Spike Train Similarity Space (SSIMS) Method Detects Effects of Obstacle Proximity and Experience on Temporal Patterning of Bat Biosonar

High duty cycle moth sounds jam bat echolocation: bats counter with compensatory changes in buzz duration

Tiger moth species vary greatly in the number of clicks they produce and the resultant duty cycle. Signals with higher duty cycles are expected to more effectively interfere with bat sonar. However, little is known about the minimum duty cycle of tiger moth signals for sonar jamming. Is there a threshold that allows us to classify moths as acoustically aposematic versus sonar jammers based on their duty cycles? We performed playback experiments with three wild-caught adult male bats, Eptesicus fuscus. Bat attacks on tethered moths were challenged using acoustic signals of Bertholdia trigona with modified duty cycles ranging from 0 to 46%. We did not find evidence for a duty cycle threshold; rather, the ability to jam the bat's sonar was a continuous function of duty cycle consistent with a steady increase in the number of clicks arriving during a critical signal processing time window just prior to the arrival of an echo. The proportion of successful captures significantly decreased as the moth duty cycle increased. Our findings suggest that moths cannot be unambiguously classified as acoustically aposematic or sonar jammers based solely on duty cycle. Bats appear to compensate for sonar jamming by lengthening the duration of their terminal buzz and they are more successful in capturing moths when they do so. In contrast to previous findings for bats performing difficult spatial tasks, the number of sonar sound groups decreased in response to high duty cycles and did not affect capture success.

Spatiotemporal patterning of acoustic gaze in echolocating bats navigating gaps in clutter

We challenged four big brown bats to maneuver through abrupt turns in narrow corridors surrounded by dense acoustic clutter. We quantified bats' performance, sonar beam focus, and sensory acquisition rate. Performance was excellent in straight corridors, with sonar beam aim deviating less than 5° from the corridor midline. Bats anticipated an upcoming abrupt turn to the right or left by slowing flight speed and shifting beam aim to "look" proactively into one side of the corridor to identify the new flightpath. All bats mastered the right turn, but two bats consistently failed the left turn. Bats increased their sensory acquisition rate when confronting abrupt turns in both successful and failed flights. Limitations on biosonar performance reflected failures to switch beam aim and to modify a learned spatial map, rather than failures to update acquisition rate.

Modulation of acoustic navigation behaviour by spatial learning in the echolocating bat Rhinolophus ferrumequinum nippon

Using echolocation, bats receive acoustic information on their surroundings, which is assumed to help them sophisticatedly navigate complex environments. In this study, to understand spatial learning and acoustic sensing in bats, we investigated how flight and echolocation control changed in Rhinolophus ferrumequinum nippon as they learnt about their surroundings in an obstacle course that they flew through repeatedly. In these experiments, two testing environments (acoustically permeable and acoustically reflective) were prepared using chains and acrylic boards as obstacles to evaluate the interactive effects of spatial learning and flight environments. We found that bats reduced the meandering width of their flights and pulse emissions, and also seemed to reduce their shifts in pulse direction as they learnt more about their environments in both conditions. Throughout all our experiments, the bats with slower flight speeds tended to emit more pulses, which suggests that the number of pulse emissions reflects the echolocation tactics of each bat. The maximum flight speed was especially increased in the acoustically permeable condition, with frequent emissions of multiple pulses (≧triplets) in the early stages of flight, suggesting that bats adjust their flight plan based on how much of their surroundings they are able to sense in advance.

Big brown bats are challenged by acoustically-guided flights through a circular tunnel of hoops

Mines and caves provide essential roosting places for bats, but often they are obstructed to prevent entry by humans. To allow bats to access their roosts, metal corrugated culvert pipes are sometimes installed. Wildlife surveys indicate, however, that bats may abandon caves having corrugated culvert entrances. Culverts may be confusing to bats due to the complex patterns of echoes returned by the regular, ring-like corrugations. We tested the hypothesis that a circular tunnel composed of successive hoops is difficult for big brown bats (Eptesicus fuscus) to navigate. Experiments challenged bats with flights through a tunnel of round plastic hoops or a corridor flanked left and right by rows of plastic hanging chains. The bats swerved sideways and left the pathway on more flights in the hoop tunnel compared to only rarely in the chain corridor. Even during successful flights through the hoops, bats changed the temporal patterning of their echolocation pulses to compress them into more sonar sound groups. From prior research, this active reaction is an indicator of a perceptually more difficult task. To allow bats access to mines through culverts without affecting their echolocation behavior, smoothing or masking the regular corrugations inside with concrete may be effective.

Adaptations in the call emission pattern of frugivorous bats when orienting under challenging conditions

Echolocating bats emit biosonar calls and use echoes arising from call reflections, for orientation. They often pattern their calls into groups which increases the rate of sensory feedback. Insectivorous bats emit call groups at a higher rate when orienting in cluttered compared to uncluttered environments. Frugivorous bats increase the rate of call group emission when they echolocate in noisy environments. In frugivorous bats, it remains unclear if call group emission represents an exclusive adaptation to avoid acoustic interference by signals of conspecifics or if it represents an adaptation that allows to orient under demanding environmental conditions. Here, we compared the emission pattern of the frugivorous bat Carolliaperspicillata when the bats were flying in narrow versus wide or cluttered versus non-cluttered corridors. The bats emitted larger call groups and they increased the call rate within call groups when navigating in narrow or cluttered environments. These adaptations resemble the ones shown when the bats navigate in noisy environments. Thus, call group emission represents an adaptive behavior when the bats orient in complex environments.

Biosonar interpulse intervals and pulse-echo ambiguity in four species of echolocating bats

In complex biosonar scenes, the delay of echoes represents the spatial distribution of objects in depth. To avoid overlap of echo streams from successive broadcasts, individual echolocation sounds should only be emitted after all echoes of previous sounds have returned. However, close proximity of obstacles demands rapid pulse updates for steering to avoid collisions, which often means emitting a new sound before all of the previous echoes have returned. When two echo streams overlap, there is ambiguity about assigning echoes to the corresponding broadcasts. In laboratory tests of flight in dense, cluttered scenes, four species of echolocating bats exhibited different patterns of pulse emissions to accommodate potential pulse-echo ambiguity. Miniopterus fuliginosus emitted individual FM pulses only after all echoes of previous pulses had returned, with no alternating between long and short intervals. Pipistrellus abramus and Eptesicus fuscus alternated between emitting long FM pulse intervals to receive all echoes before the next pulse, and short intervals to update the rapidly changing scene while accepting partial overlap of successive echo streams. Rhinolophus ferrumequinum nippon transmitted CF/FM pulses in alternating short and long intervals, usually two to four closely spaced sounds that produced overlapping echo streams, followed by a longer interval that separated echo streams. Rhinolophus f. nippon is a statistical outlier from the three FM species, which are more similar to each other. The repeated overlap of CF/FM echo streams suggests that CF components have a distinct role in rejection of clutter and mitigation of ambiguity.