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Effectiveness of acoustic lures for increasing tropical forest understory bat captures

Abstract Bats are the most diverse mammalian order second to rodents, with 1400+ species globally. In the tropics, it is possible to find more than 60 bat species at a single site. However, monitoring bats is challenging due to their small size, ability to fly, cryptic nature, and nocturnal activity. Recently, bioacoustic techniques have been incorporated into survey methods, either through passive acoustic monitoring or acoustic bat lures. Lures have been developed on the premise that broadcasting acoustic stimuli increases the number of captures in harp traps or mist nets. However, this is a relatively new, niche method. This study tested the efficacy of two commonly used acoustic bat lure devices, broadcasting two different acoustic stimuli, to increase forest understory bat captures in the tropics. This is the first time an acoustic bat lure has been systematically tested in a tropical rainforest, and the first study to compare two lure devices (Sussex AutoBat and Apodemus BatLure). Using a paired experimental design, two synthesized acoustic stimuli were broadcasted, a feeding call and a social call, to understand the importance of the call type used on capture rates and genus‐specific responses. Using an acoustic lure significantly increased capture rates, while the type of device did not impact capture rates. The two acoustic stimuli had an almost even distribution of captures, suggesting that the type of call may be less important than previously thought. Results indicate a possible deterrent effect on Rhinolophous sp., while being particularly effective for attracting bats in the genera Murina and Kerivoula. This study highlights the effectiveness of lures, however, also indicates that lure effects can vary across genera. Therefore, lures may bias survey results by altering the species composition of bats caught. Future research should focus on a single species or genus, using synthesized calls of conspecifics, to fully understand the effect of lures.

Thus, there is a need for accurate assessments of bat abundance and distribution using standardized survey methods, especially in speciesrich tropical bat communities (Francis, 1990;Nurul-Ain et al., 2017).
Acoustic lures have been successfully used to increase capture rates in birds (Schaub & Jenni, 1999) and their use in bat surveys has been recommended by some (Hill et al., 2015;Hill & Greenaway, 2005). Several studies from the temperate zone indicate that using bat lures, particularly with social calls, can increase capture rates (Braun de Torrez et al., 2017;Goiti et al., 2008;Hill et al., 2014Hill et al., , 2015Hill & Greenaway, 2005;Loeb & Britzke, 2010;Quackenbush et al., 2016;Samoray et al., 2018). Methods and results, however, differ significantly, and lure use in tropical forests has not been fully explored.
Early behavioral research on the responses of microchiropteran bats to the playback of ultrasound calls noted that some bats are attracted to the source of the sound (Barclay, 1982;Russ et al., 1998;Wilkinson & Boughman, 1998). Attraction to conspecific acoustic stimuli is widespread in other taxa such as birds (Ndlovu, 2018;Zuberogoitia et al., 2020), amphibians (Gerhardt, 1994), and even fishes (Gordon et al., 2019). Responses of bats to broadcast calls have been explained as mobbing behavior in response to a distress call (Russ et al., 1998), "eavesdropping" on conspecifics echolocation calls to locate resources (Barclay, 1982), in response to social "screeches" acting as contact calls that assist foraging between group members (Wilkinson & Boughman, 1998), for mate choice (Puechmaille et al., 2014), or some combination of the above.
Acoustic bat lures have been developed on the premise that broadcasting acoustic stimuli, generally synthesized bat calls, can attract bats toward traps and therefore increase the probability of capture. This project is the first time an acoustic lure has been systematically tested in a tropical rainforest. It is also the first time that devices from two different manufacturers have been directly compared. Further research is particularly required in the tropics as these are biodiversity hotspots and current lure research has a limited geographic range.
This project aimed to assess the effectiveness of acoustic lures, alongside harp traps and mist nets, for catching insectivorous forest understory bats (narrow-space foragers; (Schnitzler et al., 2003) in Brunei Darussalam, Borneo. Furthermore, this study compared two devices and two broadcast call types. We tested three hypotheses surrounding lure use: (1) Acoustic lures increase overall bat captures; (2) The Sussex AutoBat lure and the Apodemus BatLure device do not differ in effectiveness; and (3) Using social calls as a lure is more effective than using feeding calls.

