Echolocation Calls Research Articles

Hidden Afrotropical Bat Diversity in Nigeria: Ten New Country Records from a Biodiversity Hotspot

Knowledge of Afrotropical bat diversity is hindered by hidden diversity because of inefficient sampling techniques, limited survey effort, and taxonomic description. With 90 bat species recorded, Nigeria supports almost a third of the bat diversity known to occur in Africa, yet the country remains relatively under-sampled. Southeastern Nigeria and southwestern Cameroon are predicted to be the largest of six bat diversity hotspots in Africa, with the Cameroonian side receiving more sampling effort. This area is also recognized to support hyperdiverse mammalian assemblages and endemic species. Notwithstanding, two major protected areas on the Nigerian part of this bat diversity hotspot were hitherto not sampled, raising the likelihood of discovering new country records in these conservation priority forests. We conducted an intensive survey using harp traps and mist nets in forests of Afi Mountain Wildlife Sanctuary and Cross River National Park (Okwangwo Division). We also recorded echolocation calls of captured individuals to create the first in-country call library. We captured 36 bat species of which 10 are new country records raising the number of bat species known from Nigeria to 100. Harp trap captures represented 99% of forest understory bats sampled with 15 out of 18 species caught exclusively in traps. Rarefaction curves of captures suggest near-complete sampling with harp traps but incomplete sampling with mist nets. Eight of the 10 new country records were captured exclusively in harp traps, demonstrating the value of contemporary techniques for frequently missed species — here referred to as methodological blind-spot diversity. We report the re-identification of two museum specimens that were previously identified as Rhinolophus hillorum (R. cf. hillorum) and Miniopterus inflatus (M. villiersi) thus, our captures of the true forms confirm the presence of both species in Nigeria for the first time. We also provide noteworthy taxonomic comments on other species. To improve the efficiency of future Afrotropical bat surveys we recommend the deployment of complementary trapping techniques in identified diversity hotspots, especially sites that have been under-sampled.

Natural acoustic stimuli evoke selective responses in the hippocampus of passive listening bats

A growing body of research details spatial representation in bat hippocampus, and experiments have yet to explore hippocampal neuron responses to sonar signals in animals that rely on echolocation for spatial navigation. To bridge this gap, we investigated bat hippocampal responses to natural echolocation sounds in a non‐spatial context. In this experiment, we recorded from CA1 of the hippocampus of three awake bats that listened passively to single echolocation calls, call‐echo pairs, or natural echolocation sequences. Our data analysis identified a subset of neurons showing response selectivity to the duration of single echolocation calls. However, the sampled population of CA1 neurons did not respond selectively to call‐echo delay, a stimulus dimension posited to simulate target distance in recordings from auditory brain regions of bats. A population analysis revealed ensemble coding of call duration and sequence identity. These findings open the door to many new investigations of auditory coding in the mammalian hippocampus.

ChiroVox: a public library of bat calls.

Recordings of bat echolocation and social calls are used for many research purposes from ecological studies to taxonomy. Effective use of these relies on identification of species from the recordings, but comparative recordings or detailed call descriptions to support identification are often lacking for areas with high biodiversity. The ChiroVox website (https://www.chirovox.org) was created to facilitate the sharing of bat sound recordings together with their metadata, including biodiversity data and recording circumstances. To date, more than 30 researchers have contributed over 3,900 recordings of nearly 200 species, making ChiroVox the largest open-access bat call library currently available. Each recording has a unique identifier that can be cited in publications; hence the acoustic analyses are repeatable. Most of the recordings available through the website are from bats whose species identities are confirmed, so they can be used to determine species in recordings where the bats were not captured or could not be identified. We hope that with the help of the bat researcher community, the website will grow rapidly and will serve as a solid source for bat acoustic research and monitoring.

