By Dr. Ben Golas, V.M.D.
Colorado State University
Fort Collins, Colorado
White-nose syndrome (WNS) is a disease affecting cave-dwelling bats caused by the fungus Pseudogymnoascus destructans (Pd). WNS first appeared in New York during 2006 and has since spread across the eastern half of the United States and Canada (Figure 1), killing over 6 million bats in the past decade. This is devastating for North American ecosystems; a female little brown bat can eat her weight in bugs in a single night, which doesn’t sound like much but can mean as many as 300 to 3,000 insects per day! By consuming mosquitoes, beetles, and moths that can be pests for people and agricultural crops, bats are estimated to save the United States as much as 3.7 BILLION dollars per year in pest suppression, and this is a service that we are losing through their demise. So how can we promote bat survival in the face of such a deadly threat? Our research group is trying to identify the conditions that allow bats to survive WNS so we can identify and potentially support at-risk populations.
To save bats, the first thing we need to do is find out why some survive, and to answer that question, it behooves us to ask, how does fungal infection similar to athlete’s foot turn deadly? The WNS fungus only grows at low temperatures, between about 0 and 20 degrees Celsius. Bats lower their metabolism when they hibernate, which causes their body temperature to drop to match the cold temperatures of their hibernation roosts, which might mean just above freezing for some. When their body temperature dips low enough, the fungus makes its move, growing into them (Figure 2) and leading to increased arousal from hibernation. This could be due to itchy lesions needing grooming, evaporative water loss from fungus opening thin skin membranes, or other unknown reasons, but in any case, increased arousal means increased energy loss. An arousing bat uses as much as 67 times as much energy as it does in a full day of hibernation, so increasing the frequency of arousal can really take a toll on a bat that can only store a limited payload of fat before hibernation begins. When those fat stores run low in the middle of winter, bats may die of starvation or arouse and attempt to find food. Seeing as there are not many bugs flying around in the dead of winter, these bats die of starvation or exposure outside the roost.
Given this reason for death, our group is trying to identify how some bats can live with WNS. Despite heavy casualties in some species like the little brown bat, others, such as the big brown bat, do not appear to suffer great mortality by WNS, and even of those badly-affected species, there are still remnant populations that survive in areas where the fungus has been around for years. How is it that some bats are part of the millions of dead, but others persist? In 2016, an energetics model was published that starts to get at this phenomenon. It suggests that the microclimates (temperature and humidity) where bats hibernate can influence whether they survive or perish from infection. Bats start the winter with a certain amount of energy, and fungal growth increases the amount of energy needed to survive the winter through increased arousal. The more growth, the more energy used, with both fungal growth and the bat’s regular energy use in hibernation being dependent on local microclimate. The fungus tends to prefer warmer, wetter ends of this spectrum, so the bats that hibernate in warm wet sites are going to have more energy sapped compared to bats using colder, drier roosting areas. Some bats, like the little brown bat, require the warm, wet areas in general, making them more susceptible to mortality, while others, such as the big brown bats, have a larger range of microclimates in which they can hibernate, such that they can avoid the areas where the WNS fungus would flourish.
The exciting thing about this new energetic model is that it seemed to hold true on a species-wide, continental scale, explaining why little brown bats across North America suffer so much more mortality than big brown bats. We are interested in taking this a step further and exploring this hypothesis in individual bats. Do bats that survive in areas where WNS is common choose to use hibernation roosting areas that are not ideal for fungal growth? To explore this idea, we plan to measure microclimates in these roosts (Figure 3) and see if bats are utilizing “survival space,” areas with microclimates that are suitable for bat hibernation but not fungal growth, such that the bat is able to survive winter. We can also look for ecological traps, areas that were commonly used for hibernation before the fungus appeared but are now deadly due to fungal presence. If we can identify survival spaces and ecological traps, we may be able to artificially create environments wherein bats can safely roost, or we might even be able to dissuade bats from hibernating in areas that could be tempting but deadly ecological traps.
Bats are a great asset to us and the natural ecosystems we all share. They have already suffered terribly as WNS has spread across the continent, and likely there is much more death to come, but hope remains in those remnant populations we see persisting in the WNS death zones. By finding out how these populations are surviving, we may be able to help promote their growth as well as potentially protect those that are yet to be exposed. There are many questions yet to be answered, but I at least am optimistic that not all is lost for our North American bats, and there are many brilliant people working hard to try to find solutions for a devastating situation. If you have questions about WNS or our work, please leave them in the comments section, and thank you for reading!