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Diet of the Gray Myotis (Myotis grisescens): Variability and Consistency, Opportunism, and Selectivity

Virgil Brack Jr., Richard K. LaVal
DOI: http://dx.doi.org/10.1644/05-MAMM-A-098R1.1 7-18 First published online: 20 February 2006


Food habits of the endangered gray myotis (Myotis grisescens) were ascertained from 10,736 fecal pellets collected from 1,225 bats of known sex, age, reproductive condition, and capture locations, including 5 maternity caves and 2 dispersal caves in Missouri. Diets were compared to availability of insects in 80 light-trap samples collected concomitantly with fecal samples. Proportional availability of insects varied among locations, over the season, between seasons, and between early-evening and late-night samples. Similarly, the diet varied among locations, over time, between early and late samples, and among sample groups by sex, age, and reproductive condition. Trichopterans, coleopterans, and lepidopterans were important in the diet and in light-trap samples, but there was poor correlation between corresponding diet and light-trap samples. Plecopterans, ephemeropterans, and dipterans were occasionally common in light-trap and dietary samples, although again there was poor correlation between corresponding diet and light-trap samples. Gray myotis forage individually over long distances along streams and wooded riparian habitats. Although this habitat produces a characteristic assemblage of insect prey, proportional availability varies temporally and spatially. Thus, although specific diet samples do not match corresponding insect samples, on a broader scale, diets and insect availability do correspond. On a microscale, the gray myotis exhibited some characteristics of an opportunistic forager, feeding on readily available prey, but on a macroscale was selective, feeding in aquatic-based habitats where specific types of insect prey were abundant. Juveniles foraged more in woodlands and ate more coleopterans, which may provide a greater energy reward per unit of capture effort, than did adults. Conservation efforts should include both aquatic and wooded riparian habitats.

Key words
  • food habits
  • foraging ecology
  • gray myotis
  • insect prey
  • maternity caves
  • Myotis grisescens

The gray myotis (Myotis grisescens) roosts in caves year-round; summer maternity roosts are in warm caves and winter hibernacula are cold. Summer concentrations of males are often referred to as bachelor colonies. Caves used by bats between the maternity season and when they arrive at hibernacula are often referred to as dispersal caves. The gray myotis disperses nightly from cave roosts to forage along streams (LaVal et al. 1977; LaVal and LaVal 1980), and over and along edges of impoundments (Tuttle 1976).

The gray myotis was listed as endangered on 28 April 1976 (United States Department of the Interior 1976). Population declines in Missouri (LaVal and LaVal 1980), and presumably rangewide, were associated with human disturbance at maternity and hibernacula caves (Tuttle 1979), loss of riparian foraging habitat, and pesticide poisoning from insect prey (Clark et al. 1978, 1983). Because the species was experiencing substantive population declines, it was prudent to learn about the foraging ecology and food habits of the species (LaVal et al. 1977; LaVal and LaVal 1980).

The purpose of this study was to investigate food habits of the gray myotis at micro- and macroscales and to compare foods eaten with insect prey availability. Use of fecal analysis for the study of bat food habits is an established procedure (Kunz and Whitaker 1983) used for many species, including the gray myotis in Indiana (Brack et al. 1984; Whitaker et al. 2001), Kentucky (Lacki et al. 1995), and Alabama (Best et al. 1997). We used light traps to represent insect availability. All methods of sampling are biased; however, light traps have been used for similar studies (Brack and LaVal 1985; Lacki et al. 1995). LaVal and LaVal (1980) provided a brief summary of the food habits addressed in detail in this study. We sampled diets and insects over 2 years at 5 maternity colonies and 2 dispersal caves. We examined variation in the diet seasonally, annually, and geographically within and among groups by sex and age classes, including reproductive females (pregnant, lactating, and postlactating), juveniles, and adult males. Few studies (e.g., McWilliams 2002) have examined in detail the variability in the diet of a bat across sex, age, and reproductive groups, as well as temporally. Ultimately, we undertook this study to add to the knowledge available for management of this endangered bat.

Materials and Methods

Samples of feces from bats and insects from light traps were collected during 1978 and 1979 at maternity and dispersal caves in Missouri. Diet was ascertained from analysis of 10,736 fecal pellets collected from 1,225 bats of known sex, age, reproductive condition, and capture location. Insect availability was determined from 80 light-trap samples collected concomitantly with fecal samples in foraging habitats. Our fieldwork was conducted in accordance with what are now the animal care and use guidelines of the American Society of Mammalogists (Animal Care and Use Committee 1998).

Study areas.—Food habits and insect community composition were studied at 4 sites in Missouri in summer 1978 (Fig. 1). Maternity colonies studied were at 2 locations: Roaring Springs Cave and Bat Caves II and III. Roaring Springs is about 15 km south of Bat Cave II. Studies in late summer 1978 were completed at 2 postreproductive dispersal caves, Saloon and Twenty-three Degree caves. Saloon Cave is about 30 km south–southwest of Roaring Springs Cave and Twenty-three Degree Cave is about 10 km south of Saloon Cave. During summer 1979, food habits and insect availability were studied at 3 maternity colonies (Fig. 1), Mauss, Beck, and Holton caves. Mauss Cave is about 40 km northeast of Beck Cave, and Holton Cave is about 120 km north–northeast of Mauss Cave. A brief description of the ecological setting at each cave is provided below; more detail is given by LaVal and LaVal (1980).

Fig. 1

Locations in Missouri sampled for gray myotis food habits and insect availability.

Roaring Springs Cave, in Franklin County, is along the Meramec River, a fast-flowing, gravel-bottomed river with a narrow floodplain bordered by steep limestone bluffs. The bluffs and surrounding hills were dominated by oak–hickory (Quercus–Carya) forest, whereas forest species composition adjacent to the river was dominated by American sycamore (Platanus occidentalis), black willow (Salix nigra), eastern cottonwood (Populus deltoides), and other riparian species. Topography of the area limited agricultural land use. Bat Caves II and III are along the Bourbeuse River, a tributary of the Meramec River, in Franklin County. These 2 caves are about 1.2 km apart. Gray myotis began summer 1978 in Bat Cave II but moved to Bat Cave III in early June, about the time of parturition. In contrast to the Meramec River, the Bourbeuse River is a slow-flowing, alluvial stream with a wide floodplain devoted extensively to agriculture. The floodplain, frequently cultivated to the river's edge with little natural riparian habitat, was bounded by steep hills and limestone bluffs covered with oak–hickory forests. Habitats near Saloon and Twenty-three Degree caves were similar to habitats near Roaring Springs Cave. These caves are in limestone bluffs along fast-flowing streams in areas of restricted agriculture and forest of oaks and hickories. Both are in Franklin County. Saloon Cave is on the Meramec River upstream from Roaring Springs Cave, and Twenty-three Degree Cave is on Huzzah Creek, a tributary of the Meramec River.

