Shifting effects of host physiological condition following pathogen establishment

Abstract

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 can be seasonally variable in wildlife hosts. We used data collected over 7 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 overwinter, 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.

Keywords: Pseudogymnoascus destructans; body mass; emerging infectious disease; population impacts; white-nose syndrome; wildlife disease.

Figure 1.

The effects of body mass during early hibernation on the probability of little brown bat recapture vary with time since P. destructans arrival. In years 0–3 post P. destructans arrival, the probability a bat was recaptured overwinter increased as early hibernation mass increased. However, after WNS established (greater than 3 years since P. destructans arrival), there was no longer a clear trend between early hibernation body mass and bat survival. Solid points of early hibernation body masses during each phase show the fraction recaptured at 0.5 g bins (e.g. 9.75–10.25) and sample sizes for binned data. Shaded regions show 95% confidence intervals.

Figure 2.

(a) Fungal loads and early hibernation (November) body mass of little brown bats strongly influences the change in individual bat mass over winter. Points show individual bats captured in both early and late hibernation. Colours denote masses of bats during early hibernation and labelled lines show predictions based on the 25th (8.7 g), 50th (9.3 g), 85th (9.8 g) and 95th (11.1 g) percentiles of the early hibernation masses. Shaded regions show 95% confidence intervals. Bats that have higher initial body mass lost more weight over winter than bats with lower body mass (i.e. darker lines are higher), suggesting that bats expend fat stores accordingly over winter. In addition, fungal loads significantly modify the effect of early hibernation mass on mass lost overwinter. Bats with high infections that were heavier lose more mass than similarly infected bats that were lighter, suggesting that highly infected bats that survive to be recaptured expend fat in accordance with their infection status. (b) Average body mass of banded little brown bats in early (November) and late (March) hibernation that were recaptured (filled circles) or not recaptured (open circles) overwinter during the WNS epidemic (years 0–3) and WNS established period (years 4+) at the same sites over time (N = 5). Shaded regions show 95% confidence intervals. We found no clear support that hibernation body masses of bats increased over time when examining these data continuously (top) but marginal support categorically (electronic supplementary material, figure S1).