White-nose syndrome (WNS), caused by the emerging fungal pathogen Pseudogymnoascus desctructans, has led to a steep decline in North American bat populations since the disease was first described in 2006. Although previously thought to only threaten bat populations in northern latitudes with prolonged hibernation in cool climates, recent reports of bat mortalities in Texas [USA] and Oaxaca [southern Mexico] indicate that populations in warmer regions are also vulnerable to WNS, posing risks to ecologically and agriculturally important bats in Mexico. Pseudogymnoascus destructans can persist on cave substrates under certain microclimate conditions, even without a host. As a result, caves that are not consistently used as hibernacula may still serve as reservoirs for pathogen transmission. Our objective was to assess microclimate suitability within selected caves in northern Mexican karst systems to characterize the region’s ability to sustain sufficient growth of P. destructans and the ability of Mexican caves to act as foci for the spread of this pathogen. We measured internal temperature from 10 caves in the Sierra Madre Oriental region of Mexico and correlated monthly mean temperatures to external surface variables (elevation, latitude, monthly mean surface temperature) to predict internal cave temperatures of known caves in surrounding karst systems within the Sierra Madre Oriental region. We then assessed the potential suitability for P. descructans in these cave systems given known relationships between the fungus and cave temperatures. Our results indicate that caves within the Sierra Madre Oriental region are likely able to sustain internal microclimates that support the growth of P. destructans, with the western ridge of the range containing a high density of caves that are particularly suitable for fungal growth. Our results suggest that the Sierra Madre Oriental region is an important region to monitor for the spread of P. destructans and WNS in Mexico.

We collected internal temperature data from 10 caves across Mexico from January to December 2019 in six states and two mountain ranges: The Sierra Madre Oriental, which runs longitudinally through six states in the central-eastern part of the country, and the Eje Volcánico Transversal, a seismically active volcanic belt that runs latitudinal through twelve states in south-central Mexico. Most caves were selected from existing records provided by collaborating biologists. Two caves sampled in Nuevo Leon were new to science and first documented during an expedition survey in the summer of 2019; for these caves, data loggers were deployed at the time of the initial survey. The geographical position of the caves reflected a wide gradient of external surface temperature, annual precipitation, lithology, and elevation. All caves were visited to inspect for qualifying criteria for the study, which included: (i) potential to sustain a wintering bat population, indicated by evidence of bat presence during the winter season (e.g., presence of an individual, guano); (ii) low levels of human disturbance, avoiding potential interferences with the deployed equipment; (iii) representativeness of gradients of external factors such as elevation, climate, lithology, and spatial distribution. 

EasyLog EL-USB-2 data loggers were deployed in each cave deemed to be suitable for data collection. These data loggers recorded temperature (° C) and relative humidity (%) at 1 h intervals throughout the course of their deployment. Data loggers were deployed in the coolest room of each cave, which typically was the room furthest from the entrance of the cave. Each data logger was attached to the wall of a cave, far enough off the floor of the cave to be near an area where an overwintering bat might roost, and in an area where the equipment would be safe from disturbance throughout the time of its deployment. Each data logger was left to record data for at least one winter season. The recorded data was downloaded using EasyLog system software. We calculated monthly mean internal temperature and relative humidity for analyses on the monthly sampling scale.

External microclimate and landscape data were obtained from various open-source databases. Monthly mean surface temperatures (°C) were obtained from rasters provided by NASA Earth Observations. A digital elevation model (DEM) raster (resolution 30m) was provided by the Amazon Web Services Terrain Titles and the Open Topography global datasets from R (package “elevatr”).