Impacts of Microclimate on Probability of White-Nose Syndrome in Varying Hibernaculum Types

White-nose syndrome (WNS) continues to compromise hibernating Perimyotis subflavus (tricolored bat) populations as it spreads across their geographic range in North America. Despite the presence of the causative agent, the fungus Pseudogymnoascus destructans (Pd), in hibernacula, some tricolored bat populations located near the southern extent of their hibernating range remain unaffected by WNS. We propose that disease progression is absent as these hibernacula, mainly highway culverts, are unsuitable for fungal growth because microclimates fall outside the growth limits of the fungus. Thus, this study investigated the role of hibernaculum microclimate in influencing potential Pd growth and subsequently a hibernaculum's capacity to be an environmental reservoir. We collected microclimate data from multiple types of hibernacula (caves, tunnels, and culverts) near the southern extent where Pd and tricolored bat ranges overlap. Analysis revealed differences in microclimates among hibernaculum types, with culverts exhibiting lower humidity over winter, resulting in reduced fungal growth compared with other hibernaculum types. Caves, which maintained optimal humidity for Pd, had the highest predicted cumulative fungal growth (3.53 cm2), more than double the estimated WNS morbidity threshold (1.54 cm2). The tunnel showed a similar pattern, with a maximum growth of 3.16 cm2. In contrast, culverts, with less suitable microclimates, had the lowest predicted fungal growth (1.78 cm2), only 15% above the morbidity threshold. Although previous research primarily focused on temperature, our findings suggest that relative humidity may be a critical factor, particularly near the southern extent of the geographic range of hibernating tricolored bats. These findings emphasize the importance of considering humidity alongside temperature when evaluating hibernaculum susceptibility to fungal invasion and WNS development. Understanding the impact of microclimate on disease dynamics and bat behavior is crucial for developing effective conservation strategies, particularly for tricolored bat populations near the southern extent of susceptible hibernating populations.