| MATERIAL S AND ME THODS
The efficacy of lure use was tested in Brunei Darussalam, a small country located on the northwest coast of Borneo, over half (54%) of which is still covered by unlogged forest (Bryan et al., 2013). The study was carried out at two sites: the Kuala Belalong Field Studies Centre (KBFSC) in the Ulu Temburong National Park, and a forestfarm mosaic in Tutong District (Figure 1).  (Struebig et al., 2012;Sukri et al., 2012). The field center is located at the northern end of the park (N4.551901, E115.155703) and managed by Universiti Brunei Darussalam. The center has one main walking track, a 2.4 km loop that climbs approximately 150 m on the west ridge. This is known as the "Ashton Trail" and has the largest bat inventory recorded in Brunei (Kofron, 2002;Masmin et al., 2016;Struebig et al., 2010Struebig et al., , 2012. A total of 36 trapping nights (9 field nights x 4 traps per night) were conducted at KBFSC between January 26 and February 6, 2020. At both sites, heavy rain impeded many nights of trapping, deterring bat emergence (Geipel et al., 2019;Kunz & Parsons, 2009).
Nights on which no bats were caught were not included in the analyses; acoustic detection devices (AnaBat Walkabout, Titley-Scientific, Brendale, Australia) also indicated no detectable bat activity during these times.  (Kunz & Parsons, 2009).

| Trap placement
In contrast, the mist nets were placed parallel to the track to be less conspicuous against the vegetation and potentially more likely to catch understory bats. Each site was used as both control and treatment to remove any site bias. Morphological measurements (forearm length, weight, and photos) were taken in the field and individuals were identified to species using "Phillipps field guide to the mammals of Borneo and their ecology" (Phillipps & Phillipps, 2018). Bats were identified to genus, but pooled for analysis due to small sample size.

| Lure devices
We tested two of the three most commonly used lure devices (Aylen, 2021). The Sussex AutoBat has been tested more thoroughly in the field within the published literature (Goiti et al., 2008;Hill et al., 2014Hill et al., , 2015Hill & Greenaway, 2005;Lintott et al., 2014) compared to devices from all other manufacturers (Braun de Torrez et al., 2017;Loeb & Britzke, 2010;Quackenbush et al., 2016;Samoray et al., 2018), although a survey of lure users suggests that the Apodemus BatLure and Binary Acoustics AT100 have been used more often (Aylen, 2021).
The Apodemus BatLure uses a single Vifa XT25SC90-04 omnidirectional speaker (additional speaker available for purchase), whereas the Sussex AutoBat utilizes two SensComp Series 600 Environmental Grade Electrostatic Ultrasonic Sensor's mounted back to back, making the speaker bi-directional (see Aylen[, 2021] for more information on the available lure devices and specifications). The technical specifications of these devices are similar, despite using different speakers, with both capable of broadcasting between 20 and 100 kHz and optimized for frequencies between 40 and 60 kHz (batma nagem ent.com; David Hill, personal communication).

| Lure setup
An acoustic lure was mounted on a tripod approximately 1-1.5 m off the ground at an approximately 110 degree angle, from 6:00 to 10:00 p.m. each night to coincide with greatest bat activity ( Figure 2). The Apodemus BatLure was programmed to match the Sussex AutoBat F I G U R E 2 Trap arrangement showing the configuration of harp trap, mist net, and acoustic lure. Two sites, at least 50 m apart, were setup per night using this configuration; a four-bank harp trap across the existing track or presumed flight path, and a mist net parallel to the track. Each site had an acoustic bat lure mounted on a tripod, either an Apodemus BatLure or a Sussex AutoBat, alternating hourly (control and treatment) throughout the night from 6:00 a.m. to 10:00 p.m. Figure not to scale existing playback settings (not standard from manufacturer), as the Sussex AutoBat playback settings cannot be easily altered. The selected broadcast call played twice followed by 5 s of silence, and this repeated continuously for 15 min.
Every 15 min the broadcast call was manually changed, resulting in 1-2 min of silence. In addition, every 30 min the device was alternated between the Apodemus BatLure and the Sussex AutoBat, and every hour the lures were alternated between treatment and control traps/nets. To ensure each broadcast call and device was used during each time period, the starting device and call type was alternated nightly using two sequences (Table 1).