Mauss Cave, in Camden County, is in a limestone bluff approximately 3.3 km from the Little Niangua River, which empties into the Lake of the Ozarks 3.9 km downstream. Alternatively, bats can fly across a ridge and access the Lake of the Ozarks over a distance of 7.6 km via the Bollinger Creek drainage. The area was a mixture of hilltop agriculture and valley deciduous forest. Good riparian habitat existed at many places along the stream. However, downstream areas along the lake were extensively developed for commercial and private recreational use. Beck Cave, in Hickory County, is approximately 1 km from the Pome De Terre River. Impoundment of the river, by the Truman Dam construction site downstream, had begun during the summer of 1979, resulting in alteration of the stream's characteristics. Before impoundment, the river was swift and had a gravel bottom. In general, areas along the stream were hilly and forested and agricultural use was occasional. Holton Cave is in Boone County, approximately 1.8 km from Perche Creek, a slow-moving, mud-bottomed stream. Perche Creek can be accessed via the Coon Creek drainage. The cave was about 8 km north of Columbia, a city of 70,000. General land use in the area was agricultural; cultivated fields often extended to the banks of Perche Creek and riparian habitat was minimal. Holton Cave is about 12 km (straight line) from the Missouri River.

Bat capture and fecal analysis.—Bats were captured at cave entrances using mist nets or a harp trap. When possible, samples of each sex or age group included approximately 15 bats; bats were held individually in cloth bags to collect feces. Fecal collection, storage, and laboratory analysis followed methods of Brack and LaVal (1985). The volume of each order of insect in the diet of a bat was expressed as a percentage of that bat's total diet, regardless of the number of fecal pellets collected from that individual. As feasible, insect parts in feces were identified to taxa below ordinal level, but because they were a small part of the diet, the diet was not quantified at these levels. The size of whole insect parts in feces (femurs and wings) was recorded in 1978.

In 1978, bats were sampled weekly at Roaring Springs Cave and Bat Caves II and EQ from mid-May to mid-July. Samples were collected 18 and 25 July and 1 August at either Twenty-three Degree or Saloon (dispersal) caves. Bats were sampled once per night at the beginning and end of the season, whereas 2 samples, early evening and late night, were collected in midseason. Early-evening sampling ran from dusk to midnight, and late-night sampling ran from 0300 h to dawn, but was terminated early if target sample sizes were reached. In 1979, early-evening and late-night samples of feces and insects were collected every other week at Mauss, Beck, and Holton caves during early June–early August. During both years at all caves, samples included reproductive (pregnant, lactating, and postlactating) females and juveniles. Adult males were sampled at dispersal caves in 1978 and at Mauss and Holton caves in 1979.

Insect sample collection.—Insects were sampled with an ultraviolet light trap and sticky traps in 1978 and with light traps in 1979. Light-trap sampling and analyses followed methods of Brack and LaVal (1985). Light traps were placed along established foraging areas (LaVal et al. 1977; LaVal and LaVal 1980). Light traps were operated at 2 locations at Mauss and Beck caves, and at a single location at other caves. Insect samples were collected concurrently with bat fecal samples.

Custom-made sticky traps in the form of a cylinder with about 600 cm2 of surface area, smeared with Tanglefoot (The Tanglefoot Co., Grand Rapids, Michigan), were placed in locations similar to but apart from light traps. The catch of insects with sticky traps was too small for comparison with fecal samples (1 or 2 small insects/trap h; usually dipterans but occasionally coleopterans), and was discontinued in 1979. However, use of sticky traps is noteworthy because on 1 August a trap placed in foliage of riparian woodlands produced specimens of the Asiatic oak weevil (Cyrtepistomus castaneus), which was common in the diet but undetected in light-trap and sticky-trap samples collected along streams.

Insect and fecal data analysis.—Data analysis followed that of Brack and LaVal (1985). Briefly, insect parts in feces were identified and percentage volume was estimated at the ordinal level. Insect availability in light-trap samples was determined in grams of biomass per trap hour, and proportional availability was calculated. After arcsine transformation, analyses of variance were used to compare diets of sample groups, among light-trap samples, and among diets and light-trap samples. Cochran's C-test for homogeneity of variance was used to substantiate the need for arcsine transformation. Student–Newman–Keuls multiple-range tests were used to further elucidate intrasample differences. All statistical analyses were completed using a version of SPSS (Nie et al. 1975). Values are presented as mean ± SD, except where otherwise noted.

A diet diversity index (DDI) was calculated: DDI = 1/ΣPi2 (MacArthur 1972), where Pi is the proportion of each insect order in the diet. The product of this diversity index provides a quick estimate of the number of equally represented orders of insects.


The 6 orders of insects most common in the diet of the gray myotis, in order of decreasing abundance, were Trichoptera, Coleoptera, Lepidoptera, Plecoptera, Ephemeroptera, and Diptera (Table 1). Trichopterans were present in 90% of samples and composed ≥50% in 31% of samples, whereas coleopterans were present in 93% of samples but composed ≥50% in only 17% of samples. Lepidopterans were present in 76% of samples and dipterans were present in 39% of samples. Plecopterans were present in 76% of samples and composed ≥50% in 9% of samples, whereas ephemeropterans were present in 46% of samples but never composed as much as 50% of any sample. The orders Trichoptera, Plecoptera, Ephemeroptera, and Diptera were considered aquatic-based prey, whereas Coleoptera and Lepidoptera were terrestrial-based. Adult males in 1979 ate the most aquatic-based prey and juveniles the least. Prey of adult females was roughly half aquatic-based anti half terrestrial-based except in 1978 at dispersal caves, when it was predominately coleopterans and some lepidopterans. The types of insects consumed varied among sample groups (sex, age, date, time, and location). Remains of insects from other orders were often present in small amounts and were occasionally an important part of a sample. Hymenopterans (typically ants) occurred sporadically and rarely were important in the diet of an individual or sample. Homopteran remains were found frequently, but always in small amounts. Neuropteran remains were rare.

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Table 1

Diets of Myotis grisescens in Missouri at maternity and dispersal caves. Diets are presented as ranges over sample dates for each sex and age group by percent (%) volume by insect order. Diets by location, date, early and late samples, and sex, age, and reproductive condition are available from the author (VB).

