Female Bats Research Articles

Elevated vitamin D levels in diurnally-active female fruit bats

Animal species have evolved to enhance their survival by focusing their temporal activity on specific parts of the diurnal-nocturnal cycle. Various factors, including inter-specific competition and anti-predator behavior, as well as anthropogenic effects like light pollution, have prompted some species to expand or shift their temporal niches. Our study focuses on the temporal niche shift of the Egyptian fruit bat (Rousettus aegyptiacus) to diurnal activity in Israel.Through an extensive citizen-science study, we assessed the distribution of these bats’ diurnal activity across Israel. We also documented the sex and age of bats from a colony known for its diurnal activity and collected blood samples from them for metabolic analysis.Our findings indicate that the shift toward daytime activity predominantly takes place in urban settings and is mostly exhibited by females. We found a significant physiological effect of this temporal shift, namely: diurnal bats’ vitamin D levels were significantly higher, and their parathyroid hormone (PTH) levels were significantly lower than those of nocturnal bats.We suggest that the reproductive metabolic demands of female bats might be a key factor driving this shift to diurnal activity. We hypothesize that the increase in vitamin D, derived from sunlight hours, might play a crucial role in regulating calcium homeostasis, thus contributing to the bats’ physiological needs during the reproduction season.

Assessing the Home Range, Foraging Pattern and Roost Fidelity of Greater Short-nosed Fruit Bat (Cynopterus sphinx) in a Semi-Urban Environment

Radio telemetry is a method used in wildlife ecology to examine the movements and behaviours of animals, as well as to define their home ranges and habitat preferences. Bats, a highly diverse group of mammals, are recognized for their essential contributions to ecosystems, including seed dispersal, pollination, and pest control through their foraging activities. This study utilised radio telemetry to explore the basic foraging behaviours, range of movement, and roosting flexibility of the Cynopterus sphinx species. A total of 15 bats were equipped with compact, hand-wired, two-stage transmitter radio devices, including two harem males, nine harem females, and four non-harem males. Three groups used TRX-1000S receivers and collapsible 5-element Yagi antennas to track these radio-tagged bats. Our findings indicate that the radio-tagged female bats were the first to leave their roosts, with emergence times between 18:15 and 19:18 hrs. In contrast, harem males emerged last, following the quicker departure of non-harem males. The harem males covered an average foraging distance of 2.4 km (±0.3 km), whereas non-harem males travelled further. Both male and female bats visited multiple feeding sites nightly, with females travelling an average distance of 4.5 km (±0.9 km). Time spent at foraging sites varied among individuals. Males show higher roost fidelity than females, who frequently move between harems and roost sites. The study highlighted gender-specific differences in emergence times and distinct foraging behaviours, underlining the importance of understanding these patterns for conservation and habitat management efforts, which are crucial for supporting bats' roles as providers of ecosystem services.

Hydroelectric dam impacts shorten and delay the reproductive periods of female leaf-nosed bats in Western Amazonia

Abstract The life cycles of bats are constrained by availability of resources such as food and shelter. Disruptions limiting otherwise naturally regulated access to these resources ultimately affect the fitness of bat populations and may eventually lead to local extinctions. We sampled bats before and after an Amazonian mega-dam water filling and retrieved data on their reproductive phenology, testing if and how sex ratio, seasonal abundance, and reproductive patterns respond to impacts of the newly operating hydroelectric dam. We analyzed data from 3,298 bat captures of 49 species. Females outnumbered males, previously and after the dam implementation, and we observed seasonal variations in the abundance of bats, which was lower after the implementation of the dam for most species. All species had at least 1 reproductive peak of pregnancy and/or lactation beginning at a variable time across seasons. Most species and functional ensembles were reproductively bimodal, with exceptions being the polymodal Artibeus obscurus and unimodal Phylostomus elongatus. Operations of the hydroelectric dam affected the abundance of both sexes equally, and effects were stronger in the rainy season, altering the timing of reproduction for females, delaying and shortening pregnancy and lactation. These changes expressed in female reproductive activity may be related to the shortfall of resources because of loss of irreplaceable vegetation caused by the dam filling. As we cannot predict the roles of variation of reproductive patterns observed after the filling in the long run we reinforce the importance of long-term, continuous monitoring and management to safeguard the reestablishment of reproductive patterns of females in order to repopulate and keep bat populations healthy in remaining areas along following dam construction.

