The Relationship Between Echolocation-Call Frequency and Moth Predation of a Tropical Bat Fauna

425

The relationship between echolocation-call frequency and moth predation of a tropical bat fauna

C.R. Pavey, C.J. Burwell, and D.J. Milne

Abstract: The allotonic frequency hypothesis proposes that the proportion of eared moths in the diet should be highest in bats whose echolocation calls are dominated by frequencies outside the optimum hearing range of moths i.e., <20 and >60 kHz. The hypothesis was tested on an ecologically diverse bat assemblage in northern tropical Australia that consisted of 23 species (5 families, 14 genera). Peak frequency of signals of bats within the echolocation assemblage ranged from 19.8 to 157 kHz but was greatest between 20 and 50 kHz. A strong positive relationship existed between peak call frequency and percentage of moths in the diet for a sample of 16 bats from the assemblage representing 13 genera (R2 = 0.54, p = 0.001). The relationship remained strong when the three species with low-intensity calls were excluded. When the two species with high duty cycle, constant-frequency signals were removed, the relationship was weaker but still significant. In contrast to previous research, eared moths constituted only 54% of moth captures in light traps at bat foraging grounds, and eared moths were significantly larger than non-eared individuals. These results show that the pattern of moth predation by tropical bats is similar to that already established for bat faunas in subtropical and temperate regions. Résumé : L’hypothèse de la fréquence allotonique veut que la proportion de papillons de nuit à organes tympaniques soit maximale dans le régime alimentaire des chauves-souris dont les appels d’écholocation sont dominés par des fré- quences hors du registre d’audition optimal des papillons, c.-à-d. <20 et >60 kHz. Nous avons évalué cette hypothèse sur un peuplement écologiquement diversifié de chauves-souris dans le nord de l’Australie tropicale qui comprend 23 espèces (5 familles, 14 genres). Les pics des fréquences des signaux des chauves-souris dans l’ensemble d’écholocation varient de 19,8 à 157 kHz, mais le maximum se situe entre 20 et 50 kHz. Il existe une forte relation positive entre le pic des fréquences des appels et le pourcentage de papillons de nuit dans le régime alimentaire dans un échantillon de 16 chauves-souris du peuplement et appartenant à 13 genres (R2 = 0,54, p = 0,001). La relation demeure forte, même lorsqu’on retire les trois espèces qui ont des appels de faible intensité. Lorsqu’on retire les deux espèces qui possèdent des signaux de cycle actif de haute fréquence constante, la relation est affaiblie, mais reste encore significative. Con- trairement à certaines études antérieures, les papillons de nuit à organes tympaniques ne représentent que 54 % des captures de papillons de nuit dans les pièges lumineux aux sites d’alimentation des chauves-souris et les papillons avec organes tympaniques sont significativement plus grands que les individus sans organes tympaniques. Ces résultats mon- trent que le patron de prédation par les chauves-souris tropicales est semblable à ceux qui ont déjà été établis pour les peuplements de chauves-souris des régions subtropicales et tempérées. [Traduit par la Rédaction] Pavey et al. 433

Introduction 1998; Miller and Surlykke 2001). Almost one-half of the world’s 200 000 species of moths belong to families that Hearing has evolved in a range of insect groups including possess ears and can hear ultrasound (Scoble 1992; Fullard the Neuroptera, Orthoptera, Dictyoptera, Coleoptera, and 1998). These 92 000 species of moths have been shown to Lepidoptera (Miller and Surlykke 2001). Among moths, ul- be abundant at bat foraging grounds across the globe. Spe- trasonic hearing functions primarily as a defence against the cies from eared families contribute ≥85% of species richness echolocation calls of foraging bats (Spangler 1988; Fullard of Macrolepidoptera in light-trap catches at sites in Europe, North America, Africa, and Australia, in environments rang- Received 12 October 2005. Accepted 17 January 2006. ing from temperate woodland to upland tropical rain forest Published on the NRC Research Press Web site at http:// (Fenton and Fullard 1979; Usher and Keiller 1998; Kitching cjz.nrc.ca on 17 March 2006. et al. 2000; Schoeman and Jacobs 2003). C.R. Pavey.1 Biodiversity Conservation, Parks and Wildlife The hearing of eared moths is most sensitive to frequen- Service, P.O. Box 1120, Alice Springs, 0871, Australia. cies between 20 and 50 kHz, a range that coincides with the C.J. Burwell. Queensland Museum, P.O. Box 3300, South peak call frequencies of most echolocating bats (Fullard Brisbane, 4101, Australia. 1987, 1998). Moths from regions with high bat diversity, D.J. Milne. Biodiversity Conservation, Parks and Wildlife where the bandwidth of echolocation frequencies is greater, Service and Tropical Savannas Cooperative Research Centre, show increased sensitivity at high and low frequencies com- P.O. Box 496, Palmerston, 0831, Australia. pared with moths from less diverse bat environments 1Corresponding author (e-mail: [email protected]). (Fullard 1982). However, although some moths can hear