Acoustic clue: bringing echolocation call data into the distribution dilemma of Pteronotus (Chiroptera: Mormoopidae) complexes in Central America

Abstract In Central America, the distributional limits and the contact zones of some Pteronotus species, such as the naked-backed bats and the lesser mustached bats, are unclear. To elucidate the distributional dilemma of the species groups Pteronotus fulvus + Pteronotus davyi and Pteronotus psilotis + Pteronotus personatus in Central America, we studied the acoustic variation of their echolocation calls along the range of possible contact zones and the existence of distinct phonic groups. We performed hierarchical k-means clustering on principal components (HCPC) using acoustic samples from Mexico, Honduras, El Salvador, Nicaragua and Costa Rica to describe the global acoustic diversity, possibly overlooking differences between species groups. We assessed whether those acoustic differences were related to the geographical distance and geographical location. We found evidence of sympatry for three phonic groups within each species complex, without a clear correspondence to the known distribution of species. The changes in frequency of their echolocation calls seem to follow a similar pattern to the geographical variation in body size of the species. Future studies in Central America should include an integrative sampling of individually captured, tagged and recorded bats to help in resolution of the distribution dilemma raised here.

Positive Interactions Drive Bat Distribution in a Remote Oceanic Archipelago (Azores, Portugal)

One of the fundamental interests in ecology is understanding which factors drive species’ distribution. We aimed to understand the drivers of bat distribution and co-occurrence patterns in a remote, insular system. The two bat species known to occur in the Azores archipelago were used as a model. Echolocation calls were recorded at 414 point-locations haphazardly distributed across the archipelago. Calls were analysed and assigned to each species. Binominal generalised linear models were adjusted using different descriptors at two scales: archipelago and island. The presence of the co-occurring species was included at both scales. The results show that island isolation, habitat and climate play an essential role on the archipelago and island scales, respectively. However, the positive interaction between bat species was the most critical driver of species’ distribution at the island scale. This high co-occurrence pattern at the island scale may result from both species’ maximising foraging profit in a region where prey abundance may be highly variable. However, further research is necessary to clarify the mechanisms behind this positive interaction. Both species are threatened and lack specific management and protection measures. Maintaining this positive interaction between the two species may prove to be fundamental for their conservation.

Different songflight calls of Pipistrellus kuhlii and Pipistrellus lepidus (Vespertilionidae, Chiroptera) in Europe

We analysed and compared the structure and parameters of the songflight calls of expansive Pipistrellus kuhlii and Pipistrellus lepidus, that recently colonized Central Europe from the south and east, respectively. Bat calls were recorded mainly in urban areas of Central Europe and the Balkans, including a narrow zone of these species' recent parapatric or sympatric occurrence (around the Carpathians and the eastern part of the Pannonian Basin). The newly described songflight calls of P. lepidus consist of more elements (median 6), are longer (mean 56.4 ms) and of a higher frequency of maximum energy (mean 25.7 kHz) than those of P. kuhlii (median 3, mean 41.0 ms and mean 14.0 kHz, respectively). This finding provides new evidence that P. lepidus represents a different species, in accordance with results from previous genetic and morphological studies. Reported differences in songflight calls permit the acoustic discrimination of P. kuhlii and P. lepidus, which is not possible based on overlapping parameters of their echolocation calls. Our findings enable distributional and ecological studies of these two species, based on acoustic methods, in the context of their rapid European expansion and the local co-occurrence.

Automated classification of bat echolocation call recordings with artificial intelligence

Acoustic recorders are commonly used to remotely monitor and collect data on bats (Order Chiroptera). These efforts result in many acoustic recordings that must be classified by a bat biologist with expertise in call classification in order to obtain useful information. The rarity of this expertise and time constraints have prompted efforts to automatically classify bat species in acoustic recordings using a variety of learning methods. There are several software programs available for this purpose, but they are imperfect and the United States Fish and Wildlife Service often recommends that a qualified acoustic analyst review bat call identifications even if using these software programs. We sought to build a model to classify bat species using modern computer vision techniques. We used images of bat echolocation calls (i.e., plots of the pulses) to train deep learning computer vision models that automatically classify bat calls to species. Our model classifies 10 species, five of which are protected under the Endangered Species Act. We evaluated our models using standard model validation procedures, and performed two external tests. For these tests, an entire dataset was withheld from the procedure before splitting the data into training and validation sets. We found that our validation accuracy (92%) and testing accuracy (90%) were higher than when we used Kaleidoscope Pro and BCID software (65% and 61% accuracy, respectively) to evaluate the same calls. Our results suggest that our approach is effective at classifying bat species from acoustic recordings, and our trained model will be incorporated into new bat call identification software: WEST-EchoVision.