White-nose syndrome, winter duration, and pre-hibernation climate impact abundance of reproductive female bats.

White-nose syndrome (WNS) is an infectious disease that disrupts hibernation in bats, leading to premature exhaustion of fat stores. Though we know WNS does impact reproduction in hibernating female bats, we are unsure how these impacts are exacerbated by local climate factors. We compiled data from four southeastern U.S. states and used generalized linear mixed effects models to compare effects of WNS, pre-hibernation climate variables, and winter duration on the number of reproductive females in species across the range of WNS susceptibility. We predicted we would see a decline in the number of reproductive females in WNS-susceptible species, with the effect exaggerated by longer winter durations and pre-hibernation climate variables that lead to reductions in foraging. We found that the number of reproductive females in WNS-susceptible species was positively correlated with pre-hibernation local climate conditions conducive to foraging; however, WNS-susceptible species experienced an overall decline with the presence of WNS and as winter duration increased. Our long-term dataset provides evidence that pre-hibernation climate, specifically favorable summer weather conditions for foraging, greatly influences the reproduction, regardless of WNS status.

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.

Summer habitat for the female Tricolored Bat (Perimyotis subflavus) in Tennessee, United States

Abstract The Tricolored Bat is an imperiled species due to white-nose syndrome. There is limited information available on roosting and foraging area use of the species to support planning and management efforts to benefit recovery in the Southeastern United States. Female tricolored bats exit hibernation and allocate energy toward disease recovery, migration, and reproduction. Providing and managing for summer habitat is 1 strategy to promote recovery. We sought to: (1) determine local- and landscape-scale factors that influence female Tricolored Bat roost selection; (2) quantify land cover use in core and overall foraging areas; and (3) define foraging area size and distances traveled by female tricolored bats in Tennessee. Bats in this study roosted in trees of variable sizes, in multiple tree species with large canopy volumes, and almost always roosted in trees with dead leaf foliage suspended in the canopy. Forest plots used by bats had trees averaging 30 cm diameter at breast height, basal areas averaging 27 m2/ha, contained multiple tree species, and comprised around a 50:50 ratio of canopy and subcanopy trees. Bats did not roost in coniferous forest areas and were only located in deciduous and mixed forest areas. Bats foraged near and directly over water, in open areas, and along forest edges. This study increases our knowledge on habitat requirements of the species in a temperate region dominated by unfragmented forests and many large water bodies and serves a baseline for management and efforts to benefit survival, reproduction, and population recovery.

Using PIT tags to infer bat reproductive status and parturition date: busy nights during lactation

Abstract Passive integrated transponders (PIT tags) can aid in the collection of important demographic data for species for which other methods, such as GPS technology, are not suitable. PIT tags can be particularly suitable to monitor small and cryptic species like bats and permit inference on their behavioral ecology. Literature for several species of bats states that females change their nightly activity patterns—going out and in of the roost only once per night during gestation compared to several times during lactation. Hence, we tested whether PIT tag detection patterns could be used to infer reproductive status and parturition date of female bats. From 2017 to 2021, we recorded detections of PIT-tagged little brown (Myotis lucifugus) and northern long-eared bats (M. septentrionalis) at the entrance of 2 maternity roosts in Québec and Newfoundland, Canada. We also used the maternity roost in Québec as a case study to further evaluate the potential of this method to link behavior and demography and investigated factors affecting parturition date. We were able to infer reproductive status for 63% to 97% of tagged individuals detected during both the gestation and the lactation periods, and parturition date for 61% to 95% of reproductive individuals, depending on the year and roost. Early spring arrival at the roost and warm spring mean temperature at night were associated with earlier parturition dates. Herein, we highlight that PIT tag systems may be useful to detect changes in activity patterns of female bats and infer individual reproductive parameters, which is on the long-term less stressful for bats and easier for researchers. We demonstrate that this approach is useful to investigate intrinsic and extrinsic factors of reproductive parameters, improving our understanding of bat population dynamics resulting in more informed population management decisions.