Sex and age% Volume Trichoptera% Volume Coleoptera% Volume Lepidoptera% Volume Plecoptera% Volume Ephemeroptera% Volume Diptera
Roaring Springs Cave
Bat Caves II and III
Dispersal caves
Adult male0.0–14.347.3–69.330.4–42.90.0–
Mauss Cave
Adult male14.1–59.70.6–27.52.7–24.68.1–68.30.0–27.90.0–0.9
Beck Cave
Holton Cave
Adult male48.6–81.10.8–28.81.2–43.50.0–16.40.0–0.80.0

Light-trap samples generally contained at least 50% and often >60% combined coleopterans and lepidopterans (Table 2); coleopterans composed ≥50% of 19% of samples and lepidopterans composed ≥50% of 13% of samples. Trichopterans composed ≥50% of 18% of samples, whereas ephemeropterans and dipterans each composed ≥50% of only 1% of samples and plecopterans never composed as much as 50% of any sample. Neuropterans composed ≥50% of 3% of samples, but individual insects were generally too large for the bats to eat. Availability of insects in light traps and presence in the diet were similar in collective samples, but poorly correlated for specific, corresponding samples.

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Table 2

Insects collected in light traps. Sample sites, time of sampling, and orders of insects correspond to those in Table 1. Samples are presented as ranges over sample dates. Insect samples by location and date for early and late samples are available from the author (VB).

Time% Volume Trichoptera% Volume Coleoptera% Volume Lepidoptera% Volume Plecoptera% Volume Ephemeroptera% Volume Diptera
Roaring Springs Cave
Bat Caves II and III
Dispersal caves
Mauss Cave
Beck Cave
Holton Cave

Reproductive females.—At Roaring Springs Cave, females ate both aquatic-based (15–94%) and terrestrial-based (6–64%) prey (Table 1). Trichopterans composed up to 94% of samples (pregnant: 54.1% ± 15.9%; lactating: 37.9% ± 29.7%). Lepidopterans were important after 7 June, when they typically composed 10–50% of the diet. Coleopterans were sporadic in the diet (0–47.2%; 12.3% ± 14.7%), whereas ephemeropterans occasionally composed 15–32% of the diet (12.2% ± 11.0%). Dipterans were often absent from the diet but composed 21.9% of the diet of pregnant females on 18 May; plecopterans twice composed >10% of the diet of lactating females.

Early-evening and late-night diets of females at Roaring Springs Cave were similar (Table 3) for all orders of insects, except more ephemeropterans were present in early than in late samples (P = 0.004). Consumption of all orders of insects varied over the season, except Trichoptera (Table 3); this was not an orderly change (Table 4). When both pregnant and lactating females were present, diets of the 2 groups were similar, but consumption of lepidopterans, coleopterans, and dipterans (P = 0.001 for each) changed over sample dates (Table 5).

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Table 3

Dietary differences of reproductive female, juvenile, and adult male Myotis grisescens by sample location. Differences are presented as analysis of variance P values for time (T; early evening and late night) and date (D; season) of sampling and interactions of the 2 (D × T).

FemaleJuvenile dateMale date
Sample locationInsect orderDateTimeD × T
Roaring SpringsTrichoptera0.3640.9040.7470.000
Bat Caves II and IIITrichoptera0.0010.0010.0010.637
Dispersal cavesTrichoptera0.1770.002
Mauss CaveTrichoptera0.0210.1040.0440.9230.046
Lepidoptera0.0300.51 δ0.5840.0440.649
Beck CaveTrichoptera0.6220.2130.3070.000
Holton CaveTrichoptera0.0370.8530.253
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Table 4

Student–Newman–Kuels (SNK) tests were used to determine dates on which diets of reproductive female, juvenile, and adult male Myotis grisescens differed at each sample location. For each order of insects, dates when less was eaten (α = 0.05) are in parentheses opposing dates when more was eaten. A slash (/) separates independent sets of opposing dates.

Insect orderSNK dates of sampling (α = 0.05) for reproductive females, juveniles, and adult males
Roaring Springs Cave
TrichopteraFemale: (6 July, 12 July, 28 June, 21 June, 7 June, 30 May, 23 May, 14 June) (18 May); juvenile: (28 June, 12 July) (6 July)
ColeopteraFemale: (21 June) (6 July)/(14 June) (7 June); juvenile: (12 July) (6 July)
LepidopteraFemale: (23 May, 30 May, 18 May, 21 June) (14 June)/(6 July, 28 June) (7 June)
PlecopteraFemale: (21 June, 23 May, 14 June, 18 May, 30 May, 7 June, 12 July, 28 June) (6 July); juvenile: (12 July, 28 June) (6 July)
EphemeropteraFemale: (23 May, 12 July, 18 May, 30 May, 14 June, 6 July, 28 June, 21 June) (7 June); juvenile: (6 July, 12 July) (28 June)
DipteraFemale: (21 June) (7 June)/(6 July, 12 July, 28 June, 14 June, 30 May, 23 May, 7 June) (18 May)
Bat Caves II and III
TrichopteraFemale: (19 June, 27 June, 6 June, 11 July, 31 May, 13 June, 5 July) (17 May, 25 May)
ColeopteraFemale: (13 June) (27 June, 5 July, 25 May)
LepidopteraFemale: (17 May, 5 July, 11 July) (27 June)/(13 June) (6 June, 31 May)
PlecopteraFemale: (13 June, 17 May, 25 May, 31 May, 6 June) (19 June, 5 July)
EphemeropteraFemale: (17 May, 25 May, 31 May, 6 June, 5 July, 13 June) (11 July); juvenile: (11 July) (5 July)
DipteraFemale: (27 June, 5 July, 11 July, 19 June, 13 June) (31 May, 6 June, 25 May)/(31 May) (17 May)
Dispersal caves
TrichopteraJuvenile: (25 July, 1 August) (18 July)
LepidopteraJuvenile: (18 July, 1 August) (25 July)
Mauss Cave
ColeopteraFemale: (6 June, 19 June) (17 July, 3 August)
LepidopteraFemale: (3 August, 6 June) (3 July, 19 June); juvenile: (3 August, 17 July) (3 July)
PlecopteraFemale: (3 August, 17 July) (3 July, 6 June, 19 June)/(3 July) (6 June, 19 June)/(6 June) (19 June); juvenile: (17 July, 3 August) (3 July); male: (17 July, 3 August) (3 July, 6 June, 19 June)
EphemeropteraFemale: (3 August, 17 July, 3 July) (6 June); male: (17 July) (3 August)
DipteraFemale: (3 July, 3 August) (6 June)
Beck Cave
TrichopteraJuvenile: (4 July) (18 July, 2 August)/(18 July) (2 August)
ColeopteraFemale: (18 July) (18 June, 2 August, 4 July, 4 June)/(18 June) (4 July, 4 June); juvenile: (18 July, 4 July) (2 August)
LepidopteraFemale: (18 July) (4 July, 4 June); juvenile: (2 August) (4 July)
PlecopteraFemale: (2 August, 18 July, 4 July, 4 June) (18 June)
DipteraFemale: (18 June, 4 July, 18 July) (4 June)
Holton Cave
ColeopteraFemale: (10 June) (15 July, 4 August, 21 June)/(2 July, 15 July) (4 August, 21 June)/(4 August) (21 June)
LepidopteraFemale: (4 August, 15 July, 2 July, 10 June) (21 June)
PlecopteraFemale: (2 July, 15 July, 4 August) (10 June, 21 June)
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Table 5

Dietary differences between pregnant (P) and lactating (L) female Myotis grisescens, on dates of common occurrence, and among dates, as well as interactions between reproductive condition (P/L) and date (D). Also shown are dietary differences between lactating females and juveniles (J) on dates of common occurrence, and among dates, as well as interactions between age (L/J) and date (D). Dietary differences are P-values from analysis of variance.

