Overwinter hibernation survival
Overall, our energetic model predicted nearly ubiquitous survival of uninfected bats throughout the range of M. lucifugus with < 0.0001% of cells falling below the threshold for survival. Our results found that 95% of all uninfected bats roosting at \(4\)˚C and 98% relative humidity during hibernation would only require 0.21 g–0.60 g of fat to survive the duration of hibernation (median = 0.48 g, mean = 0.45 g, sd = 0.12 g, Figure S2). When considering the amount of fat bats had pre-hibernation, the median bat emerged with 1.85 g of body fat remaining (mean = 2.16 g, sd = 1.01 g) and the heaviest 95th percentile of bats had up to 4.05 g of fat remaining. Considering these residual fat values in the terms of days spent in hibernation, the median bat would have sufficient fat resources to survive an additional 181 days (mean = 190.83 days, sd= 45.35 days) and those bats with < 4 g of fat remaining could be capable of surviving another 280 days in those optimal roosting conditions.
When P. destructans infection was included into the optimal roosting conditions, the median value of fat required to survive hibernation was increased by 0.7 g to 1.21 g (mean = 1.16 g,sd = 0.45) and the residual fat values dropped to a median of 1.22 g (mean = 1.45 g, sd = 0.76). In total, 95% of bats were predicted to emerge with between 0.61 g and 2.93 g of fat remaining after the hibernation. Translating the fat values into days, the median value was reduced ~ 135 days to 45.63 days (mean= 55.41 days, sd = 45.35 days). Mortality of bats prior to the end of the hibernation period was predicted in 4.82 % of the cells where the survival of hibernating bats fell < 0, visible in the northeastern provinces of Canada (Figure S2).
Most of the predicted best available subterranean hibernacula were expected to have available temperatures at or above 4°C. Despite this, 32.64% of cells fell below that temperature and 6.44% of cells fell below the lower critical temperature of 2°C that M. lucifugusdefends during hibernation (Figure S3). Despite these lower than optimal temperatures, M. lucifugus was predicted to survive across most of its distribution, with only 0.5% of cells predicted to fall below the survival threshold—primarily around Denali National Park in Alaska. Median fat required to survive hibernation as an uninfected bat dropped 0.71 g to 1.11 g (mean = 1.41 g, sd = 0.93) with a nearly identical 95% interior range. When including infection withP. destructans, the hibernation energetics model predicted the median fat required for hibernation increased 0.72 g to 1.21g, (mean = 1.20 g, sd = 0.40g). Like optimal roosting conditions, infection resulted in 4.74% of cells falling below the survival threshold.
While neither of the considered hibernation conditions predict large areas to result in mortality, the increased percent of fat needed to hibernate with P. destructans highlights the metabolic consequences of infection (Figure S4). At \(4\)˚C and 98% relative humidity, bats were predicted to expend a median of 154% more body fat resources to hibernate while infected for the same winter duration (Table S4). Overall, 95% of infected bats are predicted to increase their metabolic expenditure between~66% and 195% compared to their healthy counterparts. Hibernating at the predicted best available temperature suggested similar increases in the energy expended; however, the geographic distribution of where the greatest increases occurred were different than those observed under static preferred conditions.