Timing matters: sonar call groups facilitate target localization in bats

Abstract

To successfully negotiate a cluttered environment, an echolocating bat must control the timing of motor behaviors in response to dynamic sensory information. Here we detail the big brown bat's adaptive temporal control over sonar call production for tracking prey, moving predictably or unpredictably, under different experimental conditions. We studied the adaptive control of vocal-motor behaviors in free-flying big brown bats, Eptesicus fuscus, as they captured tethered and free-flying insects, in open and cluttered environments. We also studied adaptive sonar behavior in bats trained to track moving targets from a resting position. In each of these experiments, bats adjusted the features of their calls to separate target and clutter. Under many task conditions, flying bats produced prominent sonar sound groups identified as clusters of echolocation pulses with relatively stable intervals, surrounded by longer pulse intervals. In experiments where bats tracked approaching targets from a resting position, bats also produced sonar sound groups, and the prevalence of these sonar sound groups increased when motion of the target was unpredictable. We hypothesize that sonar sound groups produced during flight, and the sonar call doublets produced by a bat tracking a target from a resting position, help the animal resolve dynamic target location and represent the echo scene in greater detail. Collectively, our data reveal adaptive temporal control over sonar call production that allows the bat to negotiate a complex and dynamic environment.

Echolocation behavior in big brown bats is not impaired after intense broadband noise exposures

“…The types and proportions of sonar sound groups emitted by big brown bats also differ depending on experimental conditions and task difficulty (Moss et al, 2006;Petrites et al, 2009;Kothari et al, 2014;Sändig et al, 2014;Knowles et al, 2015;Warnecke et al, 2016), with more groups (pulses in doublets and triplets) in more difficult tasks. Navigating through a narrow corridor surrounded by high-density clutter would be expected to result in more doublets and triplets rather than singles.…”

“…Along with short-to-long staggering, the absolute lengths of IPIs vary with the acoustic complexity of the task the bat is asked to solve. Big brown bats flying in open space emit pulses with IPIs of around 175 ms, whereas bats flying in the laboratory near or through clutter emit pulses with IPIs ranging from 29 to 100 ms in different experimental conditions (Petrites et al, 2009;Kothari et al, 2014;Warnecke et al, 2016;Wheeler et al, 2016). Bats in our experiment flew down a narrow (40 cm) curved corridor through a high-density chain array.…”

“…First, we identified 'sonar sound groups' or 'strobe groups' (Moss et al, 2006;Petrites et al, 2009;Kothari et al, 2014), which are groups of pulses with short, stable IPIs surrounded by longer IPIs. Using the criteria formulated by Kothari et al (2014), sound groups were classified as singles, doublets, triplets or quadruplets based on an 'island' criterion (between-group IPIs at least 20% larger than within-group IPIs) and a 'stability' criterion (5% tolerance within a group, for groups of three or more pulses). Because the cut-offs for these island and stability criteria are arbitrary and may not capture sound group patterning in all echolocation tasks, we also used a second metric based on the ratio of the post-IPI to the pre-IPI for each pulse (Wheeler et al, 2016).…”

“…These bats dynamically change the duration, spectrum, directional aim, number and temporal patterning of their emitted pulses, and thus the acoustic sampling of the spatial scene, to reflect the complexity of their surroundings (Surlykke and Moss, 2000;Moss et al, 2006;Moss and Surlykke, 2010;Petrites et al, 2009). In particular, the time intervals [inter-pulse intervals (IPIs)] between the individual pulses in a train are known to vary with the perceived difficulty of the experimental task and according to the bat's individual strategy for navigating the scene (Surlykke and Moss, 2000;Petrites et al, 2009;Barchi et al, 2013;Falk et al, 2014;Kothari et al, 2014;Sändig et al, 2014;Knowles et al, 2015;Wheeler et al, 2016). Changes in IPIs thus provide one index of the bat's vocal adaptations to its surroundings, particularly when steering through and foraging within cluttered acoustic environments.…”

Tight coordination of aerial flight maneuvers and sonar call production in insectivorous bats

“…Bat echolocation sound groups, clusters of calls flanked by signals at longer intervals, are produced when animals encounter challenging sonar tasks requiring figure-ground segregation and accurate measurement of target position (Kothari et al, 2014;Moss et al, 2006). These sound groups are therefore hypothesized to support high-resolution sonar information (Moss and Surlykke, 2010).…”