Can. J. Zool. 84: 425–433 (2006) doi:10.1139/Z06-010 © 2006 NRC Canada 426 Can. J. Zool. Vol. 84, 2006 over a wider range of frequencies, sensitivity falls off gradu- Study site ally above 55 kHz (Fullard 1987). Although non-eared The study region is the tropics of the Northern Territory moths have evolved a suite of behavioural responses to re- of Australia, north of the 18°S parallel. This area is domi- duce bat predation (Soutar and Fullard 2004 and references nated by eucalypt savanna woodland and encompasses therein), the ability to hear combined with a variety of de- approximately 340 000 km2. Maximum mean weekly tem- fensive flight maneouvres enables eared moths to increase perature ranges between 32 and 39 °C and mean annual rain- their chances of avoiding bat predation by up to 40% fall between 360 and 1720 mm. Rainfall is highly seasonal, (Roeder 1967; Acharya and Fenton 1999). almost all precipitation occurring from November to April. Some echolocating bats are able to avoid detection by Topographic relief is relatively low; maximum elevation is eared moths either by emitting low-intensity calls that are al- 553 m. most imperceptible to moths or by calling at frequencies above and below the optimum hearing range of moths Echolocation frequency (Fullard 1998). Such frequencies are referred to as allotonic Data on the structure and frequencies of search-phase (i.e., frequencies are mismatched between moths and bats). echolocation signals of bats were obtained from 53 sites in a The allotonic frequency hypothesis proposes that the propor- variety of environments throughout the study area. Voucher tion of eared moths in the diet will be highest in bats whose calls were taken from free-flying bats in the field. echolocation calls are dominated by frequencies outside the We recorded calls of free-flying bats using a hand-held optimum hearing range of moths, i.e., <20 and >60 kHz Anabat® II bat detector (Titley Electronics, Ballina, N.S.W.) (Fenton and Fullard 1979). connected to an Optimus CTR-115 tape recorder (Sony The hypothesis is supported by research showing a high Chrome UX tapes). The person recording the calls usually proportion of moths in the diet of bat species calling at sat on the roof of a stationary 4WD vehicle at this time. allotonic frequencies (e.g., Rydell and Arlettaz 1994) and When a bat was heard through the Anabat® speaker, the tape by a positive relationship between peak frequency of echo- recorder was manually switched on via the Anabat® tape location calls and quantity of moths incorporated in the diet switch. A spotlight (12 V, 100 W) was switched on to locate for bat assemblages with frequencies >20 kHz. The latter ev- the bat and better track its movements. On some occasions idence is available for meta-analyses of bats with specific the bat was then collected. Each specimen was identified and echolocation strategies (Jones 1992) and call frequencies numbered. We also recorded calls of bats that were released (Bogdanowicz et al. 1999), and for foraging guilds (Pavey after capture in mist nets and harp traps. and Burwell 1998) and local bat communities in temperate The Anabat® system uses frequency division and zero- and subtropical regions (Jacobs 2000; Schoeman and Jacobs crossing analysis to construct frequency/time graphs from 2003). Although some results must be treated with caution detected signals (de Oliveira 1998). The frequency range of because data were collected in different ways at different the microphone on the detector is 10–200 kHz with a peak times (Schoeman and Jacobs 2003), overall the findings sug- frequency response at 50 kHz. The system responds to the gest that the allotonic frequency hypothesis is valid for a dominant harmonic, regardless of which one it is. If another wide range of bat assemblages. However, the hypothesis has harmonic is strongly represented, the system will repeatedly not been tested for bat faunas in the tropics, a major short- jump between the two harmonics and neither will be accu- coming given that bat diversity is concentrated in tropical re- rately represented. This situation rarely occurs (de Oliveira gions (Findley 1993) and, consequently, the echolocation 1998). In this study, peak call frequency for high duty cycle assemblages experienced by moths in the tropics are ex- (i.e., constant frequency) bats is defined as the maximum tremely diverse (e.g., Fullard 1998). frequency of the call, whereas for low duty cycle species it Here we report the results of a test of the allotonic fre- is defined as the frequency at the end or at the flattest por- quency hypothesis in the tropics of the Northern Territory of tion of the call (refer to Fig. 2 of Milne 2002). Australia. The study had three aims: (1) to describe the echolocation assemblage experienced by eared moths; (2) to Dietary data assess the level of moth predation by representative species Dietary data were obtained by identifying prey in the in the bat assemblage; and (3) to assess the relative propor- stomachs of specimens collected after recording of voucher tions of eared and non-eared moths at bat foraging grounds. calls, the stomachs of specimens held in the Museum and The bat fauna we examined is diverse, both taxonomically Art Gallery of the Northern Territory, Darwin, and faecal and ecologically; it includes 5 families, 14 genera (7 genera pellets collected during the field survey. Faecal pellets were in the Vespertilionidae) and both of Australia’s endemic gen- obtained from three sources: (1) from bats placed in cloth era: Rhinonicteris Gray, 1847 (Hipposideridae) and Macro- bags for 1–2 h after capture in mist nets or harp traps (all derma Miller, 1906 (Megadermatidae) (Table 1). The study bats were captured before 2200); (2) from the bottom of area is one of relatively few regions in the Old World tropics harp-trap bags in the morning if only one species of bat was where horseshoe bats (Rhinolophidae) do not occur. caught in the harp trap during the entire night (bags were cleaned before use each evening); and (3) under clusters of Materials and methods roosting bats. Each pellet/stomach content was placed in a petri dish and Fieldwork was carried out during the late dry and early teased apart using 10% KOH and 70% ethanol. We system- wet seasons (September to January) each year from 2000 to atically searched the material for identifiable fragments 2003 inclusive, most sampling taking place from September under a low-power (6.4×–40×) binocular microscope. Prey to November (late dry season). items were identified to the lowest taxonomic level possible.

