White-nose syndrome is a fungal disease killing bats in eastern North America, but disease is not seen in European bats and is less severe in some North American species. We show that how bats use energy during hibernation and fungal growth rates under different environmental conditions can explain how some bats are able to survive winter with infection and others are not. Our study shows how simple but nonlinear interactions between fungal growth and bat energetics result in decreased survival times at more humid hibernation sites; however, differences between species such as body size and metabolic rates determine the impact of fungal infection on bat survival, allowing European bat species to survive, whereas North American species can experience dramatic decline.

Keywords

We model the growth dynamics of Pd and energetic requirements of WNS-affected hibernating bats under a range of environmental conditions. Populations of the little brown bat (Myotis lucifugus) in the northeastern United States and Canada have been more affected by WNS than any other species ( 5 ). We compare model outcomes from M. lucifugus to another species less affected by WNS in North America, the big brown bat (Eptesicus fuscus), and to two apparently unaffected European species, the serotine (Eptesicus serotinus) and greater mouse-eared bats (Myotis myotis). We model Pd growth as a function of body temperature and RH and incorporate this into an energetic model across a range of ambient temperatures. We predict mortality times, based on critical depletion of stored body fat, for specific combinations of environmental conditions. We then use climate data from within the distributions of each species to predict survival times of each species compared to spatially varying winter durations within its range, with and without fungal infection.

Hibernating bats spend the majority of winter in deep torpor (body temperature <10°C) with intermittent arousals to euthermia (35° to 38°C) ( 16 ). Euthermic arousals at low ambient temperature consume the majority of overwinter energy reserves, and bats require specific and narrow ranges of hibernaculum temperatures to survive winter on their limited energy budgets ( 16 , 17 ). Studies show that WNS mortality begins approximately 3 months into hibernation, and diseased bats typically become emaciated by late winter as arousal frequency from torpor increases ( 8 , 18 ). Thus, a common hypothesis for why bats die from WNS is that Pd infection causes them to arouse with increasing frequency and deplete their required fat reserves ( 8 , 18 , 19 ). However, support for how this hypothesis plays out in nature is largely circumstantial, and a mechanistic understanding that would allow prediction of how the WNS epidemic will spread and which species will be most affected is lacking. Few studies explicitly integrate feedback on bat arousal from temperature-dependent fungal growth ( 20 ), despite clear patterns in how temperature influences Pd growth ( 9 ) and how Pd infection increases arousal frequency ( 8 , 18 ). Furthermore, empirical evidence indicates that relative humidity (RH) influences WNS population declines ( 21 ), yet humidity-dependent fungal growth has not successfully been integrated into bat survival models. Species of bats in the northeastern United States showing greatest susceptibility to WNS are known to consistently use the wettest hibernacula ( 10 ), and conidial fungi are more likely to germinate on wet surfaces ( 22 ), suggesting that Pd may be more infectious in humid sites. Humidity has been integrated into bat hibernation models ( 20 , 23 ), but only as a factor influencing evaporative water loss (EWL) in the bat hosts and not accounting for potentially nonlinear interactions among temperature, RH, and growth of the fungus in bat skin.

The annual spread of the fungus that causes WNS, Pd, in North America. Shaded land units represent counties in the United States and voting subdistricts in Canada. Red counties show where the disease is believed to have started during the winter of 2006–2007.

Central questions in disease and evolutionary ecology relate to how interactions among hosts, their parasites, and the environment affect host-parasite dynamics and why pathogenicity may differ among individuals and species. Wildlife diseases can have major impacts on human, animal, and ecosystem health. Epizootics of rinderpest affected African ecosystems for decades ( 1 ), and chytridiomycosis and ranavirus have caused worldwide decline of amphibian populations and species extinctions ( 2 , 3 ). Of similar magnitude and potential for long-term impacts, white-nose syndrome (WNS) is one of the most rapidly spreading wildlife diseases ever recorded ( Fig. 1 ) ( 4 , 5 ). WNS is caused by the psychrophilic (cold-growing) fungus Pseudogymnoascus destructans (Pd; previously Geomyces destructans) ( 6 – 8 ), which grows at approximately 0° to 19.7°C ( 9 ) and invades the skin tissues of hibernating bats ( 10 , 11 ). European bats infected with Pd can hibernate without obvious mortality ( 12 ); however, since the discovery of WNS in North America during 2007, WNS has been diagnosed in seven species spanning 26 U.S. states and 5 Canadian provinces ( Fig. 1 ) and has killed millions of bats. Genetic evidence suggests that Pd was introduced to North America from Europe ( 7 , 13 – 15 ).

