Echolocating Big Brown Bats, Eptesicus fuscus, Modulate Pulse Intervals to Overcome Range Ambiguity in Cluttered Surroundings

Abstract

Big brown bats (Eptesicus fuscus) emit trains of brief, wideband frequency-modulated (FM) echolocation sounds and use echoes of these sounds to orient, find insects, and guide flight through vegetation. They are observed to emit sounds that alternate between short and long inter-pulse intervals (IPIs), forming sonar sound groups. The occurrence of these strobe groups has been linked to flight in cluttered acoustic environments, but how exactly bats use sonar sound groups to orient and navigate is still a mystery. Here, the production of sound groups during clutter navigation was examined. Controlled flight experiments were conducted where the proximity of the nearest obstacles was systematically decreased while the extended scene was kept constant. Four bats flew along a corridor of varying widths (100, 70, and 40 cm) bounded by rows of vertically hanging plastic chains while in-flight echolocation calls were recorded. Bats shortened their IPIs for more rapid spatial sampling and also grouped their sounds more tightly when flying in narrower corridors. Bats emitted echolocation calls with progressively shorter IPIs over the course of a flight, and began their flights by emitting shorter starting IPI calls when clutter was denser. The percentage of sound groups containing 3 or more calls increased with increasing clutter proximity. Moreover, IPI sequences having internal structure become more pronounced when corridor width narrows. A novel metric for analyzing the temporal organization of sound sequences was developed, and the results indicate that the time interval between echolocation calls depends heavily on the preceding time interval. The occurrence of specific IPI patterns were dependent upon clutter, which suggests that sonar sound grouping may be an adaptive strategy for coping with pulse-echo ambiguity in cluttered surroundings.

Adaptive sonar call timing supports target tracking in echolocating bats

“…Bats dynamically adapt sonar call parameters to extract task-relevant echo information from the environment, and the temporal patterning of sonar calls (sonar sound groups, SSGs) is a component of the bat's adaptive vocal behavior. The production of SSGs has been recorded in echolocating bats in the field and in the laboratory Surlykke, 2001, 2010;Moss et al, 2006;Petrites et al, 2009;Surlykke et al, 2009;Aytekin et al, 2010;Hiryu et al, 2010;Falk et al, 2014;Kothari et al, 2014;Sändig et al, 2014;Wheeler et al, 2016). Insectivorous bats typically track moving prey, often with erratic trajectories, and in this study we show that bats increase the production of SSGs when target motion is unpredictable.…”

“…A related finding, reported by Sändig et al (2014), showed that bats performing a wire-avoidance task increased the production of SSGs with increasing task difficulty. In a more recent study, Wheeler et al (2016) reported that big brown bats not only increased the number of SSGs, but also the number of sonar vocalizations contained in each SSG, as they encountered greater clutter along their flight path. These observations support the hypothesis that the bat's production of SSGs serves to improve its spatiotemporal resolution of objects (targets or obstacles) in the environment.…”

“…Insectivorous bats reduce the interval between sonar calls as the distance to prey decreases Kalko and Schnitzler, 1989;Moss and Surlykke, 2001), and embedded in foraging call sequences are sonar sound groups (SSGs), which are defined as clusters of echolocation signals at comparatively short and stable pulse intervals, flanked by signals at longer intervals (Moss and Surlykke, 2001;Moss et al, 2006;Kothari et al, 2014). The production of SSGs is an adaptive sonar behavior that has been reported in both laboratory and field studies of bat echolocation, in free-flying animals (Surlykke and Moss, 2000;Moss and Surlykke, 2001;Moss et al, 2006;Petrites et al, 2009;Kothari et al, 2014;Sändig et al, 2014;Falk et al, 2015;Wheeler et al, 2016) and those tracking moving prey from a stationary position (Aytekin et al, 2010;Kothari et al, 2014). Past studies have shown that bats temporally cluster echolocation calls to produce SSGs when they are engaged in tasks that require higher spatiotemporal resolution.…”

Dynamic representation of 3D auditory space in the midbrain of the free-flying echolocating bat

“…In flight, bats displayed natural adaptations in sonar behavior (Griffin, 1958;Simmons et al, 1979). Specifically, they increased echolocation pulse rate (PR) and decreased pulse duration (PD) as they approached objects or their landing points ( Figure 1E), and they also produced sonar sound groups (SSGs), or clusters of vocalizations, to inspect objects in space (Falk et al, 2014;Moss et al, 2006;Petrites et al, 2009;Sändig et al, 2014;Wheeler et al, 2016).…”

Adaptations in the echolocation behavior of fruit-eating bats when orienting under challenging conditions

“…This makes the acquisition rate highly accessible and allows us to answer questions on how bats adapt their sensory acquisition rate when orienting under different conditions. Many behavioral studies showed that bats often pattern their echolocation calls in form of groups (Amichai et al, 2015; Brinklov et al, 2011; Brinklov et al, 2009; Galambos and Griffin, 1942; Grinnell and Griffin, 1958; Kothari et al, 2018a; Luo et al, 2015; Roverud and Grinnell, 1985a; Roverud and Grinnell, 1985b; Wheeler et al, 2016; Wohlgemuth et al, 2016; Figure 1). Results from insectivorous bats led to the hypothesis that the emission of call groups may represent an adaptation to orient in complex environments (Falk et al, 2014; Fawcett et al, 2015; Kothari et al, 2014; Moss et al, 2006; Petrites et al, 2009; Sändig et al, 2014; Surlykke et al, 2009).…”