Reproductive conditionLactating females and juveniles
P/LDatePL × DL/JDateL/J × D
Roaring Springs7–21 June28 June–12 July
Bat Caves II and III6–19 June27 June–11 July

At Bat Caves II and III, lepidopterans were the most consistently abundant item in the diet of females (pregnant: 32.3% ± 25.5%; lactating: 37.4% ± 34.2%; Table 1). There was no pattern to abundance of coleopterans (6.8% ± 6.7%) or trichopterans in the diet, although trichopterans were sporadically abundant (20.6% ± 27.6%). Most consumption of dipterans was early in the season, from 17 May to 6 June. Plecopterans were absent before 19 June, but were common thereafter. Ephemeropterans, absent from the diet early in the season, composed 10–25% of most late-season samples.

At Bat Caves II and III, females ate fewer trichopterans (P = 0.017) and more plecopterans (P = 0.001) in early evening than late at night (Table 3). The relative abundance of each order of insect in the diet, except Ephemeroptera (P = 0.070), changed throughout the season. As the season progressed, females ate fewer dipterans and more plecopterans (Tables 3 and 4). When pregnant and lactating females both were present, diets were similar, but diets varied among sample dates (Table 5).

Diets of females differed between Roaring Springs Cave and Bat Caves II and III. Females from Roaring Springs Cave ate fewer lepidopterans (P = 0.018), dipterans (P = 0.001), and plecopterans (P = 0.0023), and more trichopterans (P = 0.001) and ephemeropterans (P = 0.041).

At dispersal caves, postlactating females ate predominantly coleopterans (52–87%; 68.8% ± 17.2%). Dipterans and plecopterans were not eaten; lepidopterans composed 12.4% ± 10.9%, trichopterans composed 7.4 ± 7.3%, and ephemeropterans composed 7.2 ± 6.3% of the diet. Over the 3 sample dates, the diet changed little (Table 3).

At Mauss Cave, the diet of females varied over the summer (Tables 3 and 4). Plecopterans and trichopterans comprised most of the diet of pregnant females on 6 June (Table 1). Trichopterans (49.0% ± 16.7%), coleopterans (21.0% ± 12.8%), and lepidopterans (13.7% ± 3.4%) were most important in diets of lactating females, although plecopterans composed 44.1% of the diet on 19 June. Postlactating females ate predominately trichopterans and coleopterans. Dipterans were eaten only during the first 2 sample periods (0.1–3.1%). Consumption of ephemeropterans and plecopterans decreased through the season and consumption of coleopterans was greater late in the season (P 0.011). No differences were found between diets of early-evening and late-night samples (Table 3).

At Beck Cave, coleopterans and trichopterans were each the largest part of 4 samples, whereas lepidopterans and plecopterans were each the largest part of 1 sample. Early-evening and late-night diets of females were similar (P 0.171), but proportions of lepidopterans, coleopterans, dipterans, and plecopterans varied through the season (P 0.029; Table 3). Midseason consumption of lepidopterans was high, and late-season plecopteran consumption was low (Table 4). Coleopterans and lepidopterans composed most of the diet of pregnant females (Table 1). Lactating females ate a more diverse diet, which contained similar portions of trichopterans (34.6% ± 29.7%), coleopterans (27.7% ± 27.9%), lepidopterans (18.4% ± 14.1%), and plecopterans (15.2% ± 20.7%). Trichopterans were important in the diet of postlactating females; lepidopterans and coleopterans were important on 2 August. Dipterans composed 10.8% of 1 early-season (4 June) sample and ephemeropterans composed 14.2% of the 2 August sample.

Diets of females from Holton Cave varied little between early evening and late night (Table 3), but an exception was consumption of plecopterans (P = 0.001), which were more common in early-evening samples. Like other maternity colonies, the diet of females varied over the season (Tables 3 and 4), but no pattern was discernable. Pregnant females ate plecopterans, ephemeropterans, trichopterans, and dipterans (Table 1). Trichopterans also were important in the diet of lactating females (53.7% ± 30.6%) and postlactating females. Lepidopterans (11.4% ± 9.9%), coleopterans (10.0% ± 18.2%), and plecopterans (13.1% ± 29.4%) were sporadically abundant in the diet of lactating females. Hemipterans and hymenopterans and were occasionally important in diets of individual females.

Females at Mauss, Beck, and Holton caves ate similar kinds and proportions of insects, but there were differences among localities in consumption of coleopterans (P = 0.001), plecopterans (P = 0.001), and homopterans (P = 0.021). Females in 1979 ate more coleopterans than did females in 1978, but similar proportions of coleopterans were found in 1978 and 1979 diets when 1978 samples include dispersal caves.

Juveniles.—Diets of juveniles were similar by time and year, but varied among dates (Table 3). No pattern was detected. Coleopterans and lepidopterans, and often trichopterans, were a major component of the diet (>25% in 19, 8, and 13 samples, respectively; Table 1). Dipterans and plecopterans were absent or very small parts of diets (plecopterans composed >25% of the diet in 1 sample); ephemeropterans, although frequently absent, were occasionally and erratically present in diets (≤17%). In 1978, only consumption of plecopterans (P = 0.001) varied among locations, and in 1979, diets were similar among locations. Juveniles generally consumed larger proportions of lepidopterans and coleopterans than did lactating females (Table 5).

At Roaring Springs Cave, juveniles ate predominately coleopterans (46.5% ± 14.7%) and lepidopterans (31.5% ± 6.2%). Trichopterans were most abundant on 6 July (30.7%), plecopterans on 6 July (9.9%), and ephemeropterans on 28 June (9.9%). At Bat Caves II and III, terrestrial-based prey (lepidopterans and coleopterans) composed 45.0–83.3% of the diet. Aquatic-based prey (trichopterans [14.6% ± 14.1%], plecopterans [3.3% ± 5.7%], and ephemeropterans [3.3% ± 5.7%]) composed the balance of the diet. At dispersal caves, terrestrial-based coleopterans and lepidopterans collectively composed 65.3–94.1% of the diet. Trichopterans, the only aquatic-based prey of importance, composed 5.8–32.3% of the diet.

