Female Big Brown Bats Research Articles

Hibernating female big brown bats (Eptesicus fuscus) adjust huddling and drinking behaviour, but not arousal frequency, in response to low humidity.

Many mammals hibernate during winter, reducing energy expenditure via bouts of torpor. The majority of a hibernator's energy reserves are used to fuel brief, but costly, arousals from torpor. Although arousals likely serve multiple functions, an important one is to restore water stores depleted during torpor. Many hibernating bat species require high humidity, presumably to reduce torpid water loss, but big brown bats (Eptesicus fuscus) appear tolerant of a wide humidity range. We tested the hypothesis that hibernating female E. fuscus use behavioural flexibility during torpor and arousals to maintain water balance and reduce energy expenditure. We predicted: (1) E. fuscus hibernating in dry conditions would exhibit more compact huddles during torpor and drink more frequently than bats in high humidity conditions; and (2) the frequency and duration of torpor bouts and arousals, and thus total loss of body mass would not differ between bats in the two environments. We housed hibernating E. fuscus in temperature- and humidity-controlled incubators at 50% or 98% relative humidity (8°C, 110 days). Bats in the dry environment maintained a more compact huddle during torpor and drank more frequently during arousals. Bats in the two environments had a similar number of arousals, but arousal duration was shorter in the dry environment. However, total loss of body mass over hibernation did not differ between treatments, indicating that the two groups used similar amounts of energy. Our results suggest that behavioural flexibility allows hibernating E. fuscus to maintain water balance and reduce energy costs across a wide range of hibernation humidities.

ECR Spotlight – Kristina Muise

ECR Spotlight is a series of interviews with early-career authors from a selection of papers published in Journal of Experimental Biology and aims to promote not only the diversity of early-career researchers (ECRs) working in experimental biology but also the huge variety of animals and physiological systems that are essential for the ‘comparative’ approach. Kristina Muise is an author on ‘ Hibernating female big brown bats (Eptesicus fuscus) adjust huddling and drinking behaviour, but not arousal frequency, in response to low humidity', published in JEB. Kristina conducted the research described in this article while a Master's student in Craig Willis's lab at University of Winnipeg, Canada. She is now a Veterinary Student at Royal Veterinary College, UK.

Social network characteristics and predicted pathogen transmission in summer colonies of female big brown bats (Eptesicus fuscus)

Host behavior can affect host-pathogen dynamics, and sociality is predicted to increase risk of pathogen exposure. Many species minimize costs of parasitism by only aggregating seasonally, such as during reproductive periods, but colonial species may still be limited in their potential to evade pathogens. Bats are among the most gregarious mammals and females of many temperate species form maternity colonies in summer where they communally raise pups in both natural and anthropogenic roost structures. Social network structure may differ between natural and anthropogenic roosts in ways that affect pathogen dynamics. We used social network analysis to quantify interactions of big brown bats (Eptesicus fuscus) in a tree-roosting colony, where the colony is divided among multiple trees each day, and a building colony, where most of the colony roosts together each day. We simulated transmission of a pathogen throughout both sets of networks. We tested three hypotheses: (1) network metrics differ between pregnancy and lactation; (2) changing network structure between reproductive stages influences predicted pathogen dynamics; and (3) network metrics and predicted pathogen dynamics differ between colonies of bats in trees versus buildings. Network structure was weaker for bats roosting in trees during pregnancy and lactation compared to bats roosting in a building, and our models showed that a hypothetical pathogen would spread more rapidly for bats in the building colony. Our results are important for understanding variation in social tendencies and pathogen transmission among colonies of bats and have implications for conservation and public health. Host behavior, particularly social behavior, can affect dynamics of wildlife pathogens. Bats are highly social mammals and females of temperate species form colonies in spring and early summer in tree or building roosts. Thermal characteristics of trees and buildings appear to differ in ways that affect roosting behavior and social interactions. We used social network analyses to quantify interactions of big brown bats in tree and building roosts and simulated consequences for pathogen dynamics. Network structure was weaker for bats roosting in trees with more frequent roost switching and relatively diffuse contacts across the network. Our models showed that a hypothetical pathogen could spread up to four times faster in a building colony compared to a colony of bats roosting in trees. Our results are important for understanding how sociality can influence pathogen dynamics in bats and have implications for conservation and public health.