Timing and Weather Offer Alternative Mitigation Strategies for Lowering Bat Mortality at Wind Energy Facilities in Ontario.

Simple SummaryWind farms represent one of the largest sources of anthropogenic mortality for bats. Wind proponents attempt to mitigate these effects via operational curtailment, such that wind energy is not produced on nights with low wind speeds during the late summer and fall. Our study modeled bat activity and mortality on two timescales (nightly and seasonally) and in response to a range of weather variables. We showed that bat mortality risks could be lowered and opportunities for wind energy production can be increased by focusing curtailment efforts to the periods of the night and year when bats are most active and by considering a wider range of weather variables, compared to standard curtailment strategies.Relatively high mortality of migratory bats at wind energy facilities has prompted research to understand the underlying spatial and temporal factors, with the goal of developing more effective mitigation approaches. We examined acoustic recordings of echolocation calls at 12 sites and post-construction carcass survey data collected at 10 wind energy facilities in Ontario, Canada, to quantify the degree to which timing and regional-scale weather predict bat activity and mortality. Rain and low temperatures consistently predicted low mortality and activity of big brown bats (Eptesicus fuscus) and three species of migratory tree bats: hoary bat (Lasiurus cinereus), eastern red bat (L. borealis), and silver-haired bat (Lasionycteris noctivagans). Bat activity occurred in waves with distinct peaks through the season; regardless of seasonal timing, most activities occurred in the first half of the night. We conclude that wind energy facilities could adopt a novel and more effective curtailment strategy based on weather and seasonal and nocturnal timing that would minimize mortality risks for bats while increasing the opportunities for power generation, relative to the mitigation strategy of increasing cut-in wind speed to 5.5 m/s.

Cryptic diversity of Rhinolophus lepidus in South Asia and differentiation across a biogeographic barrier

Peninsular India is an important region for mammalian diversification and harbors major biogeographic barriers. However, little is known about the role of this region in the diversification of bats though it harbors high chiropteran diversity. In this study, we used phenotypic, acoustic, and genetic markers to assess the diversification of Rhinolophus lepidus bats in South Asia. We first investigated if peninsular India is associated with speciation of R. lepidus. Further, we tested if the Palghat Gap acts as a biogeographic barrier to gene flow in this species. Our results revealed cryptic genetic diversity in peninsular India suggesting that this region holds at least one endemic species level lineage of the R. lepidus species complex.  Analyses of populations of R. lepidus across the Palghat Gap in the Western Ghats revealed clinal variation in phenotype, with bats south of this barrier being bigger and emitting echolocation calls of higher frequency. We also observed that populations on either side of the Palghat Gap have remained genetically isolated since the mid-Holocene.

Testing Accuracy and Agreement among Multiple Versions of Automated Bat Call Classification Software

ABSTRACTPassive acoustic monitoring is a common method of studying bats that involves recording echolocation calls of bats in their natural environment. Call sequences are then identified to species using automated acoustic analysis software. One limitation of acoustic software programs, particularly for long‐term monitoring efforts, is that newer versions may provide results that are not directly comparable with older versions. However, there is little available information regarding how much or in what ways the versions differ, or which versions are most accurate. We evaluated 2 software programs used for automated bat call identification by testing a common set of echolocation call files across multiple versions of each program. We quantified the level of agreement on identification results and compared accuracy rates among the versions of each program. Level of agreement varied by species, recording location, and the software versions being compared. Overall percent agreement ranged from 28–82%. Newer versions were more conservative, in that they assigned fewer species‐level identifications. However, newer versions were not substantially more accurate than older versions. Our conclusions suggest that bat researchers should be attentive to what software versions and settings are used as they plan and perform data analyses. Software developers could assist software users by providing more detailed information about their testing procedures and results, and what changes are associated with new versions. © 2021 The Wildlife Society.