Table 1. Echolocation-call characters, foraging guild, and body mass of the 23 species of insectivorous bats from tropical northern Australia included in the study. Call Peak frequency Foraging Body mass type* (kHz)† Intensity guild‡ (g)§ Emballonuridae Saccolaimus flaviventris (Peters, 1867) LDFM 20.3 High US 51.4–54.8 Taphozous kapalgensis McKean and Friend, 1979 LDFM 23.6 High US 26.0 Taphozous georgianus Thomas, 1915 LDFM 24.1 High US 24.1 Molossidae Mormopterus loriae (Thomas, 1897) LDFM 31.7 High US, BCS 7.4 Mormopterus beccarii Peters, 1881 LDFM 24.3 High US 14.8 Chaerophon jobensis (Miller, 1902) LDFM 19.8 High US 20.4 Megadermatidae Macroderma gigas (Dobson, 1880) LDFM 20.0–12.05 Low BCS 104.6 Hipposideridae Rhinonicterus aurantius (Gray, 1845) HDCF 116.0 High BCS, HCS 8.4 Hipposideros ater Templeton, 1848 HDCF 157.0 High BCS, HCS 4.2 Hipposideros diadema (E. Geoffroy, 1813) HDCF 69.1 High BCS 26.5 Hipposideros stenotis Thomas, 1913 HDCF 106.0 High BCS 5.5 Vespertilionidae Miniopterus schreibersii (Kuhl, 1817) LDFM 48.5 High BCS 11.3 Pipistrellus westralis Koopman, 1984 LDFM 46.6 High BCS 3.1 Pipistrellus adamsi Kitchener, Caputi and Jones, 1986 LDFM 43.9 High BCS 4.2 Chalinolobus nigrogriseus (Gould, 1856) LDFM 38.4 High BCS 6.0 Chalinolobus gouldii (Gray, 1841) LDFM 30.5 High BCS 9.8 Scotorepens greyii (Gray, 1843) LDFM 38.0 High BCS 6.6 Myotis macropus Gould, 1855 LDFM 40.1 High BCS 8.3 Nyctophilus arnhemensis Johnson, 1959 LDFM 47.1 Low HCS 6.6 Nyctophilus walkeri Thomas, 1892 LDFM 54.7 Low HCS 4.4 Nyctophilus bifax Thomas, 1915 LDFM 49.4 Low HCS 9.3 Nyctophilus geoffroyi Leach, 1821 LDFM 45.8 Low HCS 5.8 Vespadelus caurinus (Thomas, 1914) LDFM 59.6 High BCS 3.1 Note: Species for which dietary data were collected are in boldface type. *LDFM, low duty cycle, frequency-modulated; HDCF, high duty cycle, constant frequency. †Data from Milne (2002) and Milne et al. (2003), except Macroderma gigas (Kulzer et al. 1984; Guppy et al. 1985). ‡US, uncluttered space; BCS, background cluttered space; HCS, highly cluttered space. Guild assignments are based on Milne et al. (2004, Table 1). §Data from Churchill (1998), except Saccolaimus flaviventris (Rhodes and Hall 1997). 5Frequency data for M. gigas are for the first harmonic; each pulse typically has three or four harmonics.

We recorded whether a taxon was present in each faecal pel- as either eared or non-eared. Eared moths belonged to the let and the number of each body part present. The percent- Noctuidae, Arctiidae, Geometridae, Pyralidae, Notodontidae, age by volume of each order in each pellet/stomach content and Lymantriidae. Moths belonging to all other families were was visually estimated to the nearest 5%. classified as non-eared.

Moth capture Data analysis We assessed the proportion of eared and non-eared moths We regressed the arcsine of the mean percent volume of by light-trapping at 21 sites spread across the study area. moths in the bats’ diet against the log of their mean peak Trapping was carried out for 1 night at 19 sites and for 2 call frequency. For this analysis we included bat species for nights at the remaining 2 sites. The light trap consisted of a which we obtained dietary data from a minimum of five in- 12 V fluorescent light hung from the higher end of a white dividuals. If the sample included faecal pellets, we analysed cotton sheet (1.5 m × 2.5 m) suspended off the ground by at least five faecal pellets from each individual (Whitaker et strings tied to the corners to form a funnel, one end higher al. 1996; Schoeman and Jacobs 2003). For some species, all than the other. At the bottom of the funnel a hole was cut in dietary data came from faecal pellets collected under roost- the sheet and a plastic jar (65 mm diameter × 130 mm ing bats. In this case a sample of at least 20 faecal pellets depth) partially filled with 70% ethanol was suspended un- was analysed for each species (Whitaker et al. 1999; derneath. Insects that fell into the jar were collected the fol- Arlettaz et al. 2000). Sixteen species were included in this lowing morning. In the laboratory, we separated moths from analysis (Table 2), including representative species from each all other insects after filtering the samples througha2mm family, foraging guild, and echolocation-call type (Table 1). sieve to remove the smallest insects (mostly <3 mm in body Thirteen of the 14 genera present in the study area were rep- length). Each moth with a body length ≥3 mm was classified resented in this sample. A single representative was included