Choice Experiments Demonstrate that Male Big Brown Bats (Eptesicus fuscus) Prefer Echolocation Calls of High Copulatory Females

It generally is accepted that bats emit ultrasonic vocalizations that function for echolocation purposes as well as for communication. We tested whether male or female big brown bats (Eptesicus fuscus) responded to variation in echolocation calls of the opposite sex in a manner that would suggest calls are used in a mating context. We presented 31 female and 10 male big brown bats with ultrasonic playbacks of differentially mating (i.e., high frequency copulators = HM versus low-frequency copulators = LM) individuals of the opposite sex. We measured 1) which side of the arena each subject selected first (HM versus LM), and 2) duration spent (seconds) on each side of the arena (HM versus LM). For both of these measures (i.e., first choice and duration) male subjects were more likely to select the echolocation calls of HM females, but the same respective tests determined that female subjects did not select echolocation calls of frequently copulating males over calls of infrequently copulating males. These r...

Wound healing in the flight membranes of wild big brown bats

ABSTRACTThe flight membranes of bats are susceptible to damage (e.g., holes and tears) from a number of sources, including impacts with natural and man‐made objects, fighting between conspecifics, and attacks by predators or pathogens. Biologists routinely biopsy bat wings as a method of tissue collection for molecular research, and sometimes for the temporary identification of animals in the field. A previous study reported that captive big brown bats (Eptesicus fuscus) rapidly and completely healed flight membrane wounds. Given that limited care is provided to animals following tissue biopsy in the field, we sought to determine whether healing times for wounds from bats in captivity were applicable to bats in the wild. We measured and compared healing times of wounds in the wing and tail membranes of 50 non‐reproductive female big brown bats from a wild population in Cuba following recapture. Tail wounds healed significantly faster than wing wounds of the same size, likely because of the increased thickness and vasculature of the tail membrane. Our data are concordant with a previous laboratory study in captive big brown bats, and confirm that tail membrane biopsies are better for obtaining tissue samples for molecular work because tail wounds heal faster than wing wounds. © 2015 The Wildlife Society.

Seasonal and reproductive effects on wound healing in the flight membranes of captive big brown bats.

ABSTRACTThe flight membranes of bats serve a number of physiological functions important for survival. Although flight membrane injuries are commonly observed in wild-caught bats, in most cases the damage heals completely. Previous studies examining wound healing in the flight membranes of bats have not taken into consideration energy constraints that could influence healing times. Wound healing results in increased energy demands, therefore we hypothesized that wound healing times would be slower during periods of energy conservation and/or energy output. In this study we used an 8 mm diameter circular punch tool to biopsy the wing membranes of healthy adult female big brown bats (Eptesicus fuscus) from a captive research colony to test the hypothesis that healing times will vary with seasonal temperature changes between the summer and winter seasons, and with reproductive condition between lactating and non-reproductive females. As expected, membrane biopsies took significantly longer to heal during the winter when bats were hibernating compared to the summer when bats were active. Surprisingly, no difference in healing time was observed between lactating and non-reproductive females. The wings of most bats fully healed, although some individuals showed wound expansion demonstrating that impaired healing is occasionally observed in otherwise healthy subjects.

Sex and Season Differences in the Echolocation Pulses of Big Brown Bats (Eptesicus fuscus) and Their Relation to Mating Activity

Although several studies have reported inter- and intra-individual variation in the echolocation pulses of big brown bats, relatively few have explored its function in the context of courtship and mating. In this study, we tested for sex and seasonal differences in the echolocation pulses of male and female big brown bats, and related these differences to mating activity. We recorded the echolocation pulses of adult female and male big brown bats in a controlled laboratory environment during the mating and three non-mating seasons. Factor analysis was used to reduce the number of call variables into two principle components (PC1 and PC2). Call components related to PC2 were significantly sexually dimorphic in the mating season. However, no call components were significantly sexually dimorphic in the non-mating season. In addition, we found a significant correlation between PC1 and the mating score of male bats. There was no significant correlation between principle components and the mating score of femal...

The influence of reproductive condition and concurrent environmental factors on torpor and foraging patterns in female big brown bats (Eptesicus fuscus).