Cover Picture and Issue Information

Functional EcologyVolume 35, Issue 12 p. 2615-2617 COVER PICTURE AND ISSUE INFORMATIONFree Access Cover Picture and Issue Information First published: 06 December 2021 https://doi.org/10.1111/1365-2435.13599AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The acoustic environment can structure animal assemblages by providing or obscuring salient information. In Sedlock et al. 2021 (Func. Ecol. 2021, 35:12; https://doi.org/doi:10.1111/1365-2435.13933), the authors explore the response of wild aerial-hawking bats to a natural source of ultrasonic noise: meadow katydids (Conocephalus longipennis) that form dense aggregations of asynchronously calling males in rice paddies across Southeast Asia. They mimicked this expanse of broadband ultrasound with 100 speakers installed in a rice paddy at the International Rice Research Institute in the Philippines. This phantom chorus revealed that, while most bats were undeterred by the chorus playback, low-flying bats whose echolocation calls overlapped with the katydid chorus were pushed up away from the noise source. The katydid chorus degraded foraging habitat for bats, but possibly carved out predator-reduced space for their insect prey. This study advances our understanding of how the acoustic environment can filter animal communities by bringing it into the ultrasonic realm, and its potential role in modulating trophic interactions in an agricultural landscape. Volume35, Issue12December 2021Pages 2615-2617 RelatedInformation

Bat point counts: A novel sampling method shines light on flying bat communities.

Emerging technologies based on the detection of electro‐magnetic energy offer promising opportunities for sampling biodiversity. We exploit their potential by showing here how they can be used in bat point counts—a novel method to sample flying bats—to overcome shortcomings of traditional sampling methods, and to maximize sampling coverage and taxonomic resolution of this elusive taxon with minimal sampling bias. We conducted bat point counts with a sampling rig combining a thermal scope to detect bats, an ultrasound recorder to obtain echolocation calls, and a near‐infrared camera to capture bat morphology. We identified bats with a dedicated identification key combining acoustic and morphological features, and compared bat point counts with the standard bat sampling methods of mist‐netting and automated ultrasound recording in three oil palm plantation sites in Indonesia, over nine survey nights. Based on rarefaction and extrapolation sampling curves, bat point counts were similarly effective but more time‐efficient than the established methods for sampling the oil palm species pool in our study. Point counts sampled species that tend to avoid nets and those that are not echolocating, and thus cannot be detected acoustically. We identified some bat sonotypes with near‐infrared imagery, and bat point counts revealed strong sampling biases in previous studies using capture‐based methods, suggesting similar biases in other regions might exist. Our method should be tested in a wider range of habitats and regions to assess its performance. However, while capture‐based methods allow to identify bats with absolute and internal morphometry, and unattended ultrasound recorders can effectively sample echolocating bats, bat point counts are a promising, non‐invasive, and potentially competitive new tool for sampling all flying bats without bias and observing their behavior in the wild.

Characterization of Echolocation Calls from Insectivorous Bats in Puting Bato Cave 5, Burdeos, Polillo Island, Philippines