Table 2. Percent volume of prey categories in the diets of 16 bat species from tropical northern Australia. Sf Tg Cj Mg Ra Ha Ms Prey category (n = 10) (n =5) (n =8) (*) (n =7) (n =7) (*) Blattodea 0.75 35.6 4.2 7.9 Isoptera 3.6 7.1 Orthoptera 8.0 9.0 16.25 47.75 12.5 Orthopteroid 3.0 2.1 Hemiptera Heteroptera 16.25 16.0 5.6 2.1 9.3 Auchenorrhyncha 2.5 2.0 1.25 0.25 3.6 0.7 Stenorrhyncha Neuroptera 1.0 1.0 3.1 Coleoptera 62.5 57.0 28.1 28.5 39.2 16.7 1.1 Diptera 2.7 5.4 0.9 Trichoptera Lepidoptera 3.0 13.1 41.7 59.0 65.9 Hymenoptera 10.0 9.0 0.25 0.8 0.7 9.3 Aranae 1.0 0.2 Chilopoda 8.75 Vertebrate 13.5 Note: The 16 bat species are as follows: *Sf, Saccolaimus flaviventris; Tg, Taphozous georgianus; Cj, Chaerophon jobensis; Mg, Macroderma gigas; Ra, nigrogriseus; Cg, Chalinolobus gouldii; Sg, Scotorepens greyii; Mm, Myotis macropus;Na,Nyctophilus arnhemensis; Nw, Nyctophilus walkeri;Vc, pellets were analysed. from each genus except Nyctophilus Leach, 1821, Chalin- species (Fig. 1). All species with peak call frequencies olobus Peters, 1867, and Pipistrellus Kaup, 1829, which had >60 kHz were high duty cycle echolocators. Although we two representatives each. lack standardized estimates of bat abundance, trapping with We used standard t tests to examine differences in abun- harp traps and mist nets indicates that the most abundant dance and wing length between eared and non-eared moths. species within the study area are those calling within the 20– Values presented are means ± SE. 50 kHz frequency band. In contrast, the high duty cycle echolocators, especially Hipposideros diadema (E. Geoffroy, Results 1813) and Hipposideros stenotis Thomas, 1913, are compar- atively rare (D.J. Milne, unpublished data). The insectivorous bat fauna of the study area consisted of 23 species with widespread distributions (Table 1) and a fur- Diet ther 5 species that either occur only in the extreme south of The 16 bat species included in the dietary sample had the study area or are very rare. We cover the 23 widespread peak frequencies ranging from 19.8 to 157 kHz, thus includ- species here. All species are considered to be insectivorous ing the frequency extremes of the wider bat assemblage (Ta- or carnivorous; no partial frugivores are known from the as- ble 1). When data for the 16 species are combined, the list of semblage. prey captured includes 10 insect orders, 2 other arthropod classes (spiders (class Arachnida, order Araneae); centipedes Echolocation assemblage (class Chilopoda)) and vertebrates. Macroderma gigas was We recorded echolocation signals of all 23 widespread the only species that captured centipedes and vertebrates. In species from the study area (Table 1). These species exhibit contrast, eight species preyed on spiders, although spiders three distinct approaches to echolocation. The four hip- contributed >5% by volume to the diet of only one species, posiderid bats have high-intensity, constant-frequency calls Nyctophilus arnhemensis Johnson, 1959 (Table 2). produced at moderately high duty cycles. Five species pro- Coleoptera was the only insect order recorded in the diet duce low-intensity signals at low duty cycles. One of these of all 16 bats (Table 2). Hemiptera were taken by all species species, Macroderma gigas (Dobson, 1880), is a facultative except Hipposideros ater Templeton, 1848 and Lepidoptera echolocator that, on occasion, hunts by passive sound local- by all species except Saccolaimus flaviventris (Peters, 1867) ization using noise generated by its prey (Kulzer et al. 1984; and M. gigas. The diet of only five species consisted of Pettigrew et al. 1986). The majority of the bats in the study >50% by volume of a single insect order (Table 2). area produce high-intensity, low duty cycle signals (Table 1). The peak frequency of the signals of bats within the echo- Relationship between call frequency and diet location assemblage ranged from 19.8 to 157 kHz (Table 1). A strong positive relationship existed between peak call However, the echolocation assemblage encountered by eared frequency of all bats and percentage of moths in the diet 2 moths was greatest in the frequency range 20–50 kHz (R = 0.54, F[1,14] = 16.15, p = 0.001) (Fig. 2). The relation- (Fig. 1). Fifteen of the 23 species had peak call frequencies ship remained strong when the three species with low- within this range, including 12 of the 14 species with high- intensity calls were excluded from the analysis (R2 = 0.52, intensity, low duty cycle signals, i.e., typical aerial-hawking F[1,11] = 11.72, p = 0.006). These species were Nyctophilus © 2006 NRC Canada Pavey et al. 429

Mm Pw Pa Cn Cg Sg Na Nw Vc (*) (n =5) (n =5) (n = 11) (n =5) (n = 15) (n =5) (n =5) (n =6) 3.0 5.0 2.0 1.0 1.0 3.6 26.0 13.2 54.25 1.0 1.3 9.0 1.05 15.8 36.0 3.0

16.0 4.0 2.0 32.0 26.6 12.9 0.8 0.5 14.0 1.8 10.0 0.1 0.1 2.5 0.05 0.6 1.0 0.3 1.0 6.0 29.0 35.0 36.0 2.0 45.9 32.2 42.7 30.0 12.5 5.0 0.6 2.05 2.5 0.8 8.75 24.0 3.0 25.8 16.0 3.8 10.8 39.95 15.8 11.25 25.0 35.0 31.3 2.0 18.3 30.0 3.75 1.0 1.0 0.6 6.0 1.7

