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

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

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

Re‐weighing the 5% tagging recommendation: assessing the potential impacts of tags on the behaviour and body condition of bats

Considerable advances and breakthroughs in wildlife tracking technology have occurred in recent years, allowing researchers to gain insights into the movements and behaviours of a broad range of animals. Considering the accessibility and increase in use of tracking devices in wildlife studies, it is important to better understand the effects on these on animals. Our endeavour revisits a guideline established in 1988, which proposes that bats may encounter body condition or health problems and alter their behaviour when carrying tags weighing more than 5% of their body mass. Through a systematic literature review, we conducted a meta‐analysis to identify the impacts of tags on bats, including 367 papers from 1976 to 2023 that discussed, mentioned, employed, or quantified tagging of bats. We noted that the proportion of studies exceeding the 5% rule has not changed in recent years. However, the impact of tags was quantified in few studies for behaviour (n = 7) and body condition (n = 10) of bats. We were unable to assess whether tags weighing less or more than 5% of the bat's body mass impacted bats, but our meta‐analysis did identify that tags, irrespective of mass, affect the behaviour and body condition of bats. Although the overall magnitude of measured effects of tags on bats was small, progress has been made to advance our understanding of tag mass on bats. Naturally, there is a bias in reporting of significant results, illustrating the need of reporting results when there is no apparent effect of tags on bats. Our findings highlight the need for rigorous reporting of behaviour and body condition data associated with tagging of animals and illustrate the importance for studies comparing how tracking devices of different dimensions and masses may impact bat species to ensure research meets rigorous ethical standards

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

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

Echolocating bats prefer a high risk-high gain foraging strategy to increase prey profitability

Predators that target multiple prey types are predicted to switch foraging modes according to prey profitability to increase energy returns in dynamic environments. Here, we use bat-borne tags and DNA metabarcoding of feces to test the hypothesis that greater mouse-eared bats make immediate foraging decisions based on prey profitability and changes in the environment. We show that these bats use two foraging strategies with similar average nightly captures of 25 small, aerial insects and 29 large, ground-dwelling insects per bat, but with much higher capture success in the air (76%) vs ground (30%). However, owing to the 3–20 times larger ground prey, 85% of the nightly food acquisition comes from ground prey despite the 2.5 times higher failure rates. We find that most bats use the same foraging strategy on a given night suggesting that bats adapt their hunting behavior to weather and ground conditions. We conclude that these bats use high risk-high gain gleaning of ground prey as a primary foraging tactic, but switch to aerial hunting when environmental changes reduce the profitability of ground prey, showing that prey switching matched to environmental dynamics plays a key role in covering the energy intake even in specialized predators

Nyctalus_noctula_landing_experiment_Biosonar_parameters_extracted

Data collected on Nyctalus noctula in a large flight room at the University of Southern Denmark. The bat was trained to fly across a flight room and land on a target platform. The echolocation signals were recorded simultaneously by an array behind the target sphere as well as from an on-board acoustic tag. The folder here includes the extracted echolocation parameters recorded by the array at a sampling rate of 250 kHz

Nyctalus_noctula_target_appraoch_experiment_array-recorded

Data collected on Nyctalus noctula in a large flight room at the University of Southern Denmark. The bat was trained to fly across a flight room and land on a target platform. The echolocation signals were recorded simultaneously by an array behind the target sphere as well as from an on-board acoustic tag. This .zip-folder includes the array recorded flights sampled at 250 kHz