| Broadcast calls
Two synthesized Australasian species' calls were used as broadcast lures on each device. These were pre-programmed onto the Sussex AutoBat by Dr. Roger B. Coles (Queensland University of Technology). These species would be considered "foreign" as they do not occur in Brunei. One call was based on a social call (unknown function) of Chalinolobus nigrogriseus with two main parts: an approximately 60-ms-long sinusoidal FM sweep from 60 to 15 kHz, followed by three shorter semi-sinusoidal components ranging from

| Statistical analyses
To analyses the effect of an acoustic lure on capture rates of bats, a generalized linear mixed model ("GLMMer"; negative binomial distribution due to overdispersion) was run in Program R (R v4.0.3; R Core Team, 2019) with the R package collection "tidyverse" (Wickham et al., 2019) and "lme4" (Bates et al., 2015). Night and trap site were included as random effects. The response variable was number of bats caught per hour as treatment and control nets were changed hourly. The model included total trapping effort, including time periods with no captures, as these are true random zeros (Blasco-Moreno et al., 2019) within the sampling variability (n = 88; 11 field nights × 2 trapping sites × 4 h per night). We then analyzed the number of bats caught only when a lure was broadcasting to assess any effect of the device used or call type broadcast using Chi-squared tests (n = 24).

| RE SULTS
In total, 35 individual bats from five genera were captured. Over twice as many bats were caught when either the Sussex AutoBat or Apodemus BatLure was present (24 individuals) rather than with no lure present (11 individuals; n = 88; number of bats caught per hour; Table 2); the use of a lure increased capture rates of bats significantly (GLMM, df = 1, p = .03). Analysis on the number of bats caught only when a lure was broadcasting (n = 24) showed no significant difference in captures based on which device was broadcasting ( 2 = 3.24, df = 1, p = .07). There was also no significant difference in capture rates based on whether the social or feeding call was being broadcast ( 2 = 0.36, df = 1, p = .5). There were too few captures to statistically analyse any effect of the lure on species or genus, however, 79% of Kerivoula sp. (n = 14) were caught when a lure was present. In contrast, 71% of Rhinolophus sp. (n = 7) were caught when there was no lure present. The majority of Hipposideros sp. were caught when a lure was present (67%; n = 6), and this was equally distributed between the two broadcast calls. Two Phoniscus sp. and four Murina sp. were caught, all when a lure was broadcasting. Only 17% of bats caught (n = 35) were captured in mist nets, the remaining 83% being captured in harp traps (Table 2).