Rhinonicterus aurantius; Ha, Hipposideros ater;Ms,Miniopterus schreibersii;Pw,Pipistrellus westralis; Pa, Pipistrellus adamsi; Cn, Chalinolobus Vespadelus caurinus. n is the number of individuals. An asterisk indicates that faeces were collected below a colony of at least 10 bats. A minimum of 20 walkeri Thomas, 1892, N. arnhemensis, and M. gigas. The representative species from 13 genera were assessed). The low intensity of the calls of these species suggested that they assemblage includes species with a wide range of foraging may not have been readily detected by eared moths. When strategies and echolocation-call designs and representative the two species with high duty cycle, constant-frequency species are close to the extremes of body-mass variation calls, H. ater and Rhinonicteris aurantius (Gray, 1845), were present in microchiropteran bats, ranging from 3.1 to removed, the relationship was weaker but still significant 104.6 g (Table 1). Previous research on the relationship be- 2 (R = 0.31, F[1,12] = 5.32, p = 0.04). The three low-intensity tween bat call frequency and moth predation has consisted echolocators were included in this analysis. of either a comparison of bat diets across a wide range of The 10 vespertilionid bats included in the sample had studies without using standard methods of diet assessment peak frequencies ranging from 30.5 to 59.6 kHz, i.e., within (e.g., Jones 1992) or an investigation of local communities/ the range of best hearing of eared moths (Table 1). There guilds with richness of up to nine species from seven genera was no significant relationship between peak call frequency (Schoeman and Jacobs 2003), though typically much fewer and percentage of moths in the diet when only vespertilion- (e.g., Pavey and Burwell 1998). 2 ids were assessed (R = 0.12, F[1,8] = 1.08, p = 0.33). The results of the study corroborate the allotonic frequency hypothesis. Specifically, a strong positive relation- Moth abundance ship existed when we regressed peak call frequency against A total of 867 moths with body length ≥3 mm were cap- the percentage of moths in the diet for the 16 species in the tured during 23 nights of insect sampling. Moths belonging assemblage for which data for both variables were available. to eared families were more abundant than non-eared moths This finding shows that the pattern of incorporation of moths (eared moths, n = 469, 20.39 ± 7.23 captures/trap-night; into the diets of tropical bats is similar to that already estab- non-eared moths, n = 398, 17.30 ± 3.21 captures/trap-night). lished for bat faunas in subtropical and temperate regions However, the difference in mean abundance per trap-night (Jacobs 2000; Schoeman and Jacobs 2003). between eared and non-eared moths was not significant (t Sampling for this study was carried out within an exten- test, p = 0.70). sive area of the tropics of northern Australia and therefore The eared moths captured in the light traps were, on aver- represents a regional rather than a local bat assemblage. age, >30% larger than non-eared moths when forewing However, the bat fauna of the study area has a low β diver- length is used as a measure of body size (8.66 ± 0.16 and sity; species are widespread within the study area and there 6.64 ± 0.18 mm, respectively). This difference was highly is little turnover of species across environments (Churchill significant (t test, p < 0.0001). 1998). The average local richness at sampling sites within the study area was 6 species, with a maximum of 15 species. Discussion A shortcoming of most studies of the relationship between bat call frequency and predation on moths is that the pres- Our study is the first to examine the allotonic frequency ence of moths in the diet is determined only by identifying hypothesis in a species-rich bat assemblage in the tropics. scales in faecal samples. This method does not enable identi- The assemblage examined is diverse both taxonomically and fication of moths to family and, therefore, precludes an ecologically, containing 5 families and 14 genera (of which assessment of the proportions of eared and non-eared indi-

Fig. 1. Echolocation assemblage to which moths are exposed Fig. 2. Relationship between the peak frequency of echolocation within the study area in tropical northern Australia, displayed as signals and the mean percentage by volume of moths in the diets the number of bat species in three echolocation categories with of 16 species of bats in tropical northern Australia represented peak call frequencies in each 10 kHz frequency band. LDLI, low according to their echolocation strategy (refer to Fig. 1 for ab- duty cycle, low intensity; HDHI, high duty cycle, high intensity; breviations). For an explanation of species abbreviations see Ta- LDHI low duty cycle, high intensity. ble 2. 8 LDLI 70 Ms LDHI LDLI 7 HDHI Ha HDHI LDHI 60 6