Sex and State-Dependent Effects on Proactive Behaviors of Bent-Wing Bats Across Contexts

Synopsis Animals within a population may show distinct behavioral types that differ consistently among individuals over time and across contexts, collectively known as animal personality. Individual state variables arising from intrinsic features of organisms and their interactions with the environment may contribute to or aid in maintaining these interindividual behavioral differences. The present study examined the effects of body mass, body condition, flight morphology, and parasite load on the personality traits of bent-wing bats Miniopterus fuliginosus. We assessed the bats in three testing contexts—hole-board box (HB), tunnel-box (TB), and flight-tent (FT)—that mimicked their natural environmental settings and allowed for different locomotion modes. A principal component analysis loaded the three mutually positively correlated personality traits of the bats—boldness, activity, and exploration—in each context onto a single component of proactiveness. In accordance with the AIC criteria, sex, body mass, body condition index, and wingtip shape were selected as predictors for the proactiveness of the bats in the TB and FT tests. In the HB tests, the biomass and abundances of parasitic bat flies were additionally selected, but body condition was excluded. We found a negative effect of the body mass on the proactiveness of the female bats in both the HB and FT tests, and that on the proactiveness of the male bats in the HB tests but not so in the FT tests. The sexual differences and negative correlation between the body mass of the bats and their proactive responsiveness are consistent with the mechanism of state-dependent energy assimilation efficiency. Our results may also concur with the predicted feedback mechanism stemming from the characteristic conditions associated with the environment of the bats. This latter inference offers insights for exploring the patterns of personality traits along gradients or the seasonality of ecological conditions.

Female little brown bats require both building and natural roosts in a mountainous environment with short summers

AbstractThe little brown bat (Myotis lucifugus) is an endangered species that occurs throughout most of North America. The recovery strategy for little brown bats in Canada identifies a need to understand habitat use and protect important bat habitat across the species' Canadian range; however, bat roosting habitat requirements have not been well studied in mountainous environments within the northern part of the species' range. Our objective was to understand the use of building and natural roosts by female little brown bats in the Canadian Rocky Mountains. Over two summers, we captured little brown bats in Banff, Yoho, and Kootenay National Parks, in Alberta and British Columbia, Canada, and tracked 49 females to their day roosts. Buildings were the only roost type used by radio‐tagged bats of all reproductive stages, the dominant roost type used throughout the reproductive period, and the only structures in which we located maternity colonies. Buildings also appeared to shape the distribution of foraging female and reproductive female bats such that few were captured at long distances from towns and building clusters. Compared with bats in more southern and in non‐mountainous areas, bats in our study area had delayed juvenile weaning and volancy, began hibernation early, and had low reproductive rates. We suggest that buildings are a key habitat for female little brown bats to persist at their current levels in our study area, by providing thermal conditions that promote overwinter survival; additional reproductive delays caused by torpor use in cool natural roosts would likely reduce juvenile and adult female overwinter survival below the levels required to sustain populations. Nevertheless, building colonies were unusually small, and bats had relatively low fidelity to particular building roosts, suggesting that building roosts may not consistently meet bats' needs. Some nonreproductive and pregnant females also frequently used natural roosts, particularly after colder nights; these roosts may be important in facilitating torpor and energy conservation for these bats. Because bats are long‐lived with low reproductive output and low juvenile survival, addressing the needs of both reproductive and nonreproductive individuals is important for population persistence.

Content of stress granules reveals a sex difference at the early phase of cold exposure in mice.

