Energy compensation and received echo level dynamics in constant-frequency bats during active target approaches

This work was supported by the Semper Arden Carlsberg grant to P.T.M., by a National Science Foundation grant [1658620] to R.M. and by a National Natural Science Foundation of China [11574183] to R.M.Bats have been reported to adjust the energy of their outgoing vocalizations to target range (R) in a logarithmic fashion close to 20log10R which has been interpreted as providing one-way compensation for increasing echo levels during target approaches. However, it remains unknown how species using high-frequency calls, which are strongly affected by absorption, adjust their vocal outputs during approaches to point targets. We hypothesized that such species should compensate less than the 20log10R model predicts at longer distances and more at shorter distances as a consequence of the significant influence of absorption at longer ranges. Using a microphone array and an acoustic recording tag, we show that the output adjustments of two Hipposideros pratti and one Hipposideros armiger do not decrease logarithmically during approaches to different-sized targets. Consequently, received echo levels increase dramatically early in the approach phase with near-constant output levels, but level off late in the approach phase as a result of substantial output reductions. To improve echo-to-noise ratio, we suggest that bats using higher frequency vocalizations compensate less at longer ranges, where they are strongly affected by absorption. Close to the target, they decrease their output levels dramatically to mitigate reception of very high echo levels. This strategy maintains received echo levels between 6 and 40 dB re. 20 µPa2 s across different target sizes. The bats partially compensated for target size, but not in a one-to-one dB fashion, showing that these bats do not seek to stabilize perceived echo levels, but may instead use them to gauge target size.Publisher PDFPeer reviewe

Echolocation click source parameters of Australian snubfin dolphins (Orcaella heinsohni)

Author Posting. © Acoustical Society of America, 2018. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 143 (2018): 2564, doi: 10.1121/1.5034174.The Australian snubfin dolphin (Orcaella heinsohni) is endemic to Australian waters, yet little is known about its abundance and habitat use. To investigate the feasibility of Passive Acoustic Monitoring for snubfin dolphins, biosonar clicks were recorded in Cygnet Bay, Australia, using a four-element hydrophone array. Clicks had a mean source level of 200 ± 5 dB re 1 μPa pp, transmission directivity index of 24 dB, mean centroid frequency of 98 ± 9 kHz, and a root-mean-square bandwidth of 31 ± 3 kHz. Such properties lend themselves to passive acoustic monitoring, but are comparable to similarly-sized delphinids, thus requiring additional cues to discriminate between snubfins and sympatric species.We thank the Fitzroy Basin Association for funding fieldwork in Gladstone May 2013 as well as the Australian Marine Mammal Centre who funded J.N.S. with the Bill Dawbin Fellowship and provided fieldwork funding. P.T.M. was funded by a Sir Walter Murdoch Honorary Professorship from Murdoch University and frame grants from FNU. F.H.J. was supported by the office of naval research (N00014-1410410) and an AIAS-COFUND fellowship from Aarhus Institute of Advanced Studies, Aarhus University, under EU's FP7 programme (Agreement No. 609033)

Single-click beam patterns suggest dynamic changes to the field of view of echolocating Atlantic Spotted Dolphins (Stenella frontalis) in the wild

The study was funded by frame grants from the Danish Natural Science Foundation to P.T.M. and M.W., and by the National Oceanographic Partnership Programme via a research agreement between La Laguna University (N.A.d.S.) and the Woods Hole Oceanographic Institution (M.J.). F.H.J. was supported by the Danish Council for Independent Research | Natural Sciences, and is currently funded by a postdoctoral fellowship from the Carlsberg FoundationEcholocating animals exercise an extensive control over the spectral and temporal properties of their biosonar signals to facilitate perception of their actively generated auditory scene when homing in on prey. The intensity and directionality of the biosonar beam defines the field of view of echolocating animals by affecting the acoustic detection range and angular coverage. However, the spatial relationship between an echolocating predator and its prey changes rapidly, resulting in different biosonar requirements throughout prey pursuit and capture. Here, we measured single-click beam patterns using a parametric fit procedure to test whether free-ranging Atlantic spotted dolphins (Stenella frontalis) modify their biosonar beam width. We recorded echolocation clicks using a linear array of receivers and estimated the beam width of individual clicks using a parametric spectral fit, cross-validated with well-established composite beam pattern estimates. The dolphins apparently increased the biosonar beam width, to a large degree without changing the signal frequency, when they approached the recording array. This is comparable to bats that also expand their field of view during prey capture, but achieve this by decreasing biosonar frequency. This behaviour may serve to decrease the risk that rapid escape movements of prey take them outside the biosonar beam of the predator. It is likely that shared sensory requirements have resulted in bats and toothed whales expanding their acoustic field of view at close range to increase the likelihood of successfully acquiring prey using echolocation, representing a case of convergent evolution of echolocation behaviour between these two taxa.PostprintPeer reviewe