50 5 Ra 4 40

3 No. of species 30 Cn Pw 2 Moths in the diet (%) 20 Cg Vc 1 Cj

10 0 Mm Tg Sg

<20 Mg >150

20-30 30-40 40-50 50-60 60-70 70-80 80-90 0 90-100

100-110 110-120 0 50 100 150 200 Call frequency (kHz) Call frequency (kHz) viduals in the diet. Moths present in faecal samples from in Scandinavia that showed that species of non-eared moths bats are assumed to be mostly eared individuals because had significantly greater wingspans and wing loadings than light-trapping indicates that the majority of moths flying at eared species (Rydell and Lancaster 2000). Our study was night belong to eared families. In contrast to previous re- not structured to test this pattern, whereas Rydell and Lan- search, light-trapping during our study indicated that eared caster (2000) sampled a wide range of eared and non-eared individuals were not significantly more abundant than non- families and examined only one species per genus. Notwith- eared moths. This finding was unexpected, given that light- standing the differences in design between the two studies, trap sampling in the tropical, subtropical, and temperate re- the results from northern Australia suggest that the relation- gions of the Old World (Africa, Australia, Europe) indicates ship between smaller body size and possession of ears may that a minimum of 80% and often >90% by number of not hold for all moth faunas. moths captured are eared (Pavey and Burwell 1998; Usher A factor that needs to be considered when interpreting and Keiller 1998; Kitching et al. 2000; Schoeman and moth predation and abundance data is that bats can capture Jacobs 2003). Further, the moth fauna of another site, Chil- large numbers of non-eared moths even if the nocturnal lagoe, in eucalypt savanna woodland in northern Australia moth fauna is dominated by eared species. For example, was dominated by eared individuals (Pavey and Burwell non-eared moths were the major prey of Rhinolophus mega- 1998). phyllus Gray, 1834 (peak call frequency 67–71 kHz) at three We carried out 20 of the 23 nights of light-trap sampling sites in eastern Australia despite eared moths comprising during the months of September and October. This temporal >80% of moth captures during light-trapping at each site clumping of sampling may have resulted in the capture of (Pavey and Burwell 1998). A linear index of food selection large numbers of earless moths during mass emergences. revealed that R. megaphyllus avoided all major families of Sampling during a wider range of seasons is needed to as- eared moths and exhibited a strong preference for particu- sess whether our results are representative of the moth fauna lar non-eared families, including the Anthelidae, Lasio- of the study region. campidae, and Hepialidae. This finding suggests that the The lack of a significant difference in abundance between results of any assessment of the allotonic frequency hypothe- eared and non-eared moths in this study weakens our inter- sis using faecal pellet / stomach-content analysis to deter- pretation of the dietary data supporting the allotonic fre- mine moth predation will be coarse, irrespective of the quency hypothesis. However, the significantly larger size relative abundance of eared and non-eared moths. Further, of eared moths in our light-trap samples suggests that bats the light-trapping results from our study indicate that despite should actively select eared individuals when foraging be- the large body of previous work, it is not advisable to as- cause these are energetically more profitable and easier to sume that eared moths are more abundant than non-eared in- detect. However, this expectation is countered by the likelihood dividuals at all locations. that eared moths will be able to avoid bats more effectively. Only two species in the assemblage (S. flaviventris, The significantly greater forewing length of eared moths M. gigas) did not consume any moths (Table 2). These spe- in our light-trap samples contrasts with the results of a study cies were the only 2 in the 23-species-strong regional pool to

© 2006 NRC Canada Pavey et al. 431 have a body mass >30 g (Table 1) and both fed on hard- that determine the level of moth predation by insectivorous bodied invertebrates (S. flaviventris on Coleoptera; M. gigas bats. The long-duration, high duty cycle signals of Rhin- on Orthoptera and Coleoptera). Of the remaining species, 10 olophus species may favour moth predation because the sig- had >10% by volume of moths in the diet (Table 2). This nals enable the classification of prey. Schoeman and Jacobs pattern contrasts with dietary data from other assemblages/ (2003) suggested that large-winged insects such as moths species pools that include a high proportion of non-moth produce more prominent glints (amplitude modulations) in feeders (e.g., four of nine species; Schoeman and Jacobs echoes than smaller winged insects and this allows them to 2003). be detected more readily. As a consequence, high levels of The peak call frequencies of the 10 vespertilionid species moth predation by horseshoe bats may be the result of signal included in the sample were confined to the 30–60 kHz structure and duration, not peak frequency. Conversely, the band, which corresponds to the optimal hearing abilities of long-duration signals of rhinolophid species may make them all moth faunas assessed to date (Fullard 1998). As a conse- more apparent to tympanate moths (Waters and Jones 