At Mauss, Beck, and Holton caves, coleopterans (40.5–57.9%), trichopterans (26.9–49.6%), and lepidopterans (5.3–11.1%) dominated the diet of juveniles. Dipterans, plecopterans, and ephemeropterans were nearly absent from the diet. On 2 July at Holton Cave, homopterans composed 5.3% of the diet.

Adult males.—In 1978, males were sampled late in the season at dispersal caves (Table 1). Like females and juveniles, coleopterans (57.9% ± 11.0%) and lepidopterans (34.9% ± 6.9%) composed most of the diet. No dipterans or ephemeropterans were eaten. Small amounts of trichopterans (14.3%) and plecopterans (6.9%) were found in the 25 July diet.

At Mauss and Holton caves, trichopterans were the largest part of the diets (39.1% and 62.2%, respectively). Plecopterans were important in June samples at both caves. At Mauss Cave, males ate more plecopterans early in the season than late (Table 3). Lepidopterans and coleopterans were important late in the season at Mauss Cave but were more sporadic at Holton Cave. Ephemeropterans and homopterans were each important in the diet on 1 date at 1 cave. Dipterans composed <1% of all samples.

The diet of males at Mauss Cave was similar to diets of females and juveniles, although males consumed fewer coleopterans (P = 0.025). In contrast, males at Holton Cave ate predominantly trichopterans and different proportions of lepidopterans, coleopterans, plecopterans, and homopterans (P = 0.001 in all cases) than females. Seasonal variations were not significant.

Insect size and families of insects.—Sixty-five intact insect femurs were found in feces of adult females in 1978, including 48 coleopterans (2.8 ± 1.0 mm), 9 dipterans (3.9 ± 2.9 mm), 5 trichopterans (1.5 ± 0.6 mm), 1 ephemeropteran (2.8 mm), and 2 homopterans (2.2 mm). Seventeen intact insect wings were found, including 6 coleopterans (2.6 ±1.0 mm), 10 dipterans (3.2 ± 1.7 mm), and 1 trichopteran (5.0 mm). Realigned remains of a dipteran indicated a body length of 4.8 mm; remains of a coleopteran measured 9.7 mm. Feces from juveniles contained 61 femurs from insects of 3 orders, including Lepidoptera (1.5 mm), Coleoptera (n = 59; 3.1 ± 1.0 mm), and Diptera (3.1 mm). Feces also contained a 5.0-mm-long hymenopteran wing and 3 wings from coleopterans (6.3 ± 2.4 mm).

Insect remains were occasionally identified to family, including Corydalidae (Neuroptera); Corixidae (Hemiptera); Tipulidae, Ceratopogonidae, and Culicidae (Diptera); and Rhysodidae, Silphidae, Demestidae, Physodidae, Noteridae, Carabidae, Scarabaeidae, Elateridae, and Curculionidae (Coleoptera). Curculionidae and Elateridae were the only families frequently identified. Only the Asiatic oak weevil was identified to species.

Diet diversity index (DDI).—The DDI provides a quick estimate of the number of equally represented orders of insects. DDIs ranged from 1.00 to 7.31 (Table 6). Diets in 1978 were less diverse (DDI = 1.00–4.72) than in 1979 (DDI = 2.25–7.31). Within years and among sample locations, DDIs were similar. DDIs of early-evening and late-night samples were similar. DDIs of females from Roaring Springs and dispersal caves decreased erratically over the season. Similarly, in 1979, there was a general decrease over the season in DDIs of adult females. The DDI of adult females generally was larger than that of juveniles. DDIs of males and females were not demonstrably related.

View this table:
Table 6

Diet diversity indices of Myotis grisescens during early-evening, late-night, and combined samples for sex and age groups at maternity and dispersal caves.

Roaring Springs CaveBat Caves II and IIIDispersal caves
Mauss CaveBeck CaveHolton Cave
JuvAugust3.132.542.822.863.773.443.65,– 2.713.13
  • a Preg = pregnant; Lac = lactating; Juv = juvenile; P-Lac = postlactating.

Insect availability and comparison to diets.—At Roaring Springs Cave, the rate of capture in light traps was 0.3–468.2 g/h. Early-evening capture rates (41.0–468.0 g/h) were greater than late-night rates (0.3–11.3 g/h). Capture rates increased from 23 May to 28 June (except 21 June when it rained), and decreased thereafter. Trichopterans (4–85%) and coleopterans (1–71%) were major components of most samples (Table 2). Lepidopterans were heavily represented in midseason, late-night samples (25–72%). Dipterans composed 18% of the 14 June late sample (18%), but were a small part of most samples (0–9%). Ephemeropterans composed <10% of each sample. From 28 June to 12 July, neuropterans composed 26–30% of samples. Also caught were insects from the orders Hemiptera, Homoptera, Hymenoptera, Orthoptera, Plecoptera, and Dermaptera. Some were erratically present, others were consistently present; all were small parts of samples. Insect availability did not vary over the season (P = 0.013) or between early-evening and late-night samples (P > 0.100).

Over the season, proportions of insects in fecal samples of reproductive females at Roaring Springs Cave differed from proportional availability of insects in light-trap samples (P ≤ 0.030), except dipterans (P = 0.107). Early in the season, pregnant females ate fewer coleopterans and lepidopterans than were available in light-trap samples (P = 0.000 and P = 0.018, respectively). Pregnant females usually ate proportionately more ephemeropterans than were available in light traps (P = 0.008). Although trichopterans were common in both light-trap and diet samples, pregnant females ate more trichopterans than their proportional availability early in the season, and less late in the season (P = 0.001). Lactating, females ate fewer coleopterans than their proportional availability (P = 0.000), and ate more plecopterans (P = 0.029). Neuropterans were large parts of some light-trap samples but were not commonly eaten by lactating females (P = 0.000). Juveniles at Roaring Springs Cave ate fewer ephemeropterans, neuropterans, and plecopterans than were available (P = 0.000 for each).

The rate of catch increased over the season at Bat Caves II and III, but was less than at Roaring Springs Cave. Early-evening capture rates (2.9–129.1 g/h) were greater than late-night rates (0.4–15.2 g/h). Coleopterans composed >80% of light-trap samples through 6 June (Table 2). Lepidopterans were large components (39–97%) of 13 and 19 June samples. Trichopterans and ephemeropterans composed ≤29% of mid- to late-season samples. Plecopterans composed ≤20% of samples, and Dipterans were always present (0.1–7.7%). Neuropterans composed ≤53% of late-season samples. Hemipterans, homopterans, hymenopterans, and orthopterans were present in 1 or more samples.