Adaptive thermogenesis is a vital physiological process for small endotherms. Female animals usually are more sensitive to cold temperature due to anatomical differences. Whether there is a sex difference at a molecular level is unclear. Stress granules (SGs) are dynamic organelles in which untranslated mRNAs reside during cellular stress. We hypothesize that the prompt response of SGs to cold stress can reveal the molecular difference between sexes. By analyzing the content in SGs of brown adipose tissue (BAT) at the early phase of cold stress for both sexes, we found more diverse mRNAs docked in the SGs in male mice and these mRNAs representing an extensive cellular reprogramming including apoptosis process and cold-induced thermogenesis. In female mice, the mRNAs in SGs dominantly were comprised of genes regulating ribonucleoprotein complex biogenesis. Conversely, the proteome in SGs was commonly characterized as structure molecules and RNA processing for both sexes. A spectrum of eukaryotic initiation factors (eIFs) was detected in the SGs of both female and male BAT, while those remained unchanged upon cold stress in male mice, various eIF3 and eIF4G isoforms were found reduced in female mice. Taken together, the unique features in SGs of male BAT reflected a prompt uncoupling protein-1 (UCP1) induction which was absent in female, and female, by contrast, were prepared for long-term transcriptional and translational adaptations.NEW & NOTEWORTHY The proteome analysis reveals that stress granules are the predominant form of cytosolic messenger ribonucleoproteins of brown adipose tissue (BAT) at the early phase of cold exposure in mice for both sexes. The transcriptome of stress granules of BAT unveils a sex difference of molecular response in early phase of cold exposure in mice, and such difference prepares for a prompt response to cold stress in male mice while for long-term adaptation in female mice.

Female pond bats hunt in other areas than males and consume lighter prey when pregnant.

Animals with large energy requirements are forced to optimize their hunting strategy, which may result in differentiation of the diet between sexes and across seasons. Here, we examined spatiotemporal variation in the diet of both sexes of the Pond Bat Myotis dasycneme, a species known to have spatial segregation of sexes when the young are born and lactating. Fecal pellets were collected from live animals for a period of 15 years at various locations in the Netherlands. A total of 535 pellets were successfully analyzed by microscopy and an additional 160 pellets by DNA metabarcoding. Morphological and molecular analyses showed that the diet of pregnant and lactating pond bats differed significantly from the diet of females with no reproductive investment. Further analyses of the data showed that pregnant female pond bats are highly dependent on small prey and pupae, mainly nonbiting midges and mosquitoes (Diptera: Chironomidae and Culicidae). These insects can be found in large quantities in peatlands intersected with shallow waterways, the habitat type in which female pond bats were observed more often than males. Our results suggest that during pregnancy the spatial segregation of sexes coincides with sex-specific diets, which might reflect habitat selection based on energy requirements, in addition to lowered intraspecific competition.

Virome characterization and identification of a putative parvovirus and poxvirus in bat ectoparasites of Yunnan Province, China

Ectoparasites found on bats are known to contain important microbes. However, the viruses hosted by these obligate parasites are understudied. This has led to the near oversight of the potential role of these ectoparasites in virus maintenance and transmission from bats to other interacting species and the environment. Here, we sampled bat ectoparasites parasitizing a diverse selection of bat species in the families Rhinolophidae, Vespertilionidae, Megadermatidae, Hipposideridae and Pteropodidae in Yunnan Province, China. We show that the ectoparasite prevalence was generally higher in male compared to female bats. Most ectoparasites were found to fall within the Nycteribiidae, Spinturnicidae and Streblidae bat ectoparasite families. We subsequently applied a non-biased sequencing of libraries prepared from the pooled ectoparasites, followed by an in-silico virus-centric analysis of the resultant reads. We show that ectoparasites hosted by the sampled families of bats are found to carry, in addition to a diverse set of phages, vertebrate and insect viruses in the families Aliusviridae, Ascoviridae, Chuviridae, Circoviridae, Flaviviridae, Hepadnaviridae, Hepeviridae, Herpesviridae, Iridoviridae, Marseilleviridae, Nairoviridae, Orthomyxoviridae, Parvoviridae, Poxviridae, Reoviridae, Retroviridae, and Rhabdoviridae. We further report a partial Parvovirus VP1/VP2 gene and partial Poxvirus ubiquitin-like gene predicted by two independent next generation sequencing data analysis pipelines. This study describes the natural virome of bat ectoparasites, providing a platform for understanding the role these ectoparasites play in the maintenance and spread of viruses to other animals.

Reference genome for the Northern bat (Eptesicus nilssonii), a most northern bat species.