Unlike many other mammals, bats in temperate regions employ short bouts of torpor throughout the reproductive period to maintain a positive energy balance. In addition to decreasing energy expenditure during the day, they typically alter foraging patterns as well. It is well known that various environmental conditions influence both torpor and foraging patterns, but studies of these factors often have focussed on one element in isolation thus it is not known how the two behaviours are collectively influencing temperate bats. The objective of our study was to assess how reproductive condition and environmental factors concurrently affect energy balance in female big brown bats (Eptesicus fuscus). We equipped pregnant and lactating bats in southwest Saskatchewan, Canada with temperature-sensitive radio-transmitters. While transmitters were active, skin temperature data were collected and foraging patterns were determined using triangulation. Of the various environmental and physiological parameters used to model torpor characteristics, roost type was the most important factor. Bats roosting in trees used deeper and longer torpor bouts than those roosting in buildings. Lactating bats had a tendency to forage for longer durations than pregnant bats, and often made more foraging trips. When taken together, we found that foraging duration and torpor duration were not directly related during pregnancy, but exhibited an inverse relationship during lactation. This provides support for the hypothesis that there are physiological trade-offs for reproductive bats and suggests that how bats compensate is not entirely predictable based on current environmental conditions.

Evidence of social preferences in big brown bats (Eptesicus fuscus)

Among social species, iterative interactions may lead to social preferences among group-mates and often are associated with increased mating opportunities or improved indirect fitness benefits. Although preferential associations have been documented in multiple species, this phenomenon has never been empirically studied in bats—the second largest order of mammals, where many of the 1200+ species live in groups of tens to hundreds of individuals. Given the current understanding of the social behaviour exhibited in this species, we explored the possibility that adult female big brown bats (Eptesicus fuscus (Beauvois, 1796)) show preferential association between group-mates using a pairwise choice design with individuals from a captive colony. Focal individuals were placed in a Y-maze and were given free choice of two familiar conspecifics. We measured the time focal individuals spent in close proximity to each conspecific. Our results indicate that some bats exhibit preferential association between group-mates, as multiple individuals spent significantly more time in close proximity to one conspecific versus another, despite randomizing the position of stimulus bats between trials. Given the frequent and long-term associations between group members of this species, social preferences could play a significant role in the outcomes of their long-term fitness.

The Relationships between Behavioural Categories and Social Influences in the Gregarious Big Brown Bat (Eptesicus fuscus)

AbstractBehavioural plasticity is a critical component of natural selection leading to evolution. However, a surge of studies in the last two decades has discovered a distinct limit to behavioural plasticity, commonly referred to as behaviour types and behavioural syndromes. We set out to understand the relationships across behavioural categories in wild‐caught adult, female big brown bats and how they compare between social and solitary behaviours. Using bats sampled from four different maternity colonies, we ran a series of behavioural assays to create a behavioural profile for each individual. The behavioural profile encompassed exploratory, learning, competitive and aggressive categories. We found that Big brown bats exhibit a mean profile relatively unique to other well‐documented species, where aggression was linked to increased competitive ability but not to boldness. Our results indicate that the solitary and socially directed behaviours of individuals are not necessarily related and that behaviours pertaining to social interactions are linked most closely to learning abilities. Furthermore, we found evidence that poor body condition may be a predictor of increased social interactions and that behaviours exhibited in the presence of conspecifics are unrelated to those exhibited in solitude. These findings indicate importance of social affiliations on individual behaviours in this species and their uniqueness relative to other well‐studied taxa.

Tissue mercury concentrations and adrenocortical responses of female big brown bats (Eptesicus fuscus) near a contaminated river

Much of the research on mercury (Hg) in wild vertebrates has focused on piscivores and other animals at high trophic levels. However, recent studies indicated that insectivorous terrestrial vertebrates may also be at risk. In the present study, we examined blood and fur Hg concentrations as well as the adrenocortical responses of insectivorous big brown bats (Eptesicus fuscus) near the Hg-contaminated South River, VA and a nearby reference area. Baseline glucocorticoids and adrenocortical responses to handling have been widely used to assess the influence of environmental stressors because plasma glucocorticoids rise in response to various physical, psychological, and physiological challenges. Female bats captured at the contaminated site had 2.6 times higher blood and fur Hg concentrations than those captured at the reference site (blood: 0.11 vs. 0.04 μg/g wet weight; fur: 28.0 vs. 10.9 μg/g fresh weight). Fur Hg concentrations at the contaminated site were higher than most wild omnivorous and carnivorous mammals reported in the literature. Although fur and blood Hg concentrations were tightly correlated, fur Hg concentrations averaged 260 times higher than concentrations in blood. This suggests that fur may be an important depuration route for bats, just as it is in other mammals. Despite the high Hg concentrations in bat tissue, we did not observe any site difference in adrenocortical responses. Our results suggest that the bats at the contaminated site were exposed to Hg concentrations below those causing adverse effects on their adrenal axis.