Catch by order did not vary between early-evening and late-night samples (P ≥ 0.500), except Diptera (P = 0.032). Over the season, catch varied little (P ≥ 0.150), except Hymenoptera (P = 0.001), which was sporadic in occurrence. Light-trap samples at Bat Caves II and III were similar to samples at Roaring Springs (P ≥ 0.090) except Trichoptera (P = 0.007) and Homoptera (P = 0.001).

At Bat Caves II and III, pregnant females ate proportionately fewer coleopterans (P = 0.002) and neuropterans (P = 0.000), and more dipterans, trichopterans, ephemeropterans (P = 0.000 for each), and lepidopterans (P = 0.018) than were available in light traps. Similarly, lactating females ate fewer coleopterans (P = 0.002) and more lepidopterans (P = 0.020), ephemeropterans (P = 0.020), and plecopterans (P = 0.001) than were available. Neuropterans, although common in some light-trap samples, were not eaten by lactating females (P = 0.000); most were too large for the bats to eat. Consumption of coleopterans and lepidopterans, the 2 largest dietary items of juveniles, was similar to light-trap availability. Abundance of ephemeropterans, trichopterans, and dipterans differed (P ≤ 0.002) between diets of juveniles and light-trap samples, although each was a small part of both diet and light-trap samples.

At dispersal caves, coleopterans composed 30–83% of light-trap samples (Table 2). Lepidopterans composed 6, 1, and 45% of samples, whereas neuropterans composed 58% of the 25 July sample. Trichopterans and ephemeropterans composed <9% of each sample. Small numbers of insects from several other orders were caught.

Diets of postlactating females, juveniles, and males each differed from light-trap samples at dispersal caves, although availability and consumption of coleopterans and lepidopterans, the 2 major dietary items, was generally similar (consumption of coleopterans by females was an exception, because females sometimes ate more and sometimes less than available; P = 0.028). Neuropterans were important in light-trap samples but not in diets (P = 0.000). Ephemeropterans were important in females' diets, but not in light-trap samples (P = 0.005). Plecopterans, although generally unimportant in both diets and light traps, were present in different proportions in each (P = 0.000).

The catch of insects at Mauss Cave was 1.5–116.9 g/h during early-evening samples and 0.9–6.5 g/h during late-night samples. No trend was discernable in seasonable abundance. Lepidopterans and coleopterans were major parts of most samples but ephemeropterans (54%) and dipterans (60%) were each important in 1 early sample (Table 2). Catch of lepidopterans and plecopterans varied (P = 0.017) between early-evening and late-night samples. Ephemeropterans composed 1–10% of most light-trap samples and dipterans often composed <1% of the catch. Availability of ephemeropterans in light-trap samples were dissimilar over the season (P = 0.009). Trichopterans often composed 5–20% of light-trap samples. Over the season, insects from 11 orders were found in light-trap samples.

On 6 June, pregnant females and males at Mauss Cave ate fewer coleopterans than were available in light traps (Table 7). On other dates, availability and consumption were similar. Proportionately more lepidopterans were available than consumed by females, juveniles, and males on 17 July and 3 August. Abundance and consumption of trichopterans were usually similar. Plecopterans were overrepresented in diets of females on 19 June, and females and juveniles on 17 July and 3 August. On most sample dates, bats in all age and sex groups ate fewer ephemeropterans and dipterans than were available.

View this table:
Table 7

Light-trap samples compared to diets of pregnant (Preg), lactating (Lac), and postlactating (P-Lac) female, juvenile (Juv), and adult male Myotis grisescens. Differences, determined by analysis of variance with P values ≤ 0.05, are noted by order.

CaveDateGroupOrdinal difference (% volume diet/light trap)
Mauss6 JunePregColeoptera (17.2/53.2)
MaleColeoptera (9.2/53.2), Ephemeroptera (0.0/3.1)
19 JuneLacPlecoptera (44.1/3.9), Ephemeroptera (6.8/53.5)
3 JulyLacEphemeroptera (0.0/2.2), Diptera (0.0/0.3)
MaleEphemeroptera (0.0/2.2), Diptera (0.0/0.3)
17 JulyLacLepidoptera (8.6/55.6), Plecoptera (2.3/0.3), Ephemeroptera (0.0/1.2), Diptera (0.0/0.2)
JuvLepidoptera (10.3/55.6), Plecoptera (0.0/0.3), Ephemeroptera (0.0/1.2), Diptera (0.0/0.2)
MaleLepidoptera (16.6/55.6), Ephemeroptera (0.0/1.2)
3 AugustP-LacLepidoptera (8.7/30.5), Plecoptera (0.0/0.5), Ephemeroptera (0.0/8.1)
JuvLepidoptera (2.5/30.5), Plecoptera (0.0/0.5), Ephemeroptera (0.0/8.1), Diptera (0.0/19.4)
MaleTrichoptera (8.1/11.5), Lepidoptera (22.4/30.5), Diptera (0.0/19.4)
Beck4 JuneLacDiptera (0.0/18.3)
18 JuneLacColeoptera (10.3/37.4), Plecoptera (40.2/0.0), Diptera (0.0/0.1)
4 JulyLacEphemeroptera (0.0/4.5), Diptera (0.0/0.8)
JuvTrichoptera (12.3/29.0), Coleoptera 72.5/37.6), Lepidoptera (15.0/20.5), Plecoptera (0.0/0.1),
Ephemeroptera (0.0/14.5), Diptera (0.0/0.8)
18 JulyLacColeoptera (10.8/15.3), Lepidoptera (1.8/38.5), Ephemeroptera (0.0/3.8)
P-LacColeoptera (0.4/15.3), Lepidoptera (23.8/38.5), Ephemeroptera (0.0/3.8)
JuvLepidoptera (15.4/38.5), Plecoptera (0.0/0.2), Ephemeroptera (5.7/3.8), Diptera (3.0/5.1)
2 AugustP-LacLepidoptera (5.8/50.5), Plecoptera (0.0/0.1), Ephemeroptera 14.2/7.3)
JuvTrichoptera (44.3/4.6), Lepidoptera (1.8/50.5), Ephemeroptera (5.0/7.3)
Holton10 JunePregColeoptera (0.2/19.1), Diptera (16.8/18.5)
LacColeoptera (0.7/19.1), Ephemeroptera (0.4/42.8), Diptera (0.2/18.5)
MaleEphemeroptera (0.0/42.8), Diptera (0.0/18.5)
21 JuneMaleColeoptera (1.6/69.7), Ephemeroptera (0.0/1.0), Diptera (0.0/0.3)
2 JulyLacColeoptera (2.2/7.7), Plecoptera (0.0/0.1), Ephemeroptera (0.0/0.5)
JuvEphemeroptera (0.0/0.5)
MaleColeoptera (6.4/7.7), Ephemeroptera (0.0/0.5)
15 JulyLacColeoptera (2.8/56.7), Lepidoptera (12.2/13.1), Plecoptera (0.0/0.1), Ephemeroptera (0.0/10.6), Diptera (0.0/0.6)
JuvEphemeroptera (0.0/10.6), Diptera (0.1/0.6)
MaleEphemeroptera (0.8/10.6), Diptera (0.0/0.6)
4 AugustP-LacLepidoptera (6.2/21.1), Ephemeroptera (0.0/0.4), Diptera (0.0/0.7)
JuvLepidoptera (3.4/21.1), Diptera (0.0/0.7)
MaleColeoptera (3.6/56.5), Lepidoptera (1.2/21.1), Ephemeroptera (0.0/0.4), Diptera (0.0/0.7)