Hunting bats adjust their echolocation to receive weak prey echoes for clutter reduction

This study was funded by the Carlsberg Semper Ardens grant to P.T.M. and by the Emmy Noether program of the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation, grant no. 241711556) to H.R.G. All experiments were carried out under the following licenses: 721/12.06.2017, 180/07.08.2018, and 795/17.05.2019.How animals extract information from their surroundings to guide motor patterns is central to their survival. Here, we use echo-recording tags to show how wild hunting bats adjust their sensory strategies to their prey and natural environment. When searching, bats maximize the chances of detecting small prey by using large sensory volumes. During prey pursuit, they trade spatial for temporal information by reducing sensory volumes while increasing update rate and redundancy of their sensory scenes. These adjustments lead to very weak prey echoes that bats protect from interference by segregating prey sensory streams from the background using a combination of fast-acting sensory and motor strategies. Counterintuitively, these weak sensory scenes allow bats to be efficient hunters close to background clutter broadening the niches available to hunt for insects.Publisher PDFPeer reviewe

Open-source workflow approaches to passive acoustic monitoring of bats

The work was funded by grants to PTM from Carlsberg Semper Ardens Research Projects and the Independent Research Fund Denmark.The affordability, storage and power capacity of compact modern recording hardware have evolved passive acoustic monitoring (PAM) of animals and soundscapes into a non-invasive, cost-effective tool for research and ecological management particularly useful for bats and toothed whales that orient and forage using ultrasonic echolocation. The use of PAM at large scales hinges on effective automated detectors and species classifiers which, combined with distance sampling approaches, have enabled species abundance estimation of toothed whales. But standardized, user-friendly and open access automated detection and classification workflows are in demand for this key conservation metric to be realized for bats. We used the PAMGuard toolbox including its new deep learning classification module to test the performance of four open-source workflows for automated analyses of acoustic datasets from bats. Each workflow used a different initial detection algorithm followed by the same deep learning classification algorithm and was evaluated against the performance of an expert manual analyst. Workflow performance depended strongly on the signal-to-noise ratio and detection algorithm used: the full deep learning workflow had the best classification accuracy (≤67%) but was computationally too slow for practical large-scale bat PAM. Workflows using PAMGuard's detection module or triggers onboard an SM4BAT or AudioMoth accurately classified up to 47%, 59% and 34%, respectively, of calls to species. Not all workflows included noise sampling critical to estimating changes in detection probability over time, a vital parameter for abundance estimation. The workflow using PAMGuard's detection module was 40 times faster than the full deep learning workflow and missed as few calls (recall for both ~0.6), thus balancing computational speed and performance. We show that complete acoustic detection and classification workflows for bat PAM data can be efficiently automated using open-source software such as PAMGuard and exemplify how detection choices, whether pre- or post-deployment, hardware or software-driven, affect the performance of deep learning classification and the downstream ecological information that can be extracted from acoustic recordings. In particular, understanding and quantifying detection/classification accuracy and the probability of detection are key to avoid introducing biases that may ultimately affect the quality of data for ecological management.Publisher PDFPeer reviewe

Recreational vessels without Automatic Identification System (AIS) dominate anthropogenic noise contributions to a shallow water soundscape

