allow comparison with the echo-acoustic flow-only condition (tunnel walls with weak or strong echo-acoustic flow under complete darkness).…”

Segregating signal from noise through movement in echolocating bats

“…Bats that use constant-frequency echolocation use the prey's wing-movement in order to detect prey in cluttered (i.e., highly echoic) environments 37 . A few studies suggested that bats also move differently depending on the type of background in their environment 38,39 , and that they might alter movement in order to improve SNR 40,41 but so-far there was no consistent investigation of how bats use self-movement for segregating signal from noise and how movement based sensing interacts with adaptations of echolocation.…”