At Beck Cave, the upstream light trap usually caught insects at a higher rate than the downstream trap (1.2–62.2 g/h and 0.8–44.6 g/h, respectively). Late-night rates were less than early-evening rates. No significant differences were found in proportions of insects caught upstream and downstream. Downstream, the rate of catch decreased through the season. More ephemeropterans were caught late in the season than early (P = 0.023), although they composed only 1–17% of each catch (Table 2). Lepidopterans composed ≤87% of samples and were generally more abundant in late-night and late-season samples. During midseason, trichopterans composed ≤82% of light-trap samples. Catch in early evening was larger (P = 0.034) than late at night. Coleopterans were important in every late-night trap sample (7–77%). Dipterans composed 2% and 20% of 2 samples, but otherwise were a negligible part of the catch.

Diets of all bats at Beck Cave varied from insect availability by sample group and sample date (Table 7). During the 1st half of the season, availability and consumption of lepidopterans and ephemeropterans were similar, but later in the season, light-trap samples contained proportionately more lepidopterans (P = 0.050) and ephemeropterans (P = 0.013). Differences between diets and availability of other orders of insects were common over the season. Homopterans were consistently small parts of light-trap and dietary samples, whereas hymenopterans (generally ants) were sporadically present in both, but not on the same dates (P = 0.001), except 18 June (P = 0.780).

At Holton Cave, catch of insects in light traps was 0.1–128.0 g/h. The rate of catch increased until 2 July and decreased thereafter. Early-evening rates were greater than late-night rates. Insect capture, by order, was similar to that at Mauss and Beck caves (Table 2), except plecopterans were more common at Mauss Cave than at Holton and Beck caves (P = 0.004).

Proportions of insects captured were similar throughout the season at Holton Cave, except more dipterans (28% and 9%) were caught on 10 June than on other dates (P = 0.006). No differences were found between early-evening and late-night samples. Coleopterans were a major part of every sample

(26–91%), except 1. The 10 June late-night capture rate was the lowest recorded (0.1 g/h) and was largely ephemeropterans (86%), which composed <10% of other samples. Trichopterans and lepidopterans usually composed 10–30% of most samples. Plecopterans were never important in any sample. Hemipterans composed 16.9% of the 15 July late-night sample.

Holton Cave light-trap samples frequently contained different proportions of insects than found in bats' diets (Table 7). On 10 June, coleopterans, ephemeropterans, and dipterans were more common in light-trap samples than in diets. On 21 June, males ate fewer coleopterans and dipterans than were available. Availability and consumption of coleopterans differed on 2 July and 4 August for males, and on 2 and 15 July for lactating females. From 2 July to 4 August, ephemeropterans were underrepresented in most diets. On 15 July and 4 August, consumption of dipterans by all groups differed from availability, although dipterans were important in neither. On 15 July and 4 August, lepidopteran availability varied from consumption by several sample groups.

Summary.—The diets of gray bats were compared to availability of insects in light traps. The diet varied among locations, over time, between early and late samples, and among sample groups by sex, age, and reproductive condition. The availability of insects was similarly varied among samples. Trichopterans, coleopterans, and lepidopterans were important in both diet and light-trap samples, but there was poor correlation between corresponding diet and light-trap samples. There was similarly poor correlation between corresponding diet and light-trap samples of plecopterans, ephemeropterans, and dipterans, which were occasionally common in light-trap and dietary samples.


The gray myotis forages low over streams and impoundments (LaVal et al. 1977; LaVal and LaVal 1980; Tuttle 1976), so aquatic-based insects should be common in the diet. In Indiana, 11 individuals caught over a stream ate large numbers of trichopterans, although coleopterans, lepidopterans, and dipterans also were important in the diet (Brack et al. 1984). In contrast, the diet of a maternity colony in Indiana was composed largely of midges and other dipterans in spring and autumn, and coleopterans in summer (Whitaker et al. 2001). Brack and Mumford (1983) reported that a captive gray myotis more readily accepted and ate beetles than trichopterans and ephemeropterans. In Kentucky, although sample sizes were small, insects belonging to 9 orders were eaten, and coleopterans, trichopterans, dipterans, and lepidopterans were most frequently consumed (Lacki et al. 1995). Ephemeropterans were not eaten. In Alabama, insect remains belonging to 14 orders were found in feces collected from an unknown and varying number of gray myotis of unknown sex and age (Best et al. 1997). The most frequently represented taxa were lepidopterans, dipterans, and coleopterans.

In this study, availability of insects varied among sample locations, dates, and, sometimes, time of night. Similarly, diets varied among sample dates, locations, sometimes time of night, and among sex and age groups. Lepidopterans, coleopterans, and trichopterans were important in most diet samples and most light-trap samples, although correlation among corresponding samples was poor. Plecopterans, ephemeropterans, and dipterans were occasionally common in both light-trap and dietary samples, although again correlation in corresponding samples was poor. Hymenopterans were sporadically common in both diets and light-trap samples, but not in corresponding samples. An exception was homopterans, which often were present in very small numbers in both light-trap and diet samples. Thus, variability in diets did not coincide with variability in insect availability. Similarly, disparities between availability and consumption were apparent in Kentucky (Lacki et al. 1995) and Alabama (Best et al. 1997).

Consistent variability among diet samples, among insect samples, and between insect samples and diets at a microscale is a pattern at a macroscale. The gray myotis roosts colonially but members of a colony disperse over long distances to forage independently (LaVal and LaVal 1980), as do many species of bats (Kerth et al. 2001). The gray myotis forages in riparian habitats, over streams, and in associated terrestrial habitats, where a characteristic assemblage of insects is found, but which varies spatially and temporally (Corbet 1964), and diets vary accordingly. Thus, although specific diet samples cannot be matched to specific insect samples, similar diets and similar insect availability are found in habitats used by gray myotis in Missouri, and in Kentucky, Alabama, and Indiana.