| DISCUSS ION
Lure use had a significant positive effect on capture rates, with more bats captured when a lure was broadcasting (Table 2). Approximately two-thirds of the bats captured (24 of 35) were captured while an acoustic lure was in use. This adds to the growing pool of research highlighting the effectiveness of acoustic lure devices in surveying forest understory bats (Goiti et al., 2008;Hill et al., 2015;Hill & Greenaway, 2005;Lintott et al., 2014). The type of device made no difference to capture rates. Likewise, the type of call broadcast and feeding call versus social call also had no effect on capture rates, TA B L E 1 Setup nightly sequence of lure device, broadcast call, and time past the hour. The sequence alternated between "Sequence 1" and "Sequence 2" each field night to ensure each time block had every combination of device and call with a near even distribution between the two call types (Table 2).
This result suggests that call type may be less important than often assumed. However, since this study utilized calls modeled on two different "foreign" Australasian species, it is likely the bats are responding to the presence of a novel sound rather than out of interest in communicating with conspecifics whose calls generally differ substantially (Khan et al., 2021).
One major difference between the lure devices is that the Sussex AutoBat has a bidirectional speaker, which was likely slightly inhibited on one side through being placed at an approximately 110 degree angle (one speaker pointed more toward the ground  (Lintott et al., 2014). Calls were significantly different in studies showing no effect, and in theory, each call represents the presence of a food source, food competition, or an unknown social stimulus. These results support the theory that the primary attraction of lures is merely the presence of a novel sound, rather than a form of communication or eavesdropping, a topic that is still debated.
Our study utilized foreign Australasian calls for our lure devices.
Some studies have suggested that using a local conspecific call is most appropriate (Hill et al., 2015;Schöner et al., 2010), whereas others suggest a foreign and/or novel sound is more appropriate (Hill & Greenaway, 2005;Lintott et al., 2014;Quackenbush et al., 2016). However, even within a single region, the calls of some species can show considerable geographic variation (Sun et al., 2020) and many other factors are likely to be relevant, including the background bat activity when a lure is deployed (Lewanzik et al., 2019). In some cases, using conspecific calls has been unsuccessful in attracting the target species, having instead attracted other bats (Lintott et al., 2014). Future studies would likely benefit from focusing on one species to fully understand the effect of the lure and the most appropriate broadcast call(s), if any.
In our study, the lures were mostly ineffective at attracting horseshoe bats (Rhinolophus sp.), possibly even deterring them. It has been suggested that lures are less effective for bats of the genus Rhinolophus (Jon Flanders, personal communication), although there is no specific evidence as of yet. The two most common Rhinolophus sp. at KBFSC are Rhinolophus trifoliatus and Rhinolophus sedulous.
Of the total bats caught in this genus, 71% of them were caught when no lure was present (Table 2). Our results suggest that the lure possibly deterred them from trap sites, potentially acting as signs of intraspecific competition for foraging areas, as has been suggested with the North American vespertilionid bat Corynorhinus rafinesquii (Loeb & Britzke, 2010).
Bats in the genus Murina have been observed to be particularly attracted to lures in Japan (Hill et al., 2014). Hill et al. (2014) suggest this is due to these species being narrow-space foraging bats, making them otherwise difficult to catch with standard methods (harp traps or mist nets positioned across flyways). A high proportion of Kerivoula (79%) were caught in traps with a lure (Table 2), and this was the most abundant genus caught overall. Since both Murina and Kerivoula are "narrow space passive gleaning foragers" (Denzinger & Schnitzler, 2013) with particularly high-frequency calls compared to other species in the bat assemblage, it may have been that the acoustic stimulus indicated to these species, in contrast to the horseshoe bats, a profitable foraging site or similar source of interest. Gaining such information through eavesdropping is a known behavior in other bats (Fenton, 2003;Gillam, 2007;Ubernickel et al., 2013).
More recently, lures have also been used successfully to catch rare "open-space aerial foragers" (Braun de Torrez et al., 2017).
Given the significantly high capture rates observed elsewhere, and our results, it would be interesting to focus on this foraging guild in particular for future research as such bats are often underrepresented in faunal surveys. Furthermore, despite only two Phoniscus atrox being caught in this study, both were caught in traps with a lure and is a promising result that could potentially lead to increased captures of this near threatened species (Jayaraj, 2020).
Use of a lure to capture bats in Europe, Australia, and Japan has also been successful with 12-24.5x more captures in lure traps (Hill et al., 2014(Hill et al., , 2015Hill & Greenaway, 2005;Lintott et al., 2014). The above studies, however, did not place traps across flyways, as is often standard practice for catching forest bats (Kunz & Parsons, 2009), instead avoiding them. Avoiding flyways limits incidental catches, those likely to occur regardless of lure presence (unless the lure acts as a deterrent), however, potentially overinflates the effect of the lure compared to standard methods, that is, utilization of flyways. Appropriate trap placement is likely to impact bat captures more than incorporating a lure, with less general setup and financial cost required. Lower efficacy of lures occurs when standard trap placement is used (Quackenbush et al., 2016;Samoray et al., 2018).
While we placed mist nets against the forest edge, the lower number of bats captured in mist nets is more likely due to bats being better able to detect and avoid mist nets (Kingston et al., 2003).
A further advantage of our study is the use of synthesized calls which allows for simple standardization of methods, and increases output clarity since no background noise is included in the playback.
In general, synthesized calls are recommended for use with acoustic TA B L E 2 Number of individual bats captured over 44 trapping nights, sorted by genus. Number of bats captured without a lure, numbers caught using a lure (Apodemus BatLure vs. Sussex AutoBat), numbers caught using different types of broadcast calls (feeding vs. social), and numbers caught per trap type (harp trap or mist net) are shown. Two bats escaped traps before they could be identified. Bats were pooled for analysis due to a small sample size lures (Hill et al., 2015), and have been primarily used in many published lure studies (Goiti et al., 2008;Hill et al., 2014;Lintott et al., 2014;Quackenbush et al., 2016). We also provide full details of call parameters and the spectrograms to facilitate full use or modification in future studies. This is an important piece of information to make available for reproducibility, and to increase depth of understanding, but is not always provided in other publications.

| CON CLUS IONS
We tested three hypotheses in this study: (1)  Future research should focus on a single species or genus to fully understand the effect of the lure and broadcast stimuli, as its effectiveness appears to vary between bat genera. Manufacturers would also benefit from providing/supporting more published field tests on the appropriate methods and calls to use with their lure devices.
Lastly, our study provides evidence that the use of a lure may alter the species composition of bats caught, increasing captures of some genera, while deterring others, and thus, may bias survey results.

ACK N OWLED G EM ENTS
We would like to thank Dr. Roger Coles for providing the Sussex AutoBat lure device, and the synthesized bat calls used in this project. We especially thank the students of ECOL314 2020 (University

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are openly available at