RESULTS AND DISCUSSION

Our modified energetic model (16) is generalizable across species that prevent their body temperatures from dropping below (defend) different temperatures (T tor-min ) during overwinter hibernation. Our generalization ensures that if the temperature is at the optimal torpor temperature (T tor-min ), then energy use is minimized [minimum torpor metabolic rate (TMR min )] so that time in torpor (t tor ) is maximized, whereas if the temperature is away from T tor-min , t tor is reduced such that it is equal for given TMR above and below T tor-min (fig. S1). Our results demonstrate that bats are energetically constrained to narrow ranges of cold temperatures for surviving long winters (for example, 6 months) even in the absence of fungal infection (fig. S2). We additionally model fungal growth dynamics and incorporate these dynamics into the energetic model by assuming that Pd infection affects bats by increasing the consumption of stored fat with increasing frequency of arousal from deep torpor.

Higher arousal frequency increases overwinter mortality in the environmental conditions within which Pd grows well, especially warmer and wetter hibernacula (Fig. 2). The model predicts that hibernating M. lucifugus infected with Pd can make energy reserves last 6 months at both ambient temperatures (T a ) between 1° and 6°C and <98% RH. However, bats that defend different minimum temperatures (T tor-min ) have different optimal torpor temperatures, and this and other host traits affect the duration of winters they can survive (Fig. 2).

Fig. 2 Predicted times to deplete overwinter energy reserves for bats infected with Pd. (A to D) Model predictions in months (surface colors and contours) are shown for a range of RH percentages (88 to 99%) and ambient hibernacula temperatures (0° to 19.4°C) at which Pd grows for M. lucifugus (A), E. fuscus (B), M. myotis (C), and E. serotinus (D). The arrow in (A) shows 3.3 months at 7°C and 97% RH for M. lucifugus.

Support for our model predictions includes independent experimental observations, in which M. lucifugus experimentally infected with European and North American isolates of Pd died within 70 to 120 days at 7°C and 97% RH (8). Under those conditions, our model predicts that M. lucifugus can survive 100 days, matching the experimental results (Fig. 2). The model predicts survival to 185 days (6 months) under these same conditions without Pd infection (fig. S2). Additional evidence of model validity includes predicted surface areas infected (fig. S3) and maps of predicted overwinter survival across the distributions of affected species (Fig. 3) that qualitatively match observed disease patterns, as well as observations of greater mortality in hibernacula with warmer temperatures (21).

Fig. 3 Comparison of winter fat depletion in North American bats. (A to F) Difference between winter duration and predicted time to deplete overwinter energy reserves for M. lucifugus (A to C) and E. fuscus (D to F) within their distributions. Differences in months are shown before (A and D) and after (B and E) the arrival of the fungus Pd, the cause of WNS. Blue indicates that bats are predicted to have more than enough energy reserves to survive a typical winter (+ values), with white (no difference, all energy reserves used) and red (− values) indicating that bats are unable to survive winters with enough energy reserves to survive through hibernation. Dark gray indicates that energy reserves are greater than an 8-month upper limit in (D). Distributions of the difference between winter duration and the model’s predicted overwinter survival times are in the histograms (C and F), with median values (dashed lines) for pre-Pd (blue) and post-Pd (pink) infection and zero difference (black line) shown.

The modeled mechanism of feedback between increased fungal growth, decreased torpor time, and bat energetics driven by temperature and RH seems reasonably supported for M. lucifugus, but the robustness of this mechanism is further supported across multiple species. For example, our model predicts that E. fuscus can hibernate for >6 months in the absence of Pd infection (fig. S2) and over a larger range of environmental conditions than M. lucifugus. This trend is reflected in the extended survival times for E. fuscus when Pd infection is incorporated into the model (Fig. 2). We find that European M. myotis is limited to a smaller environmental parameter space than the other species analyzed (Fig. 2). However, when we compare predicted potential hibernation duration to estimated winter duration (hereafter “survival capacity”) of each of the four species across their distributional ranges using climate data (Figs. 3 and 4), results indicate that although M. myotis has a lower median survival capacity (+3.2 months) than E. serotinus (+4.3 months), the median survival capacities of both infected European species are higher than those predicted for both infected North American species (M. lucifugus, −1.2 months; E. fuscus, +2.2 months). Modeled survival capacities of M. lucifugus and E. fuscus without Pd infection were +1.5 and +4.9 months, respectively (Fig. 3), again indicating substantially higher overwinter survival in the absence of the fungus. Bootstrapped Harrell-Davis quantiles of predicted survival capacity times are statistically higher for E. fuscus than M. lucifugus (+0.6 month versus −3.2 months, P < 0.001; +2.1 months versus −1.0 month, P < 0.001; +3.3 months versus +0.9 month, P < 0.001).