This study was funded by FNU and Semper Ardens Carlsberg Grants to P.T.M., and support from the Danish Environmental Protection Agency to J.T.Recreational boating is an increasing activity in coastal areas and its spatiotemporal overlap with key habitats of marine species pose a risk for negative noise impacts. Yet, recreational vessels are currently unaccounted for in vessel noise models using Automatic Identification System (AIS) data. Here we conduct a case study investigating noise contributions from vessels with and without AIS (non-AIS) in a shallow coastal area within the Inner Danish waters. By tracking vessels with theodolite and AIS, while recording ambient noise levels, we find that non-AIS vessels have a higher occurrence (83%) than AIS vessels, and that motorised recreational vessels can elevate third-octave band noise centred at 0.125, 2 and 16 kHz by 47–51 dB. Accordingly, these vessels dominated the soundscape in the study site due to their high numbers, high speeds and proximity to the coast. Furthermore, recreational vessels caused 49–85% of noise events potentially eliciting behavioural responses in harbour porpoises (AIS vessels caused 5–24%). We therefore conclude that AIS data would poorly predict vessel noise pollution and its impacts in this and other similar marine environments. We suggest to improve vessel noise models and impact assessments by requiring that faster and more powerful recreational vessels carry AIS-transmitters.Publisher PDFPeer reviewe

A 2.6-gram sound and movement tag for studying the acoustic scene and kinematics of echolocating bats

This study was supported by the Carlsberg Foundation via a Semper Ardens grant, ONR, N00014-17-1- 2736; AFOSR FA9550-14-1-0398, and NSF NCS-FO:1734744 and a Human Frontiers Science Program Long-Term Fellowship to AS. These experiments were approved by The Danish Council for Experiments on Animals under permit number: 2016-15-0201-00989 and by the Johns Hopkins University Animal Care and Use Committee under protocol number BA17A107. We thank Uwe Firzlaff and Lutz Wiegrebe for their help.1. To study sensorimotor behaviour in wild animals, it is necessary to synchronously record the sensory inputs available to the animal, and its movements. To do this, we have developed a biologging device that can record the primary sensory information and the associated movements during foraging and navigating in echolocating bats. 2. This 2.6 -gram tag records the sonar calls and echoes from an ultrasonic microphone, while simultaneously sampling fine-scale movement in three dimensions from wideband accelerometers and magnetometers. In this study, we tested the tag on an European noctula (Nyctalus noctula) during target approaches and on four big brown bats (Eptesicus fuscus) during prey interception in a flight room. 3. We show that the tag records both the outgoing calls and echoes returning from objects at biologically relevant distances. Inertial sensor data enables the detection of behavioural events such as flying, turning, and resting. In addition, individual wing-beats can be tracked and synchronized to the bat's sound emissions to study the coordination of different motor events. 4. By recording the primary acoustic flow of bats concomitant with associated behaviours on a very fine time-scale, this type of biologging method will foster a deeper understanding of how sensory inputs guide feeding behaviours in the wild.PostprintPeer reviewe

Estimated communication range and energetic cost of bottlenose dolphin whistles in a tropical habitat

Author Posting. © Acoustical Society of America, 2012. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 131 (2012): 582-592, doi:10.1121/1.3662067.Bottlenose dolphins (Tursiops sp.) depend on frequency-modulated whistles for many aspects of their social behavior, including group cohesion and recognition of familiar individuals. Vocalization amplitude and frequency influences communication range and may be shaped by many ecological and physiological factors including energetic costs. Here, a calibrated GPS-synchronized hydrophone array was used to record the whistles of bottlenose dolphins in a tropical shallow-water environment with high ambient noise levels. Acoustic localization techniques were used to estimate the source levels and energy content of individual whistles. Bottlenose dolphins produced whistles with mean source levels of 146.7±6.2 dB re. 1 μPa(RMS). These were lower than source levels estimated for a population inhabiting the quieter Moray Firth, indicating that dolphins do not necessarily compensate for the high noise levels found in noisy tropical habitats by increasing their source level. Combined with measured transmission loss and noise levels, these source levels provided estimated median communication ranges of 750 m and maximum communication ranges up to 5740 m. Whistles contained less than 17 mJ of acoustic energy, showing that the energetic cost of whistling is small compared to the high metabolic rate of these aquatic mammals, and unlikely to limit the vocal activity of toothed whales.This study received support from the Danish Ph.D. School of Aquatic Sciences (SOAS), Aarhus University, DK, WWF Verdensnaturfonden and Aase & Ejnar Danielsens Foundation, the Siemens Foundation, the Faculty of Science at the University of Aarhus, DK, and the Danish Natural Science Foundation via a Steno scholarship and a logistics grant to PTM