Bald Eagle Predation on Harbor Seal Pups

and roost selection by reproductive female big brown bats (Eptesicus fuscus) roosting in rock crevices. Journal of Zoology 260:235-244. Lausen CL, Barclay RMR. 2006. Benefits of living in a building: big brown bats (Eptesicus fuscus) in rocks versus buildings. Journal of Mammalogy 87:362-370. riskin DK, pybus MJ. 1998. The use of exposed di urnal roost in Alberta by the little brown bat, My otis lucifugus. Canadian Journal of Zoology 76: 767-772. slough BG, jung TS. 2008. Observations on the nat ural history of bats in the Yukon. The Northern Review 29:127-150. Speakman JR, Hays GC. 1992. Albedo and trans mittance of short-wave radiation for bat wings. Journal of Thermal Biology 17:317-321. studier EH. 1981. Energetic advantages of slight drops in body temperature in little brown bats (Myotis lucifugus). Comparative Biochemistry and Physiology A 70:537-540. turbill C. 2006. Roosting and thermoregulatory be haviour of male Gould's long-eared bats, Nycto philus gouldi: energetic benefits of thermally un stable tree roosts. Australian Journal of Zoology 54:57-60. vaughan TA. 1987. Behavioral thermoregulation in the African yellow-winged bat. Journal of Mam malogy 68:376-378. Wilde C, Knight C, Racey P. 1999. Influence of tor por on milk protein composition and secretion of lactating bats. Journal of Experimental Zoology 284:35-41. Willis KR, Brigham RM. 2007. Social thermoregu lation exerts more influence than microclimate on

Sexual Dimorphism in Big Brown Bat (Eptesicus fuscus) Ultrasonic Vocalizations is Context Dependent

Although bats are well known for their use of ultrasound for echolocation, there is limited evidence for its use in a social context. We tested whether ultrasonic vocalizations in bats were contextually (roosting or flying) sexually dimorphic. During the reproductive season, we recorded ultrasonic signals of captive adult male and female big brown bats while the bats were flying on tether lines in the field, and compared these signals to ultrasonic vocalizations made while roosting in an anechoic chamber. Principal component analysis reduced 7 ultrasonic call descriptors to 2 components that related to frequency (PC1) and time or shape (PC2). While bats were roosting, ultrasonic call components related to time or shape and frequency were both sexually dimorphic, being increased in males in each instance. However, when bats were recorded while flying, these same call components were no longer sexually dimorphic. This finding suggests that bats are changing their ultrasonic calls in relation to functional context, making them monomorphic and utilitarian for activities such as foraging and navigation, but dimorphic in a situation when mating activity is likely.

Use of Conifer Snags as Roosts by Female Bats in Western Oregon

ABSTRACT Forest management activities influence habitat suitability for bats, and knowledge of the roosting ecology of bats is fundamental to developing strategies for conserving bats in managed forests. Information on use of roosts by multiple species of bats in a given area may provide insight into interspecific ecological patterns and could improve management prescriptions to provide habitat for bats through time across diverse ownerships and over multiple spatial scales. We investigated use of conifer snags as roosts by females of 3 species of forest‐dwelling bats in Douglas‐fir (Pseudotsuga menzesii) forests in the western Oregon Cascade Range. We radiotagged 29 female big brown bats (Eptesicus fuscus), 55 long‐legged myotis (Myotis volans), and 27 long‐eared myotis (Myotis evotis) and located 42, 105, and 24 snag roosts for the 3 species, respectively. All 3 species most frequently used Douglas‐fir snags and in similar proportions to their availability. Big brown bats and long‐legged myotis rarely roosted in stands <40 years old but age of stands used by female long‐eared myotis did not differ from those randomly available. Odds of a snag being used as a roost by big brown bats increased with diameter at breast height and decreased with distance from the capture site. Diameter of snags used for roosting and the number of small (10–50 cm dbh) snags within the 20‐m radius plot were variables in the best model for roost use by long‐legged myotis. The best model for long‐eared myotis included distance to the capture site. Odds of a snag being used by female long‐eared myotis decreased with increasing distance from the capture site. There was considerable overlap in structural characteristics and the physical context of roost snags among the 3 species, but the types of roosts used among landscapes with differing densities of snags differed among the 3 species. Although big brown bats and long‐legged myotis used only snags and live trees as roosts, long‐eared myotis used a diversity of structures and the frequency of use of these structures differed with density of snags in the landscape. Relative to other roost types, frequency of use of snags by long‐eared myotis was nearly twice as high in landscapes with high densities of snags as in those with low densities. We found that some species of bats alter selection of roosts depending on landscape context and availability of different types of roosts. Our findings demonstrate that forest managers must consider the needs of multiple bat species and the distribution of roosts in the landscape, especially where densities of snags are low and at low elevations in intensively managed landscapes.