Fig. 4 Comparison of winter fat depletion in European bats. (A to D) Difference between winter duration and predicted time to deplete overwinter energy reserves for M. myotis (A and C) and E. serotinus (B and D) within their distributions. Results in months are shown with the fungus Pd, the cause of WNS in American bats. Blue indicates that bats are predicted to have more than enough energy reserves to survive a typical winter (+ values), with white (no difference, all energy reserves used) and red (− values) indicating that bats are unable to survive winters with enough energy reserves to survive through hibernation. Distributions of the difference between winter duration and model’s predicted overwinter survival times are in the histograms (C and D), with median values (dashed red lines) and zero difference (black line) shown.

We assumed that European bat parameters were measured from populations with Pd infection. Given this assumption, our model suggests that behavioral and/or physiological traits may have evolved or been preadapted in the European species to increase survival with Pd infection, whereas in M. lucifugus and E. fuscus, such traits have not evolved. Predictions suggest that E. fuscus may be better suited to survive WNS infection than M. lucifugus. Our findings suggest that environmental conditions and basic host traits alone may explain much of the variability in disease outcomes among species of bats infected by Pd in North America and Europe.

Our results show how incorporating temperature- and humidity-dependent fungal growth into models to predict bat hibernation energetics, and thus overwinter survival, is critical because these key features of fungal ecology are inextricably linked to bat mortality from WNS. Sensitivity analysis assessing that the proportion of examined parameter space survival is predicted over after 6-month winters (16) reveals that survival is significantly affected by numerous parameters, but that our model is most responsive to fungal growth changes related to RH (Fig. 5).

Fig. 5 Sensitivity analysis of model parameters. Sensitivity analysis results using partial rank correlation coefficients (PRCCs) for the output variable predicted mortality after 6 months for a range of RH percentages (88 to 99%) and ambient hibernacula temperatures (0° to 19.4°C) for M. lucifugus. Positive PRCCs indicate that increasing a parameter increases mortality. Parameter definitions and PRCC values are in Table 2. Bat parameters are light gray, temperature-related fungal growth parameters are mid-gray, and humidity-related fungal growth parameters are dark gray. Significance at α = 0.05 is demarcated by the dashed line.

The importance of RH to Pd growth is plausible, because conidial fungi such as Pd are more likely to germinate and degrade nutrient substrates in the presence of high moisture levels in their environments (22). These observations and results suggest that individuals, colonies, or species of bats that use microhabitats with lower RH will be less susceptible to WNS. In most caves, RH reaches 100% far from entrances but can vary throughout, and RH is affected by T a , airflow, and atmospheric pressure (24). Typically, bats use hibernation sites with 90 to 100% RH (25); however, three North American species that seem less severely affected by WNS—Myotis sodalis, Myotis leibii, and E. fuscus—tend to select drier areas within hibernacula, whereas the three species most dramatically affected by WNS—M. lucifugus, Myotis septentrionalis, and Perimyotis subflavus—consistently roost in the most humid locations within hibernacula and are regularly observed with condensation on their fur (10, 21). Changes in RH are predicted to affect bat survival independent of Pd infection [for example, Cryan et al. (10) and Willis et al. (23)]. We have not incorporated RH into the bat energetic model to allow for a parsimonious modeling approach; however, we predict that increased RH will likely improve the survival of M. lucifugus in the absence of Pd by reducing EWL (26, 27), whereas our model predicts that increasing humidity in hibernacula with Pd generally decreases bat survival through WNS, a result supported by field data (21). The single most affected species, M. lucifugus, is in positive water balance at 2° and 4°C only at ≥99% RH (25), which is the ideal growing conditions for Pd. Monitoring RH and understanding the interactions between water vapor pressure and EWL (26, 27) in bat hibernacula may be a key element of future disease surveillance and research efforts, because this environmental variable might explain a considerable amount of the variability in Pd pathogenicity observed among species, hibernation environments, and continents.