The northern bat (Eptesicus nilssonii) is the most northern bat species in the world. Its distribution covers whole Eurasia, and the species is thus well adapted to different habitat types. However, recent population declines have been reported and rapid conservation efforts are needed. Here we present a high-quality de novo genome assembly of a female northern bat from Finland (BLF_Eptnil_asm_v1.0). The assembly was generated using a combination of Pacbio and Omni-C technologies. The primary assembly comprises 726 scaffolds spanning 2.0 Gb, represented by a scaffold N50 of 102 Mb, a contig N50 of 66.2 Mb, and a BUSCO completeness score of 93.73%. Annotation of the assembly identified 20,250 genes. This genome will be an important resource for the conservation and evolutionary genomic studies especially in understanding how rapid environmental changes affect northern species.

Extensive Survey and Analysis of Factors Associated with Presence of Antibodies to Orthoebolaviruses in Bats from West and Central Africa.

The seroprevalence to orthoebolaviruses was studied in 9594 bats (5972 frugivorous and 3622 insectivorous) from Cameroon, the Democratic Republic of Congo (DRC) and Guinea, with a Luminex-based serological assay including recombinant antigens of four orthoebolavirus species. Seroprevalence is expressed as a range according to different cut-off calculations. Between 6.1% and 18.9% bat samples reacted with at least one orthoebolavirus antigen; the highest reactivity was seen with Glycoprotein (GP) antigens. Seroprevalence varied per species and was higher in frugivorous than insectivorous bats; 9.1-27.5% versus 1.3-4.6%, respectively. Seroprevalence in male (13.5%) and female (14.4%) bats was only slightly different and was higher in adults (14.9%) versus juveniles (9.4%) (p < 0.001). Moreover, seroprevalence was highest in subadults (45.4%) when compared to mature adults (19.2%), (p < 0.001). Our data suggest orthoebolavirus circulation is highest in young bats. More long-term studies are needed to identify birthing pulses for the different bat species in diverse geographic regions and to increase the chances of detecting viral RNA in order to document the genetic diversity of filoviruses in bats and their pathogenic potential for humans. Frugivorous bats seem more likely to be reservoirs of orthoebolaviruses, but the role of insectivorous bats has also to be further examined.

Apparent annual survival of female eastern small-footed bats (Myotis leibii) roosting in Arkansas bridges

Abstract The Eastern Small-footed Bat (Myotis leibii) inhabits mountainous areas of the eastern United States and generally roosts in cliff faces and talus slopes. However, the difficulty of accessing these sites has limited research on this species. We studied survival rates of eastern small-footed bats by capturing bats roosting in crevices between sections of concrete guardrails on bridges in the Ozark mountains of northern Arkansas from 2014 to 2021. We accumulated 1,413 bat captures (1,050F; 363M) of 283 individual females and 198 individual males sampled over 8 years and determined apparent annual survival of female bats using Cormack–Jolly–Seber survival models. Only 1 of 30 bats tested positive for the presence of Pseduogymnoascus destructans, a fungus causing white-nose syndrome (WNS) which is responsible for significant declines in populations of some North American bat species, when all maternity colonies were sampled in the spring of 2019. Overall apparent annual survival of all females was 0.643, with first-year juvenile survival of 0.472 and adult (ages 1–6) survival of 0.744, approximately 58% greater than first-year survival. Apparent annual survival did not differ greatly among the sites or among years. We found no significant decline or increase in overall number of bats among all sites during the 8 years of study. Stability in both the number of bats encountered and their survival rates suggests that the impact of WNS on these colonies has been marginal.

Seasonal variation and host sex affect bat-bat fly interaction networks in the Amazonian savannahs.