This study indicates that a combination of selective and opportunistic feeding is used by the gray myotis, similar to studies in Kentucky (Lacki et al. 1995). Brack and LaVal (1985) found that the Indiana bat (Myotis sodalis) used a combination of selective and opportunistic feeding, and Anthony and Kunz (1977) labeled the little brown myotis (Myotis lucifugus) a selective opportunist. However, many species of Myotis are thought to feed opportunistically (Fenton and Morris 1976), and on a microscale, a multitude of intraspecific variations among sample groups of gray myotis in Missouri provides evidence of opportunistic feeding. Repeated seasonal occurrence of the Asiatic oak weevil and sporadic abundance of hymenopterans in the diet of Missouri bats also are indicative of opportunistic feeding and of feeding in a terrestrial habitat. Ants are often not available, but a swarm represents a transient, superabundant food supply used by many species of bats. Vaughan (1980) observed 2 species of Myotis opportunistically feeding on hymenopterans, which are eaten by many species of bats (Freeman 1981). The weevil is common in woodland habitats across a wide geographic area in late summer and is eaten by many species of bats (Brack 1985; Brack and Whitaker 2004).

On a macroscale, variability may indicate selective, habitat-based feeding, and the diets of little brown myotis (Anthony and Kunz 1977) and northern myotis (Myotis septentrionalisBrack and Whitaker 2001) reflect the habitats in which they forage. Bat species in a western Indiana community ate different insects, indicating a degree of selectivity (Whitaker 2004). The pattern of consumption by the gray myotis is consistent with use of aquatic-based habitats while foraging, and within these habitats, consumption of the insects encountered. Optimal foraging theory (MacArthur 1972; Schoener 1971) predicts that palatable insects of an appropriate size encountered should be eaten. Barclay and Brigham (1994) found that under natural conditions, echolocating insectivorous bats attacked all suitably sized prey because they have only a fraction of a second between prey detection and capture, which may be insufficient to distinguish among prey, even if it were beneficial. If true, habitat may be the largest factor determining the prey eaten. Across time and space, the gray myotis selectively feeds over aquatic and wooded riparian habitats and eats similar types and, proportions of prey. Space and time are important dimensions in determining how a species or population responds to a patchy environment (Wiens 1976).

Females at Roaring Spring Cave and dispersal caves in 1978, and all caves in 1979, exhibited a general though erratic decrease in diet diversity throughout the summer. Brack and LaVal (1985) reported a similar decrease in diversity over the season for male Indiana bats. Late in summer, insects may be sufficiently abundant that bats can capitalize on an abundant resource, feeding on insect swarms or concentrations, or taking easily caught prey. This would reduce the time and energy spent feeding, and is consistent with actions of an optimal forager in a patchy environment (MacArthur and Pianka 1966). The diet of females in late summer was dominated by coleopterans in 1978 and by trichopterans or trichopterans and coleopterans in 1979.

Juveniles ate a less diverse diet than did adult females. There are 3 possible explanations, which need not be exclusive: juveniles are dependant upon concentrations of abundant or swarming prey, juveniles eat proportionately more easily caught insects, and juveniles forage in a different habitat than females. Juveniles ate more coleopterans and lepidopterans, which are more common in terrestrial habitats, than did females. Notably, at the time in the season when juveniles become volant and begin foraging, the Asiatic oak weevil is abundant and it was common in the diet of all samples of bats, especially juveniles. This weevil, a relatively poor flyer, was not caught in light or sticky traps placed along waterways; it was caught in traps within the foliage of riparian woods. LaVal et al. (1977) and LaVal and LaVal (1980) reported that the gray myotis sometimes foraged in riparian woodlands and occasionally agricultural lands along streams.

The young of many species apparently accompany adult females when foraging (Brigham and Brigham 1989), but juvenile gray myotis, which apparently foraged over wooded terrestrial habitats more than adults, caught more coleopterans. Coleopterans may provide a nutritional advantage, or more likely, a greater energy payoff per unit of capture effort. Items that may affect energy payoff are ease of capture, caloric concentration, prey size, and handling costs. Cummins and Wuycheck (1971) indicated that although calories per gram of dry weight is similar for many groups of insects (often 5,000–6,000 calories/g), calories per gram of wet weight can vary dramatically (800–1,400 calories/g wet weight for ephemeropterans versus 1,900–2,800 calories/g wet weight for terrestrial coleopterans).

The little brown myotis selects larger insects within the range of manageable prey (Fenton and Morris 1976). Prey parts in feces from gray myotis ranged up to 1 cm. Although this may provide a crude indication of the range of size of prey eaten, it should not be considered indicative of the frequency of prey size consumed. Brack and Mumford (1983) reported that a single captive female gray myotis readily ate beetles, consuming insects up to 2 cm in length, while culling legs and other hard chitinous parts. Barclay and Brigham (1991) argued that large insects are relatively unavailable to bats, constraining body size of prey. Dipterans, which often are small in size, were usually uncommon in the diet, but when they were common, it was early in the season when the biomass of available insects was low. This may indicate selection against small prey, dipterans, or both when other prey is available. Whitaker et al. (2001) also found dipteran consumption high in spring, low in summer, and high in October and November.

In summary, gray myotis often forage along waterways where they typically eat readily available insect prey. Because individuals from a colony forage over large distances, temporal and spatial availability of insects varies among individuals and populations, and differs from locally observed insect populations. However, collectively, gray myotis consistently eat insects that are most abundant over space and time in the habitat where they forage, that is, aquatic and wooded riparian habitats. Juveniles feed more frequently in woodlands than do adults, where they eat more coleopterans. Conservation efforts should include both streams and riparian woodlands.


This study was made possible, in part, by Federal Aid in Wildlife Restoration Act funds under Missouri's Pittman-Robertson Project (W-13-R) and the office of endangered species (project SE-2). We thank personnel from the Missouri Department of Conservation and those who assisted with fieldwork, especially T. Zinn, M. LaVal, and B. Miller. At Purdue University, thanks go to R. Mumford and H. Weeks for computer time and G. McCabe and his students for statistical advice. G. Finni provided information on caloric content of insects. J. O. Whitaker, Jr., and T. L. Best reviewed the manuscript. Environmental Solutions & Innovations, Inc., provided financial support for manuscript preparation. We dedicate this paper to the memory of Dean Metter and Gary Finni.


  • Associate Editor was William L. Gannon.

Literature Cited

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