Social thermoregulation exerts more influence than microclimate on forest roost preferences by a cavity-dwelling bat

Nest or roost temperature (T roost) is thought to impact reproductive fitness in many endotherms but few studies have directly tested the hypothesis that naturally occurring variability in nest or roost microclimate is large enough to affect reproductive success. We conducted a field experiment to test whether roost selection by cavity-dwelling, reproductive female big brown bats (Eptesicus fuscus) is more strongly influenced by roost microclimate or a physical characteristic of roosts that facilitates social thermoregulation (i.e., cavity volume). We quantified spatial variability in T roost within different-sized, unoccupied cavities and also recorded T roost in occupied vs unoccupied roost trees. We used equations relating energy use and ambient temperature for big brown bats to calculate values of daily energy expenditure from T roost data because energy is a currency that likely affects reproductive fitness. We found no difference between maximum and minimum T roost, spatial variability in T roost, or predicted energy expenditure in more-preferred vs less-preferred roosts. However, there was a significant difference between T roost and predicted energy expenditure when we compared occupied vs unoccupied roosts. The presence of bats increased T roost by as much as 7°C, and there was a significant positive correlation between the number of bats occupying a roost, maximum daily T roost, and energy savings. We calculate that, on average, a normothermic individual would save about 6.5 kJ/day (roughly 9% of the daily energy budget) by roosting in an occupied cavity relative to roosting alone and that savings may increase to 40 kJ/day (about 53% of the energy budget) for an individual roosting in a group of 45 bats. Our findings suggest that variability in microclimate among potential roost or nest sites may be less important to some cavity-dwelling endotherms than has been suggested in previous studies. Our results reinforce the importance of sociality and social thermoregulation to the roosting ecology of forest-living bats and socially roosting or nesting endotherms in general.

Maternity roost site selection of big brown bats in ponderosa pine forests of the Channeled Scablands of northeastern Washington State, USA

Bats play a variety of ecological roles in forest ecosystems and forest management can impact habitat conditions for forest-dwelling bats. We examined the use and characteristics of roosts selected by reproductive female big brown bats ( Eptesicus fuscus) in the Channeled Scablands of northeastern Washington. We radio-tracked 14 bats to locate 36 roosts. Bats were found in colonies averaging 27 and these colonies switched roosts about every 3.7 days. Habitat variables were measured for the roost itself and at a 0.1-ha microplot and 78-ha macroplot surrounding each roost. We measured habitat variables at random 0.1-ha microplots in the vicinity of each roost and at general random 0.1-ha microplots and 78-ha macroplots. Of the 36 roosts located, 34 were in natural tree cavities; 28 were in ponderosa pine ( Pinus ponderosa) and eight in quaking aspen ( Populus tremuloides). Dead tops of live pine trees were used significantly more as roosts than pine snags. Although there were significantly more roosts in trees >30 cm in diameter and >12 m high than what was available, roosts were not always the tallest tree in the stand. A significantly greater proportion of big brown bat roosts were found in open pine, aspen and mixed-aspen pine forests and less in grasslands and closed pine than expected. Forest management strategies should protect both large diameter snags and existing dead top live trees and maintain natural population levels of biological agents that create dead tops. Restoration of historic open conditions in ponderosa pine and aspen stands will provide improved habitat for big brown bats.

FACTORS INFLUENCING MOVEMENT PROBABILITIES OF BIG BROWN BATS (EPTESICUS FUSCUS) IN BUILDINGS

We investigated movements of female big brown bats (Eptesicus fuscus) roosting in maternity colonies in buildings in Fort Collins, Colorado (USA), during the summers of 2002, 2003, and 2005. This behavior can be of public health concern where bats that may carry diseases (e.g., rabies) move among buildings occupied by people. We used passive integrated transponders (PIT tags) to mark individual bats and hoop PIT readers at emergence points to passively monitor the use of building roosts by marked adult females on a daily basis during the lactation phase of reproduction. Multi-strata models were used to examine movements among roosts in relation to ambient temperatures and ectoparasite loads. Our results suggest that high ambient temperatures influence movements. Numbers of mites (Steatonyssus occidentalis) did not appear to influence movements of female bats among building roosts. In an urban landscape, periods with unusually hot conditions are accompanied by shifting of bats to different buildings or segments of buildings, and this behavior may increase the potential for contact with people in settings where, in comparison to their more regularly used buildings, the bats may be more likely to be of public concern as nuisances or health risks.

