Understanding host persistence with emerging pathogens is essential for conserving populations. Hosts may initially survive pathogen invasions through pre-adaptive mechanisms. However, whether pre-adaptive traits are directionally selected to increase in frequency depends on the heritability and environmental dependence of the trait and the costs of trait maintenance. Body condition is likely an important pre-adaptive mechanism aiding in host survival, although it can be seasonally variable in wildlife hosts. We used data collected over seven years on bat body mass, infection, and survival to determine the role of host body condition during the invasion and establishment of the emerging disease, white-nose syndrome. We found that when the pathogen first invaded, bats with higher body mass were more likely to survive, but this effect dissipated following the initial epizootic. We also found that heavier bats lost more weight over winter, but fat loss depended on infection severity. Lastly, we found mixed support that bat mass increased in the population after pathogen arrival; high annual plasticity in individual bat masses may have reduced the potential for directional selection. Overall, our results suggest that some factors that contribute to host survival during pathogen invasion may diminish over time, and are potentially replaced by other host adaptations.

We studied the arrival and establishment of P. destructans at 24 hibernacula (caves and mines where bats spend the winter) in Virginia, Wisconsin, Illinois, and Michigan over seven years (Tables S1–S3). We visited sites twice per winter and collected data on infection status and body mass of bats. At each site, we sampled individual bats (Table S1, mean = 9.2, range: 1–50) stratified across site sections. Because sites used in this study were primarily small mines where it was possible to observe all bats present, in many instances, all individuals in the population were sampled. For each bat, we collected a standardized epidermal swab sample (Langwig et al. 2015 Proceedings B), attached a unique aluminum band, and measured body mass using a digital scale (GDealer, accuracy +/- 0.03 grams). Because common condition indices are no more effective than body mass for estimating fat stores, we did not include information on bat forearm size in order to reduce handling disturbance. At every visit, we recorded and resampled any previously banded bats present. We stored swabs in RNAlater, and samples were kept at 0 C while in the field, and then at -20 C until processing. We tested samples for P. destructans DNA using real-time PCR and quantified fungal loads (Muller et al. 2013 Mycologia). Animal handling protocols were approved by Virginia Tech IACUC (#17-180, #20-150).

We investigated the effect of bat early hibernation (November) body mass on the probability an individual was recaptured overwinter (e.g. within-winter) using a generalized linear mixed model (GLMM) with a binomial distribution and a probit link, with site as a random effect, and body mass and disease phase (epidemic = 1–3 years since pathogen arrival, or established = 4–7 years since pathogen arrival) as interacting fixed effects (Table S1, total N individuals = 775). Phases were established based on previous results demonstrating that populations approach stability by year 4 following WNS arrival. For analyses of individual survival and body mass, results were similar whether we used categorical disease phase or years since WNS as a continuous variable (Appendix) and grouping by phase maximized the number of bats in the epidemic years when mortality was high and the number of recaptured bats was low. For bats that were recaptured overwinter, we examined the effect of early winter body mass and infection on the amount of mass lost overwinter during both the epidemic and established phase using a linear mixed model with site as a random effect and the change in body mass as the response variable and fixed effects of early winter mass interacting early winter fungal loads with additional additive effect of disease phase (Table S2, total N=158). Finally, we explored changes in mass over time since the invasion of P. destructans on an individual and population level to examine both plasticity and phenotypic change. For bats that were recaptured in multiple years, we used a linear mixed model with mass as a response variable, years since WNS as a fixed effect, and bat band ID as a random effect to explore plasticity in whether individual bat mass changed over time (N=91 observations, 42 unique bands, 1–3 recapture events per bat, Appendix 4.0.3). At a population level, declines in sites with the best invasion mass data limited our ability to explore changes in mass, so we restricted our analyses to N=5 sites (Table S3) that were measured during invasion and had sufficient bats to estimate during established periods using log₁₀ mass as our response variable (logged to normalize) and years since WNS interacting with season with site as a random effect. All analyses were conducted in R v.4.1.2 using lme4.

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