1996). quence, the absence of a significant relationship between As a consequence, distances at which rhinolophid species peak call frequency and percent volume of moths in the diet can be detected by eared moths will be similar to those for for vespertilionids was expected. However, of the four bats bats emitting frequency-modulated calls at lower frequencies in the sample that captured >30% by volume of moths, two (Waters and Jones 1996). This situation would reduce the were vespertilionid species; Miniopterus schreibersii (Kuhl, ability of rhinolophid bats to capture eared moths relative to 1817) and N. walkeri. Further, of all species sampled, that of bats emitting frequency-modulated calls at similar M. schreibersii had the highest percent volume of moths in frequencies. If either interpretation is correct, it weakens the its diet. This result was unexpected, because in each of the lo- evidence in support of the allotonic frequency hypothesis cal bat communities examined in southern Africa, the highest because previous guild- and community-level assessments percentage of moths has been captured by a high duty cycle have all included rhinolophid species (Pavey and Burwell echolocator (Jacobs 2000; Schoeman and Jacobs 2003). Lepi- 1998; Jacobs 2000; Schoeman and Jacobs 2003). In contrast, doptera have previously been recorded as a prominent compo- our study demonstrates a strong positive relationship be- nent of the diet of M. schreibersii in Australia (Vestjens and tween peak signal frequency and moth predation in a re- Hall 1977). gional bat assemblage lacking horseshoe bats and, therefore, It is often argued that bats with low-intensity echolocation without concerns about conflation in patterns resulting from signals should be excluded from analyses that test the allo- signal duration. Although hipposiderid bats also produce tonic frequency hypothesis because moth ears are tuned to pure-tone calls, the hipposiderid echolocation system differs the frequencies of aerial-hawking bats, which typically are from that of rhinolophid bats in consisting of shorter dura- obligate echolocators that emit high-intensity signals (e.g., tion signals given at significantly lower duty cycles (Jones Rydell et al. 1995). In contrast, bats that emit low-intensity 1999). Further, the hipposiderid system does not appear to signals are typically gleaners that fly close to surfaces and be adapted for prey selection (Pavey et al. 2001). The signif- use prey-generated sound, and often also vision, in combina- icant positive relationship between peak call frequency and tion with echolocation to hunt prey (Fullard 1998). As a percentage of moths in the diet of our sample of 16 species consequence, low-intensity echolocation is considered an al- remained even after the two hipposiderid species were ex- ternative approach to overcoming a moth’s defences because cluded. these bats are likely to be acoustically inconspicuous to In summary, we consider that the dietary data presented moths. here support the allotonic frequency hypothesis, despite the One of the low-intensity echolocators in our sample, observation that a species calling at 48.5 kHz (M. schrei- M. gigas, is a facultative echolocator that uses passive hear- bersii) captured the highest proportion of moths. The strong ing to localize prey when gleaning from the ground (Guppy positive relationship between peak call frequency and per- and Coles 1983; Kulzer et al. 1984). However, capture of centage of moths in the diet for a sample of 16 species of flying insects, which appears to be a common behaviour bats from this tropical assemblage is similar to results ob- (Tidemann et al. 1985), involves the use of echolocation tained for bat faunas in subtropical and temperate regions. (Pettigrew et al. 1986). The other two low-intensity echo- However, the absence of a significant difference in abun- locators studied were long-eared bats (genus Nyctophilus). dance between eared and non-eared moths in light-trap sam- Observations on two Nyctophilus species not included in our ples during our study was unexpected and weakens the case dietary sample, Nyctophilus geoffroyi Leach, 1821 and Nyc- in support of the allotonic frequency hypothesis, which as- tophilus gouldi Tomes, 1858, indicate that these species do sumes that the majority of moths available to foraging bats use prey-generated sound, vision, and echolocation to cap- are eared. An inability to separate eared and non-eared ture insects in the laboratory (Grant 1991; Hosken et al. moths in dietary samples is a shortcoming of this study and 1994). However, gleaning appears to be rarely used by either most previous tests of the hypothesis. species in the field, aerial hawking being the dominant foraging strategy (O’Neill and Taylor 1986; Brigham et al. Acknowledgements 1997). Despite the differences in call intensity between M. gigas, N. arnhemensis, and N. walkeri and the remaining Owen Seeman, Robert Raven, Geoff Montieth, Andrew 13 species sampled, the strength of the relationship between Amey, Patrick Couper, and Steve van Dyck provided assis- peak call frequency and moth predation did not change when tance with identification of prey fragments in the faecal sam- these three species were removed from the analysis. ples. Paul Horner and Gavin Dally of the Northern Territory Call frequency and intensity may not be the only factors Museum and Art Gallery provided access to specimens in