Avoidance of predator chemical cues by bats: an experimental assessment

[Predation places selection pressure on the behaviour of organisms, but the effect of predation risk on bat behaviour has gone largely untested. Using a Y-maze, we tested the hypothesis that female big brown bats ( Eptesicus fuscus ) avoid olfactory cues from known predators — raccoons ( Procyon lotor ) and black rat snakes ( Elaphe obsoleta ). These chemical cues are most likely encountered by bats during the assessment of potential roost sites, so avoidance of predator cues could affect roost selection. We found that big brown bats do not avoid cues from raccoon urine and there was a non-significant trend for bats to choose the arm with black rat snake cues. In a second experiment, bats were exposed to olfactory and potentially auditory cues of live black rat snakes. Bats tended to avoid the arm with live snakes, although results were non-significant. This suggests olfactory cues may not be representative of live snakes, or that auditory cues may be more important than olfactory cues for predator recognition. During late spring, female bats often avoid torpor for thermoregulatory or reproductive reasons. Therefore, bats may be alert to the approach of predators and be less responsive to chemical cues compared to other times of the year., Predation places selection pressure on the behaviour of organisms, but the effect of predation risk on bat behaviour has gone largely untested. Using a Y-maze, we tested the hypothesis that female big brown bats ( Eptesicus fuscus ) avoid olfactory cues from known predators — raccoons ( Procyon lotor ) and black rat snakes ( Elaphe obsoleta ). These chemical cues are most likely encountered by bats during the assessment of potential roost sites, so avoidance of predator cues could affect roost selection. We found that big brown bats do not avoid cues from raccoon urine and there was a non-significant trend for bats to choose the arm with black rat snake cues. In a second experiment, bats were exposed to olfactory and potentially auditory cues of live black rat snakes. Bats tended to avoid the arm with live snakes, although results were non-significant. This suggests olfactory cues may not be representative of live snakes, or that auditory cues may be more important than olfactory cues for predator recognition. During late spring, female bats often avoid torpor for thermoregulatory or reproductive reasons. Therefore, bats may be alert to the approach of predators and be less responsive to chemical cues compared to other times of the year.]

Variation in body composition of female big brown bats (Eptesicus fuscus) during lactation

Most small mammals support the nutritional requirements of milk production by increasing food intake. However, when nutrient intake is low, maternal body reserves may be mobilized to maintain adequate milk output. We examined patterns of body composition, including dry matter, fat, protein, and mineral content in big brown bats, Eptesicus fuscus, during lactation. Concentrations of fat and phosphorus were markedly lower in lactating mothers during week three of lactation than during the first two weeks, but these constituents rebounded to previous levels in the fourth and fifth week. Rapid recovery from fat depletion suggests that females are able to adjust to changes in demands for energy. The decrease in phosphorus during mid-lactation suggests bone demineralization, but an interspecific comparison of adult concentrations of minerals prevalent in bone suggests that mineral concentrations may never reach critically low levels in reproductively active females.

ROOST SELECTION BY FOREST-LIVING FEMALE BIG BROWN BATS (EPTESICUS FUSCUS)

Previous studies of forest-dwelling bats have identified physical features of trees and forests that correlate with the presence of bats by comparing roost sites to paired, randomly selected sites. This method may be limited if the absence of bats from random sites cannot be confirmed. Our purpose was to address roosting ecology of female big brown bats (Eptesicus fuscus) using a different approach. We quantified relative use of trees with 3 different types of cavity openings (long crevices, multiple holes, or single holes) and compared the relative use of these potential roosts to the availability of each roost type in the study area. Bats used trees with multiple holes and crevices significantly more often than expected based on their availability and trees with single holes less often than expected. Crevice roosts had significantly larger cavities than did single holes and roosting-group size was positively correlated with cavity volume. No relationship was found between cavity volume and tree height or stem diameter of roost trees, 2 variables that have been reported to correlate with roost selection in other studies of forest bats. Examination of our data suggests that the volume of roost cavities may be an important selection criterion for colonial, forest-living bats and that standard interpretations of the roost versus random-tree approach may not accurately identify patterns of roost selection in some systems.