Variability of the approach phase of landing echolocating Greater Mouse-eared bats

Wild bats briefly decouple sound production from wingbeats to increase sensory flow during prey captures

Active sensing animals such as echolocating bats produce the energy with which they probe their environment. The intense echolocation calls of bats are energetically expensive, but their cost can be reduced by synchronizing the exhalations needed to vocalize to wingbeats. Here, we use sound-and-movement recording tags to investigate how wild bats balance efficient sound production with information needs during foraging and navigation. We show that wild bats prioritize energy efficiency over sensory flow when periodic snapshots of the acoustic scene are sufficient during travel and search. Rapid calls during tracking and interception of close prey are decoupled from the wingbeat but are weaker and comprise <2% of all calls during a night of hunting. The limited use of fast sonar sampling provides bats with high information update rates during critical hunting moments but adds little to their overall costs of sound production despite the inefficiency of decoupling calls from wingbeats.

Reduction of emission level in approach signals of greater mouse-eared bats (Myotis myotis): No evidence for a closed loop control system for intensity compensation

Bats lower the emission SPL when approaching a target. The SPL reduction has been explained by intensity compensation which implies that bats adjust the emission SPL to perceive the retuning echoes at the same level. For a better understanding of this control mechanism we recorded the echolocation signals of four Myotis myotis with an onboard microphone when foraging in the passive mode for rustling mealworms offered in two feeding dishes with different target strength, and determined the reduction rate for the emission SPL and the increase rate for the SPL of the returning echoes. When approaching the dish with higher target strength bats started the reduction of the emission SPL at a larger reaction distance (1.05 ± 0.21 m) and approached it with a lower reduction rate of 7.2 dB/halving of distance (hd), thus producing a change of echo rate at the ears of + 4 dB/hd. At the weaker target reaction distance was shorter (0.71 ± 0.24 m) and the reduction rate (9.1 dB/hd) was higher, producing a change of echo rate of—1.2 dB/hd. Independent of dish type, bats lowered the emission SPL by about 26 dB on average. In one bat where the echo SPL from both targets could be measured, the reduction of emission SPL was triggered when the echo SPL surpassed a similar threshold value around 41–42 dB. Echo SPL was not adjusted at a constant value indicating that Myotis myotis and most likely all other bats do not use a closed loop system for intensity compensation when approaching a target of interest. We propose that bats lower the emission SPL to adjust the SPL of the perceived pulse-echo-pairs to the optimal auditory range for the processing of range information and hypothesize that bats use flow field information not only to control the reduction of the approach speed to the target but also to control the reduction of emission SPL.

Acoustic mirrors as sensory traps for bats

Sensory traps pose a considerable and often fatal risk for animals, leading them to misinterpret their environment. Bats predominantly rely on their echolocation system to forage, orientate, and navigate. We found that bats can mistake smooth, vertical surfaces as clear flight paths, repeatedly colliding with them, likely as a result of their acoustic mirror properties. The probability of collision is influenced by the number of echolocation calls and by the amount of time spent in front of the surface. The echolocation call analysis corroborates that bats perceive smooth, vertical surfaces as open flyways. Reporting on occurrences with different species in the wild, we argue that it is necessary to more closely monitor potentially dangerous locations with acoustic mirror properties (such as glass fronts) to assess the true frequency of fatalities around these sensory traps.

Description and clustering of echolocation signals of Commerson's dolphins (Cephalorhynchus commersonii) in Bahía San Julián, Argentina

Commerson's dolphins (Cephalorhynchus commersonii) inhabit coastal waters of Southern South America and Kerguelen Islands. Limited information exists about the acoustic repertoire of this species in the wild. Here, echolocation signals from free-ranging Commerson's dolphins were recorded in Bahía San Julián, Argentina. Signal parameters were calculated and a cluster analysis was made on 3180 regular clicks. Three clusters were obtained based on peak frequency (129, 137, and 173 kHz) and 3 dB bandwidth (8, 6, and 5 kHz). The 428 buzz clicks were analyzed separately. They consisted of clicks emitted with a median inter-click interval of 3.5 ms, peak frequency at 131 kHz, 3 dB bandwidth of 9 kHz, 10 dB bandwidth of 18 kHz, and duration of 56 μs. Buzz clicks were significantly shorter and with a lower peak frequency and a broader bandwidth than most of the regular clicks. This study provided the first description of different echolocation signals, including on- and off-axis signals, recorded from Commerson's dolphins in the wild, most likely as a result of animals at several distances and orientations to the recording device. This information could be useful while doing passive acoustic monitoring.