Sensitivity analyses revealed that host traits had smaller effects on survival than changes in humidity-dependent growth parameters (Table 2 and Fig. 5). However, torpor is already constrained thermally (fig. S2) (16), and host traits and species distributions interact to explain species-specific differences observed in survival times (Figs. 2 and 3). Decreasing time in torpor and increasing euthermic times also increased mortality in the sensitivity analysis as expected. With respect to host traits, survival was most sensitive to changes in body mass (BM), with increasing mass decreasing mortality (possibly due to increased fat available and decreased thermal conductance), followed by the lower temperature at and above which bats are thermally neutral (T tor-min ) and minimal resting metabolic rate (RMR) occurs, T lc . However, there was little natural variation in T lc [coefficient of variation (CV), 0.01] among species (Table 1), and greater variation in other parameters that significantly affected the model results (Fig. 5) [for example, TMR min (CV, 1.21) and T tor-min (CV, 0.66)] suggests that these may be higher priority for study because they might provide greater predictive ability regarding differences in survival between species. Attempts to predict the necessary host trait parameters (for example, T lc and TMR min ) from more frequently measured parameters (for example, BM and T tor-min ) were unsuccessful (r2 = 0.01 to 0.84), highlighting a more general need to study bats further.

Table 1 Winter energy expenditure model and parameters related to fungal growth. Data are given for little brown (M. lucifugus), big brown (E. fuscus), greater mouse eared (M. myotis), and serotine (E. serotinus) bats with the calculated CV for each parameter that was varied. View this table:

Table 2 Sensitivity analysis results for the E. fuscus–Pd model. Positive PRCCs indicate that increasing a parameter increases mortality. Default parameter values are in Table 1 and were sampled from uniform distributions from the default values to a minimum of 10% lower than default values. View this table:

Our model results predict that those species more likely to survive Pd in North America will have a combination of physical and behavioral traits, including larger body sizes and hibernation in drier sites and/or colder sites. The results suggest that within populations, there will be strong selection pressure for these traits.

Our results indicate that we can predict bat mortality using a model that incorporates bat traits, fungal growth components, and environmental conditions. We recognize, however, that other factors can play a role in the disease process. Increased metabolic rates may occur through alternative mechanisms in the absence of increased arousal frequency (28). In addition to energy expenditure, other mechanisms that may lead to decreased survival of WNS-affected bats include altered physiological processes during winter, such as fungal damage to wing membranes potentially disrupting blood circulation, water and electrolyte balance, or immune function (10, 29–32). All or some of these processes may influence arousals from torpor, but we were able to explain and predict bat mortality using a parsimonious host energetic model, and some of these additional aspects may be captured phenomenologically in our model, because Pd growth is what influences arousal (8, 18). The overwhelming effect of increased arousals depleting energy reserves is persuasive given that one arousal bout of M. lucifugus hibernating at 5°C consumes the same amount of fat energy as 67 days spent in torpor (17).

Previous studies have modeled the temporal dynamics of WNS as it spreads (33). We assume that because Pd is a saprobic fungus, it can persist in the underground environments inhabited by overwintering bats. Therefore, we did not model transmission dynamics within the bat population itself. Host density and social behaviors likely influence the transmission dynamics of WNS (21) and could be incorporated into modeling frameworks along with host-specific fungal growth as more data become available (34). Our mechanistic model may provide a framework for potentially integrating species distribution modeling (16), climate change, and the impacts of WNS (for example, fig. S4). However, such efforts may require a better understanding of how to scale macroscale environmental changes, such as climate change, to microscale environmental change within caves—our analyses suggest that simple scaling (for example, fig. S4) may not be representative (35, 36).

The results of our model paint a bleak picture for the American bat species M. lucifugus, as predicted increased arousal frequency across most of their distribution suggests that they will struggle to survive (Fig. 3). In conclusion, our model results allow us to understand the interactions between host and pathogen at the individual level, and their interaction with the environment, as well as allow us to scale these up to regional levels to understand and predict widespread species survival or decline. These results are important steps toward integrating knowledge gained through infection traits, host biology, and climate to increase understanding of WNS disease outcomes in bats, and they demonstrate the value of modeling dynamics of the “epidemiologic triad” (environment, pathogen, and host) to predict species-specific differences in infection outcome.