© 2006 NRC Canada 432 Can. J. Zool. Vol. 84, 2006 their care. Constructive comments on the manuscript were Kulzer, E., Nelson, J.E., McKean, J.L., and Moehres, F.P. 1984. received from James Fullard and two reviewers. Prey-catching behaviour and echolocation in the Australian ghost bat, Macroderma gigas (Microchiroptera: Megadermatidae). Aust. References Mammal. 7: 37–50. Miller, L.A., and Surlykke, A. 2001. How some insects detect and Acharya, L., and Fenton, M.B. 1999. Bat attacks and moth defen- avoid being eaten by bats: tactics and countertactics of prey and sive behaviour around street lights. Can. J. Zool. 77: 27–33. predator. Bioscience, 51: 570–581. Arlettaz, R., Godat, S., and Meyer, H. 2000. Competition for food Milne, D.J. 2002. Key to the bat calls of the Top End of the North- by expanding pipistrelle bat populations (Pipistrellus pipis- ern Territory. Parks and Wildlife Commission of the Northern trellus) might contribute to the decline of lesser horseshoe bats Territory Tech. Rep. 71. Parks and Wildlife Commission, Palm- (Rhinolophus hipposideros). Biol. Conserv. 93: 55–60. erston, Northern Territory, Australia. Bogdanowicz, W., Fenton, M.B., and Daleszczyk, K. 1999. The re- Milne, D.J., Reardon, T.B., and Watt, F. 2003. New records for lationships between echolocation calls, morphology and diet in the Arnhem sheathtail bat Taphozous kapalgensis (Chiroptera : insectivorous bats. J. Zool. (Lond.), 247: 381–393. Emballonuridae) from voucher specimens and Anabat record- Brigham, R.M., Francis, R.L., and Hamdorf, S. 1997. Microhabitat ings. Aust. Zool. 32: 439–445. use by two species of Nyctophilus bats: a test of ecomorphology Milne, D.J., Armstrong, M., Fisher, A., Flores, T., and Pavey, C.R. theory. Aust. J. Zool. 45: 553–560. 2004. A comparison of three survey methods for collecting bat Churchill, S. 1998. Australian bats. Reed New Holland, Sydney, echolocation calls and species accumulation rates from nightly Australia. Anabat recordings. Wildl. Res. 31: 1–7. de Oliveira, M.C. 1998. Anabat system practical guide. Depart- O’Neill, M.G., and Taylor, R.J. 1986. Observations on the flight ment of Natural Resources, Brisbane, Queensland, Australia. patterns and foraging behaviour of Tasmanian bats. Aust. Wildl. Fenton, M.B., and Fullard, J.H. 1979. The influence of moth hear- Res. 13: 427–432. ing on bat echolocation strategies. J. Comp. Physiol. A, 132: Pavey, C.R., and Burwell, C.J. 1998. Bat predation on eared moths: 77–86. a test of the allotonic frequency hypothesis. Oikos, 81: 143–151. Findley, J.S. 1993. Bats: a community perspective. Cambridge Uni- Pavey, C.R., Grunwald, J.E., and Neuweiler, G. 2001. Foraging versity Press, Cambridge. habitat and echolocation behaviour of Schneider’s leaf nosed Fullard, J.H. 1982. Echolocation assemblages and their effects on bat, Hipposideros speoris, in a vegetation mosaic in Sri Lanka. moth auditory systems. Can. J. Zool. 60: 2572–2576. Behav. Ecol. Sociobiol. 50: 209–218. Fullard, J.H. 1987. Sensory ecology and neuroethology of moths Pettigrew, J., Baker, G.B., Baker-Gabb, D., Baverstock, G., Coles, and bats: interactions in a global perspective. In Recent ad- R., Conole, L., Churchill, S., Fitzherbert, K., Guppy, A., vances in the study of bats. Edited by M.B. Fenton, P.A. Racey, Hall, L., Helman, P., Nelson, J., Priddel, D., Pulsford, I., Rich- and J.M.V. Rayner. Cambridge University Press, Cambridge. ards, G., Schultz, M., and Tidemann, C.R. 1986. The Australian pp. 244–272. ghost bat, Macroderma gigas at Pine Creek, Northern Territory. Fullard, J.H. 1998. The sensory coevolution of moths and bats. In Macroderma, 2: 8–19. Comparative hearing: insects. Edited by R.R. Hoy, A.N. Popper, Rhodes, M.P., and Hall, L.S. 1997. Observations on yellow-bellied and R.R. Fay. Springer Handbook of Auditory Research, Springer- sheath-tail bats Saccolaimus flaviventris (Peters, 1867) (Chiroptera: Verlag, New York. pp. 279–326. Emballonuridae). Aust. Zool. 30: 351–357. Grant, J.D.A. 1991. Prey location by two Australian long-eared Roeder, K.D. 1967. Nerve cells and insect behaviour. 2nd ed. Har- bats, Nyctophilus gouldi and N. geoffroyi. Aust. J. Zool. 39: 45– vard University Press, Cambridge, Mass. 56. Rydell, J., and Arlettaz, R. 1994. Low-frequency echolocation en- Guppy, A., and Coles, R.B. 1983. Feeding behaviour in the Austra- ables the bat Tadarida teniotis to feed on tympanate moths. lian ghost bat, Macroderma gigas (Chiroptera: Megadermatidae) Proc. R. Soc. Lond. B Biol. Sci. 257: 175–178. in captivity. Aust. Mammal. 6: 97–99. Rydell, J., and Lancaster, W.C. 2000. Flight and thermoregulation Guppy, A., Coles, R.B., and Pettigrew, J.D. 1985. Echolocation and in moths were shaped by predation from bats. Oikos, 88: 13–18. acoustic communication in the Australian ghost bat, Macro- derma gigas (Microchiroptera: Megadermatidae) in captivity. Rydell, J., Jones, G., and Waters, D. 1995. Echolocating bats and Aust. Mammal. 8: 299–308. hearing moths: who are the winners? Oikos, 73: 419–424. Hosken, D.J., Bailey, W.J., O’Shea, J.E., and Roberts, J.D. 1994. Schoeman, M.C., and Jacobs, D.S. 2003. Support for the allotonic Localisation of insect calls by the bat Nyctophilus geoffroyi frequency hypothesis in an insectivorous bat community. (Chiroptera: Vespertilionidae): a laboratory study. Aust. J. Zool. Oecologia (Berl.), 134: 154–162. 42: 177–184. Scoble, M.J. 1992. The Lepidoptera: form, function and diversity. Jacobs, D.S. 2000. Community level support for the allotonic fre- Oxford University Press, Oxford. quency hypothesis. Acta Chiropt. 2: 197–207. Soutar, A.R., and Fullard, J.H. 2004. Nocturnal anti-predator adap- Jones, G. 1992. Bats vs. moths: studies on diets of rhinolophid and tations in eared and earless Nearctic Lepidoptera. Behav. Ecol. hipposiderid bats support the allotonic frequency hypothesis. In 15: 1016–1022. Prague studies in mammalogy. Edited by I. Horácek and V. Spangler, H.G. 1988. Moth hearing, defense, and communication. Vohralik. Charles University Press, Prague, Czech Republic. Annu. Rev. Entomol. 33: 59–81. pp. 87–92. Tidemann, C.R., Priddel, D.M., Nelson, J.E., and Pettigrew, J.D. Jones, G. 1999. Scaling of echolocation call parameters in bats. J. 1985. Foraging behaviour of the Australian ghost bat, Macro- Exp. Biol. 202: 3359–3367. derma gigas (Microchiroptera: Megadermatidae). Aust. J. Zool. Kitching, R.L., Orr, A.G., Thalib, L., Mitchell, H., Hopkins, M.S., 33: 705–713. and Graham, A.W. 2000. Moth assemblages as indicators of en- Usher, M.B., and Keiller, S.W.J. 1998. The macrolepidoptera of vironmental quality in remnants of upland Australian rain forest. farm woodlands: determinants of diversity and community struc- J. Appl. Ecol. 37: 284–297. ture. Biodivers. Conserv. 7: 725–748.

Vestjens, W.J.M., and Hall, L.S. 1977. Stomach contents of forty- Whitaker, J.O., Jr., Neefus, C., and Kunz, T.H. 1996. Dietary varia- two species of bats from the Australasian region. Aust. Wildl. tion in the Mexican free-tailed bat (Tadarida brasiliensis mexi- Res. 4: 25–35. cana). J. Mammal. 77: 716–724. Waters, D., and Jones, G. 1996. The peripheral auditory character- Whitaker, J.O., Jr., Issac, S.S., Marimuthu, G., and Kunz, T.H. istics of noctuid moths: responses to the search-phase echo- 1999. Seasonal variation in the diet of the Indian pygmy bat, location calls of bats. J. Exp. Biol. 199: 847–856. Pipistrellus mimus, in southern India. J. Mammal. 80: 60–70.