Acoustic recording is rarely applied in studying Philippine bats, which is reflected by the scarcity of related publications in the country. As a result, the echolocation calls of many local bat species are still unknown, and a reliable bat call library does not exist. Here, we described the echolocation call of some insectivorous bats in Polillo Island and used the call characteristics to define each species. We captured and recorded the echolocation calls of cave-dwelling bats in Puting Bato Cave 5 in October and November 2019. Using BatSound, we measured the spectral and temporal call characteristics and determined the call structure and duty cycle of all recorded individuals. Then, we applied multivariate discriminant function analysis (DFA) to classify the calls into species and identify the call parameter with the highest discriminating power. In total, we recorded 104 individual bats from five species belonging to four families. All species from families Miniopteridae and Vespertilionidae produced steep frequency-modulated (FM) sweeps, while the species belonging to families Hipposideridae and Rhinolophidae emitted calls with constant frequency and FM components. Among the recorded species, Hipposideros coronatus had the highest peak frequency record (> 150 kHz), while Myotis macrotarsus had the lowest (< 40 kHz). Meanwhile, the longest call duration was recorded from Rhinolophus arcuatus (> 40 ms), while Miniopterus paululus produced the shortest call (< 3 ms). The DFA perfectly classified the calls into the correct species and peak frequency was the most important predictor among the call parameters measured. Our results provide strong evidence that insectivorous bats in Puting Bato Cave 5 produce species-specific echolocation calls, and acoustic recording can be a reliable method to enhance research on these cave-dwelling bats

Acoustic Context Modulates Natural Sound Discrimination in Auditory Cortex through Frequency-Specific Adaptation.

Sound discrimination is essential in many species for communicating and foraging. Bats, for example, use sounds for echolocation and communication. In the bat auditory cortex there are neurons that process both sound categories, but how these neurons respond to acoustic transitions, that is, echolocation streams followed by a communication sound, remains unknown. Here, we show that the acoustic context, a leading sound sequence followed by a target sound, changes neuronal discriminability of echolocation versus communication calls in the cortex of awake bats of both sexes. Nonselective neurons that fire equally well to both echolocation and communication calls in the absence of context become category selective when leading context is present. On the contrary, neurons that prefer communication sounds in the absence of context turn into nonselective ones when context is added. The presence of context leads to an overall response suppression, but the strength of this suppression is stimulus specific. Suppression is strongest when context and target sounds belong to the same category, e.g.,echolocation followed by echolocation. A neuron model of stimulus-specific adaptation replicated our results in silico The model predicts selectivity to communication and echolocation sounds in the inputs arriving to the auditory cortex, as well as two forms of adaptation, presynaptic frequency-specific adaptation acting in cortical inputs and stimulus-unspecific postsynaptic adaptation. In addition, the model predicted that context effects can last up to 1.5 s after context offset and that synaptic inputs tuned to low-frequency sounds (communication signals) have the shortest decay constant of presynaptic adaptation.SIGNIFICANCE STATEMENT We studied cortical responses to isolated calls and call mixtures in awake bats and show that (1) two neuronal populations coexist in the bat cortex, including neurons that discriminate social from echolocation sounds well and neurons that are equally driven by these two ethologically different sound types; (2) acoustic context (i.e., other natural sounds preceding the target sound) affects natural sound selectivity in a manner that could not be predicted based on responses to isolated sounds; and (3) a computational model similar to those used for explaining stimulus-specific adaptation in rodents can account for the responses observed in the bat cortex to natural sounds. This model depends on segregated feedforward inputs, synaptic depression, and postsynaptic neuronal adaptation.

Functional differences in echolocation call design in an adaptive radiation of bats.