Fine-tuned echolocation and capture-flight of Myotis capaccinii when facing different-sized insect and fish prey

Formerly thought to be a strictly insectivorous trawling bat, recent studies have shown that Myotis capaccinii also preys on fish. To determine whether differences exist in bat flight behaviour, prey handling and echolocation characteristics when catching fish and insects of different size, we conducted a field experiment focused on the last stage of prey capture. We used synchronized video and ultrasound recordings to measure several flight and dip features as well as echolocation characteristics, focusing on terminal buzz phase I, characterized by a call rate exceeding 100 Hz, and buzz phase II, characterized by a drop in the fundamental well below 20 kHz and a repetition rate exceeding 150 Hz. When capturing insects, bats used both parts of the terminal phase to the same extent, and performed short and superficial drags on the water surface. In contrast, when preying on fish, buzz I was longer and buzz II shorter, and the bats made longer and deeper dips. These variations suggest that lengthening buzz I and shortening buzz II when fishing is beneficial, probably because buzz I gives better discrimination ability and the broader sonar beam provided by buzz II is useless when no evasive flight of the prey is expected. Additionally, bats continued emitting calls beyond the theoretical signal-overlap zone, suggesting that they might obtain information even when they have surpassed that threshold, at least initially. This study shows that M. capaccinii can regulate the temporal components of its feeding buzzes and modify prey capture technique according to the target.

Spatial variation in the echolocation calls of the little brown bat (Myotis lucifugus)

We studied spatial variation in echolocation call structure of the little brown bat (Myotis lucifugus (LeConte, 1831)) by analysing calls recorded from free-flying individuals at 1 site in Haida Gwaii, British Columbia, 1 site in Chautaqua, New York, and 20 sites along the Hudson River, New York. We controlled for factors that are often thought to lead to interspecific variation in echolocation calls (habitat, ontogeny, presence of conspecifics, recording techniques, ambient conditions), which allowed us to focus on the effect of spatial scale on call structure. As predicted, we found that at small scales (up to 1 km), there was significant geographic variation, likely owing to roost-specific signatures and group foraging activities. At intermediate scales (2–500 km), we found no differences in call structure, suggesting that populations within this area are part of a single hibernating and breeding population. Finally, echolocation call structure differed at the continental scale (>1000 km) likely because of little genetic exchange among sampled populations. Our results highlight the importance of considering the magnitude of spatial scale when examining variation in echolocation call structure.

Echolocating bats emit terminal phase buzz calls while drinking on the wing

Echolocating bats are known to produce terminal buzz calls during pursuit and capture of airborne prey, however the use of buzz calls while drinking on the wing has not been previously investigated. In this study I recorded the first empirical evidence that bats produce terminal phase buzz calls while drinking on the wing. Every drinking pass recorded during this study was characterised by a terminal buzz which bats emitted immediately prior to touching the water surface with their mouth. The characteristic frequency (the frequency at the end or flattest portion of the pulse) of echolocation call sequences containing drinking buzzes varied from 25kHz to 50kHz, suggesting multiple bat species present at the study site emit buzzes while drinking on the wing. As feeding buzz calls appear to be ubiquitous among echolocating bat taxa, the prevalence of drinking buzzes clearly warrants further investigation. Drinking buzzes could potentially be used to document rates of drinking by bats in the same way that feeding buzzes are used to infer foraging activity.

Coordination of bat sonar activity and flight for the exploration of three-dimensional objects

The unique combination of flight and echolocation has opened the nocturnal air space as a rich ecological niche for bats. By analysing echoes of their sonar emissions, bats discriminate and recognize three-dimensional (3-D) objects. However, in contrast to vision, the 3-D information that can be gained by ensonifying an object from only one observation angle is sparse. To date, it is unclear how bats synchronize echolocation and flight activity to explore the 3-D shape of ensonified objects. We have devised an experimental design that allows creating 3-D virtual echo-acoustic objects by generating in real-time echoes from the bat's emissions that depend on the bat's position relative to the virtual object. Bats were trained to evaluate these 3-D virtual objects differing in their azimuthal variation of either echo amplitude or spectral composition. The data show that through a very effective coordination of sonar and flight activity, bats analyse an azimuthal variation of echo amplitude with a resolution of approximately 16 dB and a variation of echo centre frequency of approximately 19%. Control experiments show that the bats can detect not only these variations but also perturbations in the spatial arrangement of these variations. The current experimental paradigm shows that echolocating bats assemble echo-acoustic object information – acquired sequentially in flight – to reconstruct the 3-D shape of the ensonified object. Unlike previous approaches, the recruitment of virtual objects allows for a direct quantification of this reconstruction success in a highly controlled experimental approach.

How greater mouse-eared bats deal with ambiguous echoic scenes

Echolocating bats have to assign the received echoes to the correct call that generated them. Failing to do so will result in the perception of virtual targets that are positioned where there is no actual target. The assignment of echoes to the emitted calls can be ambiguous especially if the pulse intervals between calls are short and kept constant. Here, we present first evidence that greater mouse-eared bats deal with ambiguity by changing the pulse interval more often, in particular by reducing the number of calls in the terminal group before landing. This strategy separates virtual targets from real ones according to their change in position. Real targets will always remain in a constant position, and virtual targets will jitter back and forth according to the change in the time interval.