Bats are the second-most diverse group of mammals in the world, and bat flies are their main parasites. However, significant knowledge gaps remain regarding these antagonistic interactions, especially since diverse factors such as seasonality and host sex can affect their network structures. Here, we explore the influence of such factors by comparing species richness and composition of bat flies on host bats, as well as specialization and modularity of bat-bat fly interaction networks between seasons and adult host sexes. We captured bats and collected their ectoparasitic flies at 10 sampling sites in the savannahs of Amapá State, northeastern region of the Brazilian Amazon. Despite female bats being more parasitized and recording greater bat fly species richness in the wet season, neither relationship was statistically significant. The pooled network could be divided into 15 compartments with 54 links, and all subnetworks comprised >12 compartments. The total number of links ranged from 27 to 48 (for the dry and wet seasons, respectively), and female and male subnetworks had 44 and 41 links, respectively. Connectance values were very low for the pooled network and for all subnetworks. Our results revealed higher bat fly species richness and abundance in the wet season, whereas specialization and modularity were higher in the dry season. Moreover, the subnetwork for female bats displayed higher specialization and modularity than the male subnetwork. Therefore, both seasonality and host sex contribute in different ways to bat-bat fly network structure. Future studies should consider these factors when evaluating bat-bat fly interaction networks.

First observation of Lethocerus cordofanus (Hemiptera: Belostomatidae) preying on Afronycteris nana (Chiroptera: Vespertilionidae)

AbstractGiant water bugs (Hemiptera: Belostomatidae) are key predators in freshwater ecosystems and have been reported to feed on several species of vertebrates, including fishes, amphibians and reptiles. Here, we report the opportunistic predation of an adult female vesper bat (Afronycteris nana) by a giant water bug (Lethocerus cordofanus) in a temporary pond in a rice paddy in Guinea‐Bissau, West Africa. To our best knowledge, this is the first instance of natural predation upon a mammal by a giant water bug to be documented in a scientific report.

Individual Variation in Parturition Timing within and among Years for a Bat Maternity Colony.

In monoestrous species, the timing of reproduction can have important impacts on offspring survival. For heterotherms in temperate areas, parturition timing is constrained by cold weather survival strategies, such as hibernation and torpor. Female bats that are year-round residents of temperate regions, such as little brown myotis (Myotis lucifugus), invest significantly in parental care resulting in sharp changes in behavior immediately following parturition. These behavior changes may include increases in nighttime roost revisits, which can be used to identify parturition dates for individual bats that have been passive integrated transponder (PIT) tagged and use monitored roosts. Using a system of tagged bats and monitored roosts in Pynn's Brook and Salmonier Nature Park Newfoundland, Canada, we estimated parturition dates for 426 female M. lucifugus in at least one year, based on changes in nighttime roost revisit patterns, and quantified the variation in parturition dates within years among individuals, and within individuals among years. Overall, we report on a wide variation in parturition dates within years among individuals as well as year-to-year variations, both across the population and within individuals. Spring weather conditions appeared to be important influences on parturition timing. Changes in spring and summer temperature and extreme weather events, as expected due to ongoing climate change, may impact parturition timing, and therefore, offspring survival of temperate bats.

Overwinter Mass Loss of Townsend's Big-Eared Bats in Five Caves

Quantifying overwinter mass loss in bats is important for understanding hibernation energetics, habitat conservation, and the ability of bats to persist with novel pathogens. Townsend's big-eared bat (Corynorhinus townsendii) is a species of conservation concern. Little is known about overwinter mass loss of this bat in western North America. We conducted a retrospective analysis to quantify overwinter mass loss of 362 females and males during 1987 and 1988 in 5 caves in an important area for the conservation of this bat in western North America. Although body mass of 13 females in cave C62 averaged 1.1 g heavier than 12 females in cave C54 when captured in October, all those females recaptured in March lost similar percentages of body mass (x̄ = 22.6%) over winter. In cave C27, body mass of 6 females averaged 2.6 g heavier than 8 males when captured in October. Those females and males recaptured in March lost similar percentages of body mass (x̄ = 19.1%) over winter. In caves C27 and C54, mass of 96 male bats decreased by a mean of 21.1% between October and March. Across all five study caves, mass of 227 female bats decreased by a mean of 23.7% between October and March. Our results indicate that females are heavier than males when entering hibernation, and that females generally lose more mass than males during hibernation. Moreover, female bats entering hibernation in our study area weighed more than female bats of this species in other studies when entering hibernation. Our data provide researchers in western North America with mass loss estimates for female and male Townsend's big-eared bats.