All organisms have specialized systems to sense their environment. Most bat species use echolocation for navigation and foraging, but which and how ecological factors shaped echolocation call diversity remains unclear for the most diverse clades, including the adaptive radiation of neotropical leaf‐nosed bats (Phyllostomidae). This is because phyllostomids emit low‐intensity echolocation calls and many inhabit dense forests, leading to low representation in acoustic surveys. We present a field‐collected, echolocation call dataset spanning 35 species and all phyllostomid dietary guilds. We analyze these data under a phylogenetic framework to test the hypothesis that echolocation call design and parameters are specialized for the acoustic demands of different diets, and investigate the contributions of phylogeny and body size to echolocation call diversity. We further link call parameters to dietary ecology by contrasting minimum detectable prey size estimates (MDPSE) across species. We find phylogeny and body size explain a substantial proportion of echolocation call parameter diversity, but most species can be correctly assigned to taxonomic (61%) or functional (77%) dietary guilds based on call parameters. This suggests a degree of acoustic ecological specialization, albeit with interspecific similarities in call structure. Theoretical MDPSE are greatest for omnivores and smallest for insectivores. Omnivores significantly differ from other dietary guilds in MDPSE when phylogeny is not considered, but there are no differences among taxonomic dietary guilds within a phylogenetic context. Similarly, predators of non‐mobile/non‐evasive prey and predators of mobile/evasive prey differ in estimated MDPSE when phylogeny is not considered. Phyllostomid echolocation call structure may be primarily specialized for overcoming acoustic challenges of foraging in dense habitats, and then secondarily specialized for the detection of food items according to functional dietary guilds. Our results give insight into the possible ecological mechanisms shaping the diversity of sensory systems, and their reciprocal influence on resource use.

Bats (Plecotus auritus) use contact calls for communication among roost mates

Communication between group members is mediated by a diverse range of signals. Contact calls are produced by many species of birds and mammals to maintain group cohesion and associations among individuals. Contact calls in bats are typically relatively low-frequency social calls, produced only for communication. However, echolocation calls (higher in frequency and used primarily for orientation and prey detection) can also facilitate interaction among individuals and location of conspecifics in the roost. We studied calling behaviour of brown long-eared bats (Plecotus auritus) during return to maternity roosts in response to playbacks of social and echolocation calls. We hypothesised that calling by conspecifics would elicit responses in colony members. Bat responses (inspection flights and social calls production) were significantly highest during social call and echolocation call playbacks than during noise (control) playbacks. We suggest that social calling in maternity roosts of brown long-eared bat evolved to maintain associations among roostmates, rather than to find roosts or roostmates, because this species is strongly faithful to roosts and the social groups and roosts are stable over time and space. Living in a stable social group requires recognition of group members and affiliation of social bonds with group members, features that may be mediated by vocal signals.

Acoustic deterrents influence foraging activity, flight and echolocation behaviour of free-flying bats.

ABSTRACTAcoustic deterrents have shown potential as a viable mitigation measure to reduce human impacts on bats; however, the mechanisms underpinning acoustic deterrence of bats have yet to be explored. Bats avoid ambient ultrasound in their environment and alter their echolocation calls in response to masking noise. Using stereo thermal videogrammetry and acoustic methods, we tested predictions that: (i) bats would avoid acoustic deterrents and forage and social call less in a ‘treated airspace’; (ii) deterrents would cause bats to fly with more direct flight paths akin to commuting behaviour and in line with a reduction in foraging activity, resulting in increased flight speed and decreased flight tortuosity; and (iii) bats would alter their echolocation call structure in response to the masking deterrent sound. As predicted, overall bat activity was reduced by 30% and we recorded a significant reduction in counts of Pipistrellus pygmaeus (27%), Myotis spp. (probably M. daubentonii) (26%), and Nyctalus spp. and Eptesicus spp. (68%) passes. Pipistrellus pygmaeus feeding buzzes were also reduced by the deterrent in relation to general activity (by 38%); however, social calls were not (only 23% reduction). Bats also increased their flight speed and reduced the tortuosity of their flight paths, and P. pygmaeus reduced echolocation call bandwidth and start frequency of calls in response to deterrent playback, probably owing to the masking effect of the sound. Deterrence could therefore be used to remove bats from areas where they forage, for example wind turbines and roads, where they may be under threat from direct mortality.

Passive acoustic monitoring reveals the role of habitat affinity in sensitivity of sub‐tropical East Asian bats to fragmentation

AbstractHydropower infrastructure represents a major driver of habitat loss and insular fragmentation worldwide, mostly across the tropics and sub‐tropics. Despite growing evidence of dam‐induced impacts on biodiversity, the effects of insular habitat fragmentation on species assemblages remain poorly understood, particularly for East Asian vertebrates. Here, we assess how insectivorous bats respond to forest fragmentation in Thousand Island Lake, a mega‐hydroelectric dam in East China. Bat assemblages were surveyed across 36 land‐bridge islands of different sizes and degrees of isolation, using AudioMoth recorders. Echolocation calls were classified into sonotypes, each corresponding to either single or multiple species, which were further classified according to their habitat affinities into forest or open‐space foragers. Based on 22 875 five‐min recordings from 108 detector‐nights, we recorded 15 bat sonotypes, eight of which we classified as forest sonotypes (2329 bat passes) and seven as open‐space sonotypes (52 277 bat passes). Overall, sonotype richness increased with island area, but only above a certain threshold (34 ha). Habitat affinity played an important role in ensemble‐level responses to fragmentation; forest sonotype richness increased with island area, whereas open‐space sonotype activity was higher in more isolated islands. Our results highlight the relevance of particularly large fragments (>1000 ha) to maintain area‐sensitive forest bat species. However, islands below 34 ha in size and those more isolated from the mainland are also of conservation value as they, correspondingly, harbour a variable (but substantial) number of species and concentrate higher activity of open‐space foragers. These findings further demonstrate that acoustic sampling methods, as the one presented here, are able to provide key information for evidence‐based policies aimed at halting the ongoing wave of dam‐induced biodiversity loss.

Shazam for bats: Internet of Things for continuous real‐time biodiversity monitoring

Biodiversity surveys are often required for development projects in cities that could affect protected species such as bats. Bats are important biodiversity indicators of the wider health of the environment and activity surveys of bat species are used to report on the performance of mitigation actions. Typically, sensors are used in the field to listen to the ultrasonic echolocation calls of bats or the audio data is recorded for post-processing to calculate the activity levels. Current methods rely on significant human input and therefore present an opportunity for continuous monitoring and in situ machine learning detection of bat calls in the field. Here, we show the results from a longitudinal study of 15 novel Internet connected bat sensors—Echo Boxes—in a large urban park. The study provided empirical evidence of how edge processing can reduce network traffic and storage demands by several orders of magnitude, making it possible to run continuous monitoring activities for many months including periods which traditionally would not be monitored. Our results demonstrate how the combination of artificial intelligence techniques and low-cost sensor networks can be used to create novel insights for ecologists and conservation decision-makers.

Echolocation call frequencies of bats vary with body temperature and weather conditions

Many animals vary their acoustic signals with weather conditions to ensure sufficient range of signal transmission as atmospheric attenuation increases (the atmospheric attenuation change hypothesis). However, few studies have considered the effects of weather conditions on the vocalizations of endotherms via the effects on their physiological condition (the physiological change hypothesis). In the present study, we directly linked weather conditions to changes in bats' body temperatures and to their echolocation call frequencies. We found a significantly positive correlation between body temperatures and call frequencies among individuals of the great leaf-nosed bat, Hipposideros armiger , in the wild. We then monitored changes in body temperature and call frequency of H. armiger over an hour at an ambient temperature of about 11 °C. The bats emitted echolocation calls of lower frequencies with decreasing body temperature caused by decreased ambient temperature, supporting the physiological change hypothesis. We also investigated changes in body temperature and echolocation call frequencies for H. armiger from summer to winter. The bats’ body temperatures decreased with decreasing ambient temperature. However, echolocation call frequencies increased significantly. A linear mixed-effects model suggested that the change in atmospheric attenuation rather than body temperature had a significant effect on the call frequency, supporting the atmospheric attenuation change hypothesis. Overall, our results suggested that weather conditions can affect the echolocation signals of bats via the effects on their body temperature and on atmospheric attenuation. When both effects coexist, weather-induced adjustments in the echolocation signals could be more likely to be driven by the atmospheric attenuation effect. • Call frequency is correlated with body temperature among individuals in H. armiger . • Body temperature and call frequency decrease at low (constant) ambient temperature. • Seasonal variation in weather conditions causes a decrease in body temperature. • Call frequency is negatively